Background: The effect of previous natural pandemic H1N1 (H1N1 pdm09) influenza infection on the immunogenicity to subsequent inactivated influenza vaccination in children has not been well studied. We aimed to evaluate the effect of H1N1 pdm09 natural infection and vaccination on the immunogenicity to subsequent 2010-2011 seasonal inactivated influenza vaccination in children.

Methods: From October 2010 to May 2011, we conducted an open-label, multi-center study in children aged 6 months -18 years in Korea. We measured antibody titers with a hemagglutination-inhibition (HI) assay at baseline, 1 month, and 6 months after vaccination with trivalent split or subunit vaccines containing H1N1 pdm, A/H3N2, and B. The subjects were classified into 4 groups depending on the presence of laboratory-confirmed H1N1 pdm09 infection and/or vaccination in the 2009-2010 season; Group I: vaccination (-)/infection(-), Group II: vaccination (-)/infection(+), Group III: vaccination (+)/infection(-), Group IV: vaccination (+)/infection(+).

Results: Among the subjects in group I, 47 subjects who had a baseline titer >1:10 were considered to have an asymptomatic infection. They were included into the final group II (n = 80). We defined the new group II as the infection-primed (IP) group and group III as the vaccine-primed (VP) group. Seroconversion rate (57.5 % vs 35.9 %, p = 0.001), seroprotection rate at 6 months after vaccination (70.8 % vs 61.8 %, p = 0.032), and GMT at 1 month after vaccination (129.9 vs 66.5, p = 0.002) were significantly higher in the IP group than in the VP group. In the 9-18 year-old group, seroconversion rate and immunogenicity at 1 and 6 months were significantly higher in the IP group than in the VP group. However in the 1-7 year-old age group, there was no significant difference between the two groups.

Conclusions: Previous H1N1 pdm09 infection appears to have positive effects on immunogenicity of subsequent inactivated influenza vaccines against H1N1 pdm09 in older children.