Post-pandemic seroprevalence of pandemic influenza A (H1N1) 2009 infection (swine flu) among children <18 years in Germany.

We determined antibodies to the pandemic influenza A (H1N1) 2009
virus in children to assess: the incidence of (H1N1) 2009
infections in the 2009/2010 season in Germany, the proportion of
subclinical infections and to compare titers in vaccinated and
infected children. Eight pediatric hospitals distributed over
Germany prospectively provided sera from in- or outpatients aged 1
to 17 years from April 1(st) to July 31(st) 2010. Vaccination
history, recall of infections and sociodemographic factors were
ascertained. Antibody titers were measured with a sensitive and
specific in-house hemagglutination inhibition test (HIT) and
compared to age-matched sera collected during 6 months before the
onset of the pandemic in Germany. We analyzed 1420 post-pandemic
and 300 pre-pandemic sera. Among unvaccinated children aged 1-4 and
5-17 years the prevalence of HI titers (≥1∶10) was 27.1% (95% CI:
23.5-31.3) and 53.5% (95% CI: 50.9-56.2) compared to 1.7% and 5.5%,
respectively, for pre-pandemic sera, accounting for a serologically
determined incidence of influenza A (H1N1) 2009 during the season
2009/2010 of 25,4% (95% CI : 19.3-30.5) in children aged 1-4 years
and 48.0% (95% CI: 42.6-52.0) in 5-17 year old children. Of
children with HI titers ≥1∶10, 25.5% (95% CI: 22.5-28.8) reported
no history of any infectious disease since June 2009. Among
vaccinated children, 92% (95%-CI: 87.0-96.6) of the 5-17 year old
but only 47.8% (95%-CI: 33.5-66.5) of the 1-4 year old children
exhibited HI titers against influenza A virus (H1N1) 2009.
Serologically determined incidence of influenza A (H1N1) 2009
infections in children indicates high infection rates with older
children (5-17 years) infected twice as often as younger children.
In about a quarter of the children with HI titers after the season
2009/2010 subclinical infections must be assumed. Low HI titers in
young children after vaccination with the AS03(B)-adjuvanted split
virion vaccine need further scrutiny.