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Sickness absence trajectories among

young employees in their first full-time

employment and subsequent long-term

sickness absence: a Danish register-

based cohort study

Jeppe Karl Sørensen

Jimmi Mathisen

Ute Bültmann

Maria Melchior

Naja Hulvej Rod

Jacob Pedersen

Tea Lallukka

Reiner Rugulies

1,2

Karl Bang Christensen

Stephen Stansfeld

1,8

Ida Madsen

BMJ Public Health: first published as 10.1136/bmjph-2024-002046 on 16 January 2025. Downloaded from https://bmjpublichealth.bmj.com on 22 January 2025 by guest. All rights, including for text and data

mining, AI training, and similar technologies, are reserved.

Background

Sickness absence (SA) poses an important

and costly societal and public health challenge. However,

no previous studies have investigated SA trajectories

among individuals in their first full-time employment,

delving into the contribution of early SA to future long-

term absence among young employees. We aimed to

identify SA trajectories in young full-time employees and

explore them as risk markers for subsequent long-term

SA (LTSA).

Method

In a nationwide register-based cohort of

91 633 young employees (aged 15–30) entering the

►

Additional supplemental

material is published online only.

Danish labour market between 2010 and 2018, we

To view, please visit the journal

applied group-based multi-trajectory modelling to

online (https://doi.org/10.1136/

identify SA trajectories based on daily information

bmjph-2024-002046).

on SA during the first year of full-time employment.

Proportional hazard models were used to examine

associations between trajectory groups and

Received 10 October 2024

subsequent 2-year risk of LTSA after the first year (>30

Accepted 20 December 2024

days).

Results

A model with three distinct SA trajectory

groups was selected: (1) no or few shorter SA spells

(59% for women, 62% for men), (2) frequent shorter

spells (36% for women, 34% for men) and (3) frequent

longer spells (6% for women, 4% for men). A clear

social and health gradient in SA trajectories was

identified with individuals with frequent shorter or

frequent longer SA spells having lower education and a

history of mental health problems. Compared to having

no or few shorter spells, belonging to trajectories with

employer(s)) 2025. Re-use

frequent shorter or longer spells was associated with

permitted under CC BY-NC.

higher risk of subsequent LTSA (fully adjusted HRs

Published by BMJ Group.

ranging from 1.82 to 2.21).

For numbered affiliations see

Conclusion

Around 40% of young employees’ SA

end of article.

was characterised by frequent shorter or longer

spells during the first year of full- time employment,

Correspondence to

constituting an early risk marker for future LTSA.

Dr Jeppe Karl Sørensen;

More attention should be paid to young individuals

[email protected]

To cite:

Sørensen JK,

Mathisen J, Bültmann U,

et al.

Sickness absence trajectories

among young employees in

their first full-time employment

and subsequent long-term

sickness absence: a Danish

study.

BMJ Public Health

2025;3:e002046. doi:10.1136/

bmjph-2024-002046

ABSTRACT

WHAT IS ALREADY KNOWN ON THIS TOPIC

⇒

Extant knowledge suggests a recurrent nature of

sickness absence (SA), with both frequent short-

er and longer spells of SA being associated with

an increased risk of long-term work absenteeism.

However, knowledge regarding young employees at

the start of their work life remained limited and only

a limited number of studies had employed a multidi-

mensional approach, considering both the frequency

and duration of SA.

WHAT THIS STUDY ADDS

⇒

In this study, we used a nationwide register-based

population including 91 633 young employees fol-

lowed from their first year of full-time employment.

Daily records on SA frequency and duration were

used to identify three distinct SA trajectories. A clear

social and health gradient was observed among

individuals belonging to trajectories of SA charac-

terised by either frequent shorter or frequent longer

spells. These identified patterns served as crucial

early risk markers for future long-term SA (LTSA).

HOW THIS STUDY MIGHT AFFECT RESEARCH,

PRACTICE OR POLICY

⇒

From a societal and public health perspective, it

is noteworthy that a clear association between

frequent shorter SA and LTSA was already identi-

fied during the first year of full-time employment.

Considering the labour shortages across various oc-

cupational groups, this underscores the importance

of determining factors contributing to elevated SA

patterns among young employees. To enhance ex-

isting knowledge, greater attention should be given

to understanding why some young individuals, at the

beginning of their work life, tend to experience high-

er rates of sick absence than others.

Sørensen JK,

et al.

