title: Authors' reply to Makiyama and colleagues.
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Sweden jonasludvigsson@yahoo.com.
authors.forename: Jonas F
authors.initials: JF
authors.lastname: Ludvigsson
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Sweden.
authors.forename: Peter
authors.initials: P
authors.lastname: Ström
authors.affiliation: Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Sweden.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Lundholm
authors.affiliation: Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska Institutet, Sweden.
authors.forename: Sven
authors.initials: S
authors.lastname: Cnattingius
authors.affiliation: Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska Institutet, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Ekbom
authors.affiliation: Unit of Infectious Diseases, Department of Medicine, Karolinska Institutet, Sweden.
authors.forename: Åke
authors.initials: Å
authors.lastname: Örtqvist
authors.affiliation: Medical Product Agency, Uppsala, Sweden.
authors.forename: Nils
authors.initials: N
authors.lastname: Feltelius
authors.affiliation: Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska Institutet, Sweden.
authors.forename: Fredrik
authors.initials: F
authors.lastname: Granath
authors.affiliation: Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska Institutet, Sweden.
authors.forename: Olof
authors.initials: O
authors.lastname: Stephansson
mesh_terms: D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000401:mortality* / Q000517:prevention & control*; D008297:Male; D011159:Population Surveillance; D011247:Pregnancy; D035781:Siblings; D014611:Vaccination / Q000379:methods*