cb‐Hviid 2008.

Study characteristics
Methods	Cohort study ‐ by using data from the Civil Registration System and considering all children born in Denmark between 1 January 1991 and 31 December 2003, the present study investigates the association between MMR immunisation and hospitalisation with asthma diagnosis and use of anti‐asthma medication with a person‐time cohort design.
Participants	For the analysis of association between MMR vaccination and asthma hospitalisation, all those born in Denmark between 1 January 1991 and 31 December 2003, aged between 1 and 5 years, have been considered within the time period from 1 January 1992 and 31 December 2004 (N = 871,234). Children contributed to person‐time follow‐up from 1 year of age until age of 5, or until 31 December 2004, death, or disappearance/emigration. Follow‐up resulted in 2,926,406 person‐years. Due to several reasons, 15,914 children terminated their follow‐up prematurely (5455 because of death, 10,159 emigrated, and 300 disappeared). Follow‐up length for the analysis of use of anti‐asthma medication reached from 1 January 1996 to 31 December 2004, as data about medical prescription were available only from 1996. A total of 600,938 children contributed to follow‐up, corresponding to 1,858,199 person‐years. Follow‐up was prematurely terminated for 12,552 children (4681 due to death, 7710 due to emigration, and 161 disappeared).
Interventions	Dates of MMR vaccination were obtained from the National Board of Health (in Denmark routine childhood vaccination may be administered by GPs only, who must report them to the National Board of Health). Used preparation contains strain Moraten measles strain, Jeryl Lynn mumps strain, and Wistar RA 27/3 rubella strain. Authors report that 85% of the 871,234 participants in the cohort for asthma hospitalisation and 84% of those considered for anti‐asthma medication (n = 600,938) received MMR before end of follow‐up. MMR vaccination status was considered as time‐varying variable, and individuals could contribute to person‐time as both unvaccinated and vaccinated participants.
Outcomes	Asthma hospitalisation (from the Danish National Hospital Register) Anti‐asthma medication (from the Danish Prescription Drug Database)
Funding Source	Government
Notes	There is no information about the time considered between vaccination and disease onset or use of medication (i.e. authors do not provide a definition of MMR‐vaccinated and not‐vaccinated status).
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS ‐ exposed cohort selection	Low risk	Adequate ‐ Danish civil registration system ‐ probable selection bias
PCS/RCS ‐ non‐exposed cohort selection	Low risk	Adequate ‐ drawn from the same population
PCS/RCS ‐ comparability	Low risk	Age and calendar period, sex, child’s place of birth, child’s birthweight, mother’s country of birth, mother’s age at birth of child, birth order, and infant vaccine compliance
PCS/RCS ‐ assessment of outcome	Low risk	Adequate ‐ hospitalisations record
Summary Risk of Bias assessment	Low risk	Plausible bias is unlikely to have seriously altered the results.