BMJ Public Health

2025;3:e002046. doi:10.1136/bmjph-2024-002046

BEU, Alm.del - 2024-25 - Bilag 179: Orientering om videnskabelig artikel fra Det Nationale Forskningscenter for Arbejdsmiljø (NFA), fra beskæftigelsesministeren

BMJ Public Health

who, at the beginning of their work life, show increased SA

patterns.

INTRODUCTION

Sickness absence (SA) poses an important societal and

public health challenge, leading to financial consequences

due to workers’ compensation and decreased produc-

tivity.

1 2

Studies have highlighted the recurring nature of

SA, linking past SA to future SA.

3–5

This pattern has been

confirmed in multiple cohort studies reporting both

shorter (often defined as SA spells <7 days) and longer

spells (≥7 days) of SA to be potentially early risk markers

of long-term SA (LTSA) (>30 days) and disability.

6–8

Most

SA research has modelled SA in one dimension, either

focusing on the frequency of SA spells or the duration

of SA. In 2020, Duchemin and Hocine conducted a

scoping review, highlighting the current state of statistical

methods for modelling SA data.

They proposed that a

more nuanced approach, involving a multidimensional

model that considers both the frequency and duration

of SA, would advance our comprehension of the nature

of SA.

In Denmark, a large number of young individuals

(63%) are employed in marginal part-time jobs while

being enrolled in high school or university.

10–13

In contrast

to young individuals in full-time employment, workers in

marginal part-time jobs exhibit overall fewer days of SA,

possibly owing to increased flexibility in shifting or swap-

ping schedules. Previous studies have highlighted age-

related differences in SA patterns, such as shorter spells

among young employees and longer spells among older

ones.

12 14

However, despite these insights, most research

on SA has predominantly focused on midlife or older

employees in stable jobs,

15 16

leaving a substantial gap

in understanding of the recurrent nature of SA among

young employees.

A broader understanding of SA patterns as potential

early risk markers for long-term absenteeism among

younger employees could provide important insights for

future public health strategies.

Therefore, the present

study aims to (1) identify distinct SA trajectories among

young employees in their first full-time employment, (2)

describe these distinct SA trajectories characteristics and

(3) assess whether they may function as early risk markers

for subsequent risk of LTSA.

MATERIALS AND METHODS

Study design and participants

We used data from the Danish Work Life Course Cohort

study,

an open register-based inception cohort including

579 114 young Danish employees who entered the Danish

labour market for the first time between 2010 and 2018,

while being aged 15–30 and followed up in national regis-

ters until 2020. Using the unique Danish social security

number, we linked individual-level data on daily SA from

the combined Danish Register of Work Absence (RoWA).

This database includes information on public employees

and an annual sample of employees from medium to large

private companies (≥10 employees).

Consequently, we

were able to follow 301 778 individuals in RoWA for at

least 1 year between 2010 and 2018. We excluded individ-

uals who emigrated (n=397), received disability pension

(n=150) or died (n=0) before or during their first year

of employment. We also removed individuals because of

unknown information on their sex (n=46). To identify

those in their first full-time employment, we excluded

employees with contracts of less than 32 work hours per

week (n=209 552). The final study population consisted of

91 633 individuals, with the baseline year defined as their

first full-time employment year between 2010 and 2018.

Sickness absence

From RoWA, we included data on all-length SA due to own

sickness.

In the baseline year, we recorded the monthly

number of SA spells (≥1 day) and the monthly number of

days with SA during 12-month follow-up from 1 January

to 31 December. The prospective outcome of LTSA was

defined in the Danish Labour Market Account

between

2011 and 2020 as payment of SA benefits for more than

30 days in a 2-year period after the first year of full-time

employment. In Denmark, if employees are unable to

work due to illness or injuries, the employer is obligated

to pay wages from the first day of absence. After 30 days

of SA, the employer can request reimbursement for SA

benefits from the municipality.

Covariates

From Danish administrative registers,

we included

data on sex (women, men), age, migration background

(Danish, immigrant, descendent of immigrants), cohab-

itation (single, cohabited) and highest educational

attainment (primary, secondary, vocational training,

bachelor or higher). From the National Patient Register

and the Psychiatric Central Research Register,

identified individuals who had been diagnosed with

any mental disorder using the International Classifi-

cation of Diseases (ICD)-10 system (ICD-10: F00–F99)

before labour market entry. As an indicator of health

status, we included information on the annual number

of health services used within primary healthcare

(eg, general practitioner, physiotherapists, chiroprac-

tors) 1 year before baseline from the National Health

Service Register.

