Influenza virus infections from 0 to 2 years of age: A birth cohort study

Background/purpose: Influenza vaccine has been recommended in Finland since2007 for all children of 6e35 months of age and in 2009 for those 6 months against pandemicinfluenza. We investigated the incidence of influenza and vaccine effectiveness in a birthcohort of children in 2008e2011.Methods: We followed 923 children from birth to 2 years of age for respiratory tract infections.A nasal swab sample for PCR for influenza A and B viruses was taken at the onset of acute respiratoryinfections. Samples were collected either at the study clinic or at home by parents.Vaccination data was retrieved from the health registries.Results: Vaccination coverage of children aged 6e23 months was 22e47% against seasonalinfluenza and 80% against the A(H1N1)pdm09 virus in the pandemic season 2009e2010. During3 influenza seasons, 1607 nasal swab samples were collected. Influenza was confirmed in 56(6.1%) of 923 children (16 A(H1N1), 14 A(H3N2), and 26 B viruses). The incidence of influenzawas 5.1% in 2008e2009, 2.7% in 2009e2010, and 5.0% in 2010e2011. Effectiveness of the adjuvantedvaccine against the pandemic influenza A(H1N1)pdm09 was 97% (95% confidence interval,76e100%). Three children with influenza were hospitalized.Conclusion: The yearly incidence of seasonal influenza was 5% in this cohort of very young childrenwith variable influenza vaccine coverage. Adjuvanted vaccine against the pandemic influenzawas highly effective. Both seasonal and pandemic influenza cases were mostly nonsevere.