title: To rule out confounding, observational studies of influenza vaccine need to include analyses during the "preinfluenza period".
authors.forename: Michael L
authors.initials: ML
authors.lastname: Jackson
authors.forename: Noel S
authors.initials: NS
authors.lastname: Weiss
authors.forename: Jennifer C
authors.initials: JC
authors.lastname: Nelson
authors.forename: Lisa A
authors.initials: LA
authors.lastname: Jackson
mesh_terms: D000328:Adult; D000368:Aged; D015986:Confounding Factors, Epidemiologic; D006801:Humans; D007252:Influenza Vaccines; D007251:Influenza, Human / Q000209:etiology / Q000517:prevention & control*
chemical_list: D007252:Influenza Vaccines