At last, we included information on

the employment sector (public, private), occupation

(professionals, semiprofessional and clerical, routine)

and industry (knowledge work, private service, care

work and education, industrial work, building and

construction, other). See online supplemental material

appendix 1, (table S1) for grouping of occupation and

industry.

Statistical analyses

Identifying distinct SA trajectories

To identify distinct SA trajectories within the first year

of full-time employment, we used the group-based

mining, AI training, and similar technologies, are reserved.

Sørensen JK,

et al.

BMJ Public Health

2025;3:e002046. doi:10.1136/bmjph-2024-002046

BEU, Alm.del - 2024-25 - Bilag 179: Orientering om videnskabelig artikel fra Det Nationale Forskningscenter for Arbejdsmiljø (NFA), fra beskæftigelsesministeren

BMJ Public Health

multi-trajectory model (GBMTM) with the PROC TRAJ

procedure developed by Nagin

et al

for SAS 9.4.

detailed description of the applied model is presented

in online supplemental material appendix 2. Briefly,

GBMTM is a statistical method for analysing longitudinal

data to identify distinct groups sharing similar trajecto-

ries over time. Unlike traditional approaches, GBMTM

examines multiple outcomes simultaneously and aims to

identify subgroups with comparable trajectories across

time.

Among women and men separately, we modelled

SA trajectories using GBMTM, including simultaneous

information on the monthly number of SA spells and

the monthly number of SA days within the baseline

year. To find the optimal number of SA trajectories,

we used a random subsample of 5000 women and 5000

men and compared model fit for GBMTMs with one to

eight SA trajectories. Following previous research and

current recommendations,

we used three key factors

to compare different statistical models and to assess how

well the chosen model fitted our data: (1) the Bayesian

information criteria (BIC; lower values indicate better

model fit); (2) the average group probability (average

values above 70% indicating a better model where more

individuals are assigned group membership with high

probability) and (3) a sufficient number of individuals in

each trajectory group to ensure comparison (all groups

should contain approximately 2% of the population or

more). The best fitting model was chosen by authors JKS,

KBC and IEHM based on these criteria. At last, the best-

fitting model was applied to the full study population,

and individuals were assigned to the SA trajectory group

to which they had the highest probability of belonging.

Characteristics of SA trajectories

We investigated the SA trajectory groups for explanatory

variables by comparing group prevalence across different

factors within the baseline year. First, we compared yearly

SA spells and yearly SA days across SA groups. Second,

we compared prevalence across sociodemographic (age,

migration background, cohabitation and highest educa-

tional attainment), health (history of mental disorder

before labour market entry, and yearly number of health

service use) and occupational factors (employment

sector, occupation and industry). At last, we explored the

first weekday of each SA spell within SA group member-

ship, as we wanted to explore if the prevalence of the

starting weekdays would be higher among individuals

with frequent shorter spells of SA.

SA trajectories as an early risk marker for LTSA

To determine whether SA trajectories during the first

year of full-time employment could serve as an early

risk marker for LTSA, we estimated Hazard Ratio (HR)

and 95% CIs for the risk of LTSA, using a Cox propor-

tional hazard model.

Fulfilment of the proportional

hazards assumption was assessed by visual inspection of

the log–log survival plots. We assessed the prospective

risk of LTSA during a 2-year follow-up, from 1 January of

the year following baseline to 31 December 3 years after

baseline. Calendar time was used as the underlying time

axis, and participants were censored at the first instance

of migration (n=4336, defined in the Danish Civil Regis-

tration System

), disability pension (n=13, defined as

outpayment of disability benefits in the Danish Labour

Market Account

), death (n=38, defined in Danish Death

) or end of follow-up (31 December 2019).

Associations were presented as crude LTSA cases per

1000 person-years and additionally adjusted for poten-

tial confounders in three models. In model 1, associa-

tions were adjusted for age (continuous). In model 2, we

further included the employment sector (public, private)

and years since labour market entry (continuous) to

account for the uneven distribution of public and private

employees in RoWA and cohort design. In model 3, we

further included migration background (Danish, immi-

grant, descendent of immigrant), cohabitation (single,

cohabited), educational attainment (primary, secondary,

vocational training, bachelor or higher), history of any

mental disorder (no, yea), annual health services used (0,

1–3, 4–7, 8–15, 16+), occupation (professionals, semipro-

fessionals and clerical, routine) and industry (knowledge

work, private service, care work and education, industrial

work, building and construction, other or unknown) to

explore to what degree associations could be explained

by additional potential confounders. All analyses were

conducted separately for women and men.

