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

Abstract

<p>Background/purpose: Influenza vaccine has been recommended in Finland since<br />2007 for all children of 6e35 months of age and in 2009 for those 6 months against pandemic<br />influenza. We investigated the incidence of influenza and vaccine effectiveness in a birth<br />cohort of children in 2008e2011.<br />Methods: We followed 923 children from birth to 2 years of age for respiratory tract infections.<br />A nasal swab sample for PCR for influenza A and B viruses was taken at the onset of acute respiratory<br />infections. Samples were collected either at the study clinic or at home by parents.<br />Vaccination data was retrieved from the health registries.<br />Results: Vaccination coverage of children aged 6e23 months was 22e47% against seasonal<br />influenza and 80% against the A(H1N1)pdm09 virus in the pandemic season 2009e2010. During<br />3 influenza seasons, 1607 nasal swab samples were collected. Influenza was confirmed in 56<br />(6.1%) of 923 children (16 A(H1N1), 14 A(H3N2), and 26 B viruses). The incidence of influenza<br />was 5.1% in 2008e2009, 2.7% in 2009e2010, and 5.0% in 2010e2011. Effectiveness of the adjuvanted<br />vaccine against the pandemic influenza A(H1N1)pdm09 was 97% (95% confidence interval,<br />76e100%). Three children with influenza were hospitalized.<br />Conclusion: The yearly incidence of seasonal influenza was 5% in this cohort of very young children<br />with variable influenza vaccine coverage. Adjuvanted vaccine against the pandemic influenza<br />was highly effective. Both seasonal and pandemic influenza cases were mostly nonsevere.<br /></p>