pcb Louik 2013.

Methods	Retrospective cohort study. The effect of immunisation with an influenza vaccine containing pH1N1 during pregnancy on preterm birth was assessed comparing vaccinated and non‐vaccinated women.
Participants	Study population belonged to those enrolled in a large surveillance‐based, case‐control study carried out by the Slone Epidemiology Center at Boston University. Children‐cases with major structural defects were identified from participating hospitals in the areas surrounding Philadelphia and San Diego as well as by means of birth defect registries in New York State and Massachusetts. Controls were normal‐formed infants randomly selected within the same study hospitals. For the purposes of the present study, only mothers of controls (without malformations) who delivered during the 2009‐10 and 2010‐11 seasons are included. Only mothers of singleton, live‐born infants, who were immunised not after the 37th gestation week, were included. Altogether, 951 women were included, 378 of whom received influenza vaccine.
Interventions	Exposure to a pH1N1‐containing vaccine during pregnancy within the seasons 2009‐10 and 2010‐11. Exposure was ascertained by means of a computer‐assisted phone interview administered 6 months after delivery and eventually verified by examining the vaccination records. A woman was considered exposed if she had received a pH1N1‐containing vaccination. Time of exposure was considered within 1st trimester (until 14th gestation week), 2nd trimester (gestation weeks 15 to 28), and 3rd trimester (from week 29 through delivery). Women whose reported time of exposure could not be attributed to 1 of the trimesters were excluded. Not‐exposed participants should have last menstrual date within the range of last menstrual date reported by exposed participants.
Outcomes	Preterm delivery: defined as delivery at gestational age less than 37 weeks
Notes	Funding source ‐ industry Drs Louik, Chambers, Jones, Schatz, and Mitchell and Mr Kerr receive research support from Novartis Vaccines and Diagnostics (NVD) for an unrelated study of a meningitis vaccine. Dr Mitchell serves as a member of an advisory committee for a pregnancy registry for a multiple sclerosis agent conducted by Biogen‐Idec and as an unpaid consultant to NVD on matters unrelated to influenza vaccines. Drs Chambers and Jones receive support from GlaxoSmithKline Bio for an unrelated study of human papilloma virus vaccine. Drs Chambers and Jones receive support for unrelated research projects from various pharmaceutical companies: Abbott, Amgen, Bristol‐Myers Squibb, GlaxoSmithKline, Parr, Pfizer, Janssen, Roche Genentech, Sanofi Genzyme, Sandoz, and Teva. Dr Schatz has received research support for projects unrelated to the current study from Aerocrine, Genentech, GlaxoSmithKline, MedImmune, and Merck. Dr Schatz is also a research consultant on subjects unrelated to the current study for Amgen, Boston Scientific, and GlaxoSmithKline. Ms Pyo and Dr Ahrens have no conflicts to disclose.
Risk of bias
Bias	Authors' judgement	Support for judgement
PCS/RCS ‐ selection exposed cohort All outcomes	Unclear risk	Cohort consists of the control population of case‐control studies in which case population is represented by mothers of children born with major defects.
PCS/RCS ‐ selection non‐exposed cohort All outcomes	Low risk	Drawn from the same source as the exposed one
PCS/RCS ‐ comparability All outcomes	Low risk	Taken into account
PCS/RCS ‐ assessment of outcome All outcomes	Low risk	Secure records
Summary assessment	Unclear risk	Unclear risk of bias