Supplementary analyses

We conducted three sensitivity analyses. First, we investi-

gated effect modification by a history of mental disorder

(yes/no) for the association between SA trajectories and

LTSA, as mental health has been identified as an impor-

tant predictor of SA.

Second, we analysed the associa-

tion between SA trajectories and LTSA among public

employees, as RoWA only include a yearly sample of

private employees from companies with more than 10

employees. Third, we stratified analyses by the occupa-

tional group, to explore if SA trajectories served as an

early risk marker for LTSA differently across industries.

Patient and public involvement

Key stakeholders contributed to the development of

research questions and the study design by providing

input from the early stages of the grant application.

While patients were not involved in setting the research

question or the outcome measures, nor in developing

plans for the design, conduct, or implementation of the

study, the National Research Centre for the Working

Environment incorporated patient and public involve-

ment through established advisory boards as part of its

research and dissemination strategy. The results of the

study will be discussed with key stakeholder represent-

atives and disseminated to the Danish public. This will

be achieved by providing Danish summaries of the study

results published online, delivering these summaries

mining, AI training, and similar technologies, are reserved.

Sørensen JK,

et al.

BMJ Public Health

2025;3:e002046. doi:10.1136/bmjph-2024-002046

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to key stakeholders via newsletters, and presenting the

results at public seminars.

RESULTS

Table 1

shows the study population characteristics of the

91 633 young individuals in their first year with full-time

employment (baseline year), including a similar propor-

tion of women (n=44 720, 48.8%) and men (n=46 913,

51.2%) with a mean age of 22.5 years (SD 4.3) and 22.7

years (SD 4.4), respectively. A large proportion (women:

40.9%, men: 41.7%) had low educational attainment with

only primary school, whereas 17.2% (women) and 19.4%

(men) had high education with a bachelor’s degree or

higher degree. Most women worked in semiprofessional

and clerical occupations (54.8%), including sales and

service work, and most men worked in a routine occupa-

tion (43.5%).

SA trajectories during the first year with full-time

employment

We selected a model with three distinct SA groups based

on our criteria of low BIC, high average group probability

(>70%) and at least 2% in each trajectory (online supple-

mental material appendix 3 (table S3 and figure S1)).

Table 2

and

figure 1

present SA characteristics during

first full-time employment across the three identified

SA groups. Among women (58.6%) and men (61.8%),

the most common SA groups were labelled ‘no or few

shorter SA spells’, characterised by either no SA or few

short-term SA spells of one to 2 days during their first

year of full-time employment (table

and

figure 1).

second SA group was labelled ‘frequent shorter SA spells’

and comprised approximately one-third of the women

(35.9%) and men (33.8%). This group was character-

ised as having more frequent SA spells spanning one or

2 days. Among women, the yearly average number of SA

spells within the first year of employment was 5.6 with

an average length of 3.1 days. Among men, the yearly

average number of SA spells was 4.2 with an average

length of 2.5 days. A small proportion (women: 5.5%,

men: 4.4%) was assigned to the third SA group labelled

‘Frequent longer SA spells’. This group was characterised

by a higher number of longer SA spells with on average

14.0 and 11.2 yearly SA spells with an average length of

4.7 and 4.1 SA days among women and men, respectively.

Characteristics of individuals within the SA groups

Among both women and men, individuals with frequent

shorter or frequent longer SA were, compared with

individuals with no or few shorter SA spells, younger

(figure

2A),

less educated (figure

2B)

and more often

had a history of mental disorders (figure

2C).

Frequent

longer SA spells, compared with no or few shorter SA

spells, were more common within care work and educa-

tion and among men working within building and

construction (online supplemental material appendix 3

(table S5 and figure S2)). Across the three SA groups,

most SA spells started on a Monday (women: 25.5%

and men: 28.7%) and we found overall no indication of

differences in the start day across SA trajectories or sex

(online supplemental material appendix 3 (figure S3)).

SA groups and subsequent LTSA

We identified 2386 (5.3%) and 1573 (3.4%) cases of

at least one LTSA during 2 years of follow-up (mean

follow-up 1.5 years) among women and men, respectively.

Among both women and men, SA group membership was

associated with LTSA (table

3).

