cb Ritzwoller 2005

Methods	Retrospective cohort study of effectiveness of influenza vaccine Data collection from electronic medical records and immunisation registry database Vaccination status was included as a time‐varying variable using a multivariate Cox proportional hazard model to estimate a HR, this was used because patients continued to be vaccinated during the influenza season Vaccine efficacy (VE) was calculated as 1 minus HR Chronic medical conditions included
Participants	Children aged 6 to 23 months
Interventions	Vaccine not specified (see 2003 included strains below) 2003 to 2004 season will include A/New Caledonia/20/99‐like (H1N1), A/Moscow/10/99‐like (H3N2) and B/Hong Kong/330/2001‐like viruses. For the A/Moscow/10/99‐like (H3N2) virus, US manufacturers will use the antigenically equivalent A/Panama/2007/99 (H3N2) virus and for the B/Hong Kong/330/2001‐like virus, they will use either B/Hong Kong/330/01 or the antigenically equivalent virus B/Hong Kong/1434/02
Outcomes	ILI for FV children versus UV P&I for FV versus UV
Funding Source	Government/Industry
Notes	Circulating strain of A (H3N2) Data collected during peak of influenza activity
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS‐Selection Exposed cohort	Low risk	Selected group, secure record
PCS/RCS‐Selection Non Exposed cohort	Low risk	Same methods of the exposed cohort
PCS/RCS‐Comparability	High risk	Insufficient description
PCS/RCS‐Assessment of Oucome	Unclear risk	Record linkage
Summary assessments	High risk	Some doubt arises from the real comparability of the cohorts