Women in the SA group

with frequent shorter SA and women in the SA group

with frequent longer SA had, compared with women

in the SA group with no or few shorter SA spells, HRs

(95% CI) of LTSA of 2.14 (1.96 to 2.33) and 2.55 (2.20

to 2.96), respectively (model 2). Among men, the corre-

sponding HRs of LTSA were 2.23 (2.01–2.48) (frequent

shorter SA) and 2.89 (2.39–3.50) (frequent longer SA),

respectively (model 2). In fully adjusted analysis, associ-

ations attenuated but remained significant: HR of 1.82

(1.66–1.99) and 1.90 (1.63–2.22) among women and HR

of 1.90 (1.70–2.11) and 2.21 (1.82–2.69) among men for

frequent shorter and frequent longer SA spells compared

with no or few shorter SA spells, respectively (model 3).

Supplementary analyses

In supplementary analyses (online supplementary mate-

rial appendix 4 (table S6)), we found that SA groups

of frequent shorter and frequent longer SA spells were

associated with a higher risk of LTSA among both women

and men in those individuals with and without a history

of mental disorder when compared with individuals in

the SA group with no or few shorter SA spells. Among

individuals with a history of mental disorders, the asso-

ciations were weaker with HRs of 1.51 (1.18–1.93) and

1.50 (1.10–2.05) for frequent shorter SA spells, and 1.44

(1.02–2.04) and 2.15 (1.35–3.41) for frequent longer

SA spells for women and men, respectively. Stratified by

industries, all categories showed similar associations.

DISCUSSION

In a cohort of 91 633 young Danish employees in their

first full-time employment, we sought to identify distinct

SA trajectories and examine their associations with subse-

quent LTSA. We found that most of the young employees

were categorised as having no or few shorter SA spells and

around one-third were categorised as having frequent

shorter SA spells. A small proportion of individuals (5%)

were categorised as having frequent longer SA spells.

We found a strong social and health gradient within SA

groups with low age, low educational attainment and a

history of mental disorders being more prevalent among

individuals with both frequent shorter and longer SA

spells compared with individuals with no of few shorter

SA spells. In prospective analyses, we found frequent

shorter and frequent longer SA trajectories to be associ-

ated with subsequent higher risk of LTSA during 2 years

of follow-up.

mining, AI training, and similar technologies, are reserved.

Sørensen JK,

et al.

BMJ Public Health

2025;3:e002046. doi:10.1136/bmjph-2024-002046

BEU, Alm.del - 2024-25 - Bilag 179: Orientering om videnskabelig artikel fra Det Nationale Forskningscenter for Arbejdsmiljø (NFA), fra beskæftigelsesministeren

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mining, AI training, and similar technologies, are reserved.

Table 1

Characteristics of young (aged 15–30) Danish individuals in their first full-time employment (baseline) between 2010

and 2018 across sociodemographic, health and occupational factors for women (n=44 720) and men (n=46 913)

Women

Baseline-year characteristics

Total

Age in years: mean (SD)

15–19

20–25

>25

Educational level

Primary

Secondary

Vocational training

Bachelor or higher

Unknown

Cohabitation

Single

Cohabited

History of hospitalisation with any mental disorder

Yes

Annual health service use

1–3

4–7

8–15

16+

Employment sector

Public

Private

Occupation

Professionals

Semiprofessional and clerical

Routine

Unknown

Industry

Knowledge work

Private service

Care work and education

Industrial work

Building and construction

Other or unknown

Sickness absence (SA) characteristics

Yearly SA spells: mean (SD)

Yearly SA days: mean (SD)

SA, sickness absence; SD, standard deviation.

Men

100.0

22.5 (4.3)

29.3

37.6

33.1

40.9

22.1

16.6

17.2

3.1

42.0

58.0

92.5

7.5

17.4

12.5

17.1

24.2

28.7

95.4

4.6

21.0

54.8

24.1

0.1

12.0

43.6

34.8

7.1

0.5

2.1

3.3 (3.8)

10.7 (24.1)

46 913

13 033

17 446

16 434

19 579

9575

6707

9119

1933

22 300

24 613

43 320

3593

16 623

10 862

8785

6777

3861

43 456

3 457

9625

16 833

20 411

7924

21 422

7656

6743

2462

706

100.0

22.7 (4.4)

27.8

37.2

35.0

41.7

20.4

14.3

19.4

4.1

47.5

52.5

92.3

7.7

35.4

23.2

18.7

14.4

8.2

92.6

7.4

20.5

35.9

43.5

0.1

16.9

45.7

16.3

14.4

5.2

1.5

2.2 (3.0)

6.0 (14.9)

44 720

13 125

16 808

14 787

18 312

9893

7434

7684

1397

18 767

25 953

41 352

3368

7783

5610

7648

10 829

12 844

42 665

2055

9402

24 510

10 761

5357

19 500

15 561

3156

227

919

Sørensen JK,

et al.

BMJ Public Health

2025;3:e002046. doi:10.1136/bmjph-2024-002046

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mining, AI training, and similar technologies, are reserved.

Table 2

Sickness absence (SA) characteristic across SA trajectory groups among young Danish women (n=44 720) and men

(n=46 913) in their first full-time employment between 2010 and 2018

N (%)

Women

No or few shorter SA spells

Frequent shorter SA spells

Frequent longer SA spells

Men

No or few shorter SA spells

Frequent shorter SA spells

Frequent longer SA spells

SD, standard deviation.

SA spells

Mean (SD)

0.96 (1.10)

5.55 (2.28)

13.95 (3.97)

0.51 (0.75)

4.20 (1.95)

11.25 (3.51)

SA days

Mean (SD)

1.41 (1.68)

17.39 (20.77)

65.14 (59.07)

0.71 (1.06)

10.41 (9.45)

46.59 (46.70)

Length of SA spells

Mean (SD)

1.48 (2.44)

3.13 (3.95)

4.67 (4.44)

1.38 (2.90)

2.48 (2.53)

4.14 (4.35)

26 207 (58.6)

16 037 (35.9)

2476 (5.5)

28 997 (61.8)

15 859 (33.8)

2057 (4.4)

Comparison with the previous literature

To the best of our knowledge, this study is the first to

investigate SA trajectories among young individuals in

their first full-time employment. The study population

consisted of relatively young employees (mean age 23)

with mostly primary or secondary education, consistent

with some of the young individuals entering their first

full-time employment between educations. Consequently,

findings from prior studies on SA trajectories in midlife

and older employees make direct comparison with our

study difficult. However, previous research on employees

of all ages has found similar trajectories.

Similar to our

finding, a study representative of Finnish working-aged

employees identified three distinct SA groups character-

ised as low (54%), slowly increasing (34%) and high (12%)

SA periods, with clear differences across education and

health.

This study was, however, limited to only model

trajectories based on the dimension of SA (frequency).

Figure 1

Raincloud plot of sickness absence (SA) spells and SA days across SA groups for young Danish women (n=44 720)

and men (n=46 913) in their first full-time employment between 2010 and 2018.

Sørensen JK,

et al.

BMJ Public Health

2025;3:e002046. doi:10.1136/bmjph-2024-002046

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Figure 2

Prevalence of sickness absence (SA) groups across age (A), educational level (B) and history of mental disorder

before labour market entry (C) for young Danish women (n=44 720) and men (n=46 913) in their first full-time employment

between 2010 and 2018.

Only few studies have investigated SA trajectories using

a multidimensional approach. One study analysing

data from Finish public employees revealed that young

employees (aged 20–34) experiencing frequent short

SA spells had a significantly higher risk of subsequent

LTSA due to mental disorders.

This result corresponds

to the findings from our study indicating that frequent

shorter and frequent longer SA spells may be an impor-

tant early risk marker for LTSA among young employees.

We encourage future studies to identify SA trajectories

separately for younger employees and to model SA using

both frequency and duration when possible.

SA trajectories and subsequent LTSA

In this study, we identified a group of young individuals

with frequent shorter and frequent longer spells of SA

who, compared with individuals with no or few shorter

SA spells, had a markedly higher risk of LTSA two years

after their first full-time employment. We know from

previous studies that longer spells of SA are associated

with future risk of LTSA in working-aged employees (aged

18–59).

The results of this study enhance the existing

knowledge

7 30

by showing that both frequent shorter and

frequent longer SA spells are important early risk markers

for LTSA among young workers in full-time employment,

Sørensen JK,

et al.

BMJ Public Health

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mining, AI training, and similar technologies, are reserved.

Table 3

Association between sickness absence (SA) groups and 2-year risk of long-term sickness absence (LTSA) among

young Danish women (n=44 720) and men (n=46 913) in their first full-time employment between 2010 and 2018

Women

No or few shorter

SA spells

LTSA

cases

Cases per

1000 PY

HR (95% CI)

Crude

HR (95% CI)

Model 1

HR (95% CI)

Model 2

HR (95% CI)

Model 3

40 509

964

1205

217

23.8

50.6

60.6

1.00

2.17 (2.00 to

2.36)

2.62 (2.26 to

3.04)

1.00

2.10 (1.92 to

2.29)

2.48 (2.13 to

2.88)

1.00

2.14 (1.96 to

2.33)

2.55 (2.20 to

2.96)

1.00

1.82 (1.66 to

1.99)

1.90 (1.63 to

2.22)

Frequent shorter SA

spells

23 825

Frequent longer SA

spells

3581

Men

No or few shorter

SA spells

44 910

662

781

130

14.7

33.5

43.8

1.00

2.26 (2.04 to

2.51)

2.98 (2.48 to

3.59)

1.00

2.23 (2.00 to

2.47)

2.89 (2.39 to

3.49)

1.00

2.23 (2.01 to

2.48)

2.89 (2.39 to

3.50)

1.00

1.90 (1.70 to

2.11)

2.21 (1.82 to

2.69)

Frequent shorter SA

spells

23 334

Frequent longer SA

spells

2966

Model 1: adjusted for age. Model 2: further adjusted for the employment sector, and years since labour market entry. Model 3: further

adjusted for migration background, cohabitation, educational attainment, history of hospitalisation with any mental disorder, health service

use, occupation and industry.

PY, person-years.

even after adjustment for socioeconomic position and

health. However, some questions still remain about the

causality of the associations. To expand the existing

literature further, identifying underlying predictors of

different SA trajectories is of great interest. According to

the International Classification of Functioning, Disability

and Health, absence from work due to sickness can be

seen as a temporary impairment of workability, which is

not only a biological and medical phenomenon but also a

phenomenon affected by individual functioning, partic-

ipation, and environmental factors.

Individual factors

such as work environmental factors, illness behaviour

and family situation

might be important predictors of

SA and potential long-term consequences. To expand

our knowledge, future studies should focus on exploring

the reasons why some young individuals at the beginning

of their work life more often call in sick than others.

Strengths and limitations

We were able to identify more than 90 000 younger

employees in their first full-time employment between

2010 and 2018. With data on daily SA for all public

employees and a sample of private employees, we

were able to identify SA trajectories based on both the

frequency and duration of SA spells using the GBMTM

approach, which has been suggested to enhance the

understanding of the nature of SA trajectories.

Further-

more, the large study population made subgroup anal-

ysis across individual health and industry possible.

However, this study was limited to only a sample of private

employees from companies with 10 or more employees,

and hence findings from this study are not generalisable

to all private employees. Additionally, information on the

start and end date of employment was not available and

hence we were not able to account for some employees

starting their first full-time employment during follow-up.

However, we accounted for months with unemployment

benefits during follow-up (see online supplemental

material appendix 2) to account for some individuals not

being at risk of SA while not working. Furthermore, SA

might differ across occupational groups. In some occu-

pational groups, such as knowledge workers, employees

with reduced workability might be able to avoid SA by

working with less intensity and in some occupational

groups, such as care workers or teachers, employees

might be encouraged to call in sick to minimise the

spread of infections to patients, colleagues or students.

Such occupational differences might have affected the

trajectory group membership. However, supplementary

stratified analyses by occupational group indicated an

overall similar association between SA trajectories and

subsequent risk of LTSA. While register data are compre-

hensive and large, it is a limitation that we were not able

to control for potentially important covariates such as

health behaviour, which might to some degree explain

some of the association between the SA group and the

subsequent risk of LTSA. However, after additionally

controlling for sociodemographic and socioeconomic

positions and health markers, the associations attenu-

ated but remained statistically significant. At last, in this

study, we only focused on first full-time employees with

a working time of 32 hours or more a week. We know

from previous research that many young individuals are

Sørensen JK,

et al.

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working in marginal part-time jobs while they are stud-

ying.

10–13

Compared with young full-time employed,

young marginal part-time workers have overall fewer days

with SA,

which might be attributed to increased flexi-

bility in swapping or altering shifts when being unable

to work due to sickness. Hence, as the group of young

employees is very diverse, results from this study should

not be generalised to young employees in marginal part-

time work.

Ethics approval

Not applicable.

Provenance and peer review

Not commissioned; externally peer reviewed.

Data availability statement

Data may be obtained from a third party and are not

publicly available. The data underlying this article cannot be shared publicly due to

data protection regulation. All data are stored in a protected server environment at

Statistics Denmark and can be accessed only by researchers who are authorised

by Statistics Denmark and approved by the National Research Centre for the

Working Environment. Please contact JKS ([email protected]) or IM ([email protected]) for

details.

Supplemental material

This content has been supplied by the author(s). It has

not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been

peer-reviewed. Any opinions or recommendations discussed are solely those

of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and

responsibility arising from any reliance placed on the content. Where the content

includes any translated material, BMJ does not warrant the accuracy and reliability

of the translations (including but not limited to local regulations, clinical guidelines,

terminology, drug names and drug dosages), and is not responsible for any error

and/or omissions arising from translation and adaptation or otherwise.

Open access

This is an open access article distributed in accordance with the

Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which

permits others to distribute, remix, adapt, build upon this work non-commercially,

and license their derivative works on different terms, provided the original work is

properly cited, appropriate credit is given, any changes made indicated, and the

use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

Author note

JM is currently employed at Novo Nordisk A/S and affiliated as a

guest researcher at the Section of Epidemiology at the University of Copenhagen.

The work by JM on this manuscript has been conducted solely in the capacity as a

guest researcher at the University of Copenhagen. Novo Nordisk A/S had no role in

any part of the study.

ORCID iDs

Jeppe Karl Sørensen

http://orcid.org/0000-0002-0789-8752

Jimmi Mathisen

http://orcid.org/0000-0002-7391-7296

Ute Bültmann

http://orcid.org/0000-0001-9589-9220

Tea Lallukka

http://orcid.org/0000-0003-3841-3129

Maria Melchior

http://orcid.org/0000-0002-2377-619X

Naja Hulvej Rod

http://orcid.org/0000-0002-6400-5105

Jacob Pedersen

http://orcid.org/0000-0003-4429-3485

Stephen Stansfeld

http://orcid.org/0000-0001-8716-3897

Reiner Rugulies

http://orcid.org/0000-0002-7752-131X

Karl Bang Christensen

http://orcid.org/0000-0003-4518-5187

Ida Madsen

http://orcid.org/0000-0003-3635-3900

mining, AI training, and similar technologies, are reserved.

CONCLUSION

Most young employees’ (both women and men) SA were

characterised by no or few shorter SA spells during their

first year of full-time employment. One-third had more

frequent shorter SA spells and around 5% had frequent

longer SA spells. We found a social and health gradient

in SA trajectories, with individuals with frequent shorter

or frequent longer SA spells having lower education and

a history of mental health problems. Prospective anal-

yses indicated that SA trajectories within the first full-

time employment are important early risk markers for

subsequent risk of LTSA. To facilitate effective action

and intervention based on these findings, more attention

should be paid to young individuals who, at the start of

their working life, already show increased SA.

Author affiliations

National Research Centre for the Working Environment, Copenhagen, Denmark

Section of Epidemiology, Department of Public Health, University of Copenhagen,

Copenhagen, Denmark

Department of Health Sciences, Community and Occupational Medicine, University

Medical Center Groningen, University of Groningen, Groningen, The Netherlands

Department of Public Health, University of Helsinki, Helsinki, Finland

Institut Pierre Louis d'Épidémiologie et de Santé Publique (IPLESP), Equipe de

Recherche en Epidémiologie Sociale (ERES), Sorbonne Université, INSERM, Paris,

France

Centre for Psychiatry and Mental Health, Barts and the London School of Medicine,

Queen Mary University of London, London, UK

Section of Biostatistics, Department of Public Health, University of Copenhagen,

Copenhagen, Denmark

National Institute of Public Health, University of Southern Denmark, Copenhagen,

Denmark

Maria Melchior @mkmelchior

Contributors

JKS conceived the idea and designed the study with support

from JM, KBC and IM. JKS did the data linkage, data cleaning and the trajectory

analyses. JKS, KBC and IM evaluated the trajectory models. JKS and IM had access

to all the data. JKS wrote the first draft of the manuscript. All authors (JKS, JM,

UB, TL, MM, NHR, JP, SS, RR, KBC and IM) discussed the results and contributed to

the final manuscript. All authors have seen and approved the final text. JKS is the

guarantor of this study and accepts full responsibility for the finished work and the

conduct of the study, had access to the data and controlled the decision to publish.

Funding

This study was supported by grants from the Danish Work Environment

Research Fund (grant number 17-2014-03) and the Social Insurance Institution

of Finland (grant no. 29/26/2020). The funders of the study had no role in study

design, data collection, data analysis, data interpretation, writing of the report or

decision to submit for publication.

Competing interests

None declared.

Patient and public involvement

Patients and/or the public were involved in the

design, or conduct, or reporting, or dissemination plans of this research. Refer to

the Materials and methods section for further details.

Patient consent for publication

Not applicable.

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