Abstract
BackgroundWe 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.Methodology/Principal FindingsEight pediatric hospitals distributed over Germany prospectively provided sera from in- or outpatients aged 1 to 17 years from April 1st to July 31st 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.ConclusionSerologically 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 AS03B-adjuvanted split virion vaccine need further scrutiny.
Highlights
In Germany, a total of approximately 223,000 laboratoryconfirmed symptomatic cases of pandemic influenza A (H1N1) 2009 infections have been notified from April 1st 2009 until March 31st 2010, accounting for a cumulative incidence of 272 per 100,000
Hemagglutination inhibition test results – vaccinated children and children after laboratory-confirmed influenza A (H1N1) 2009 infection All 6 children 1 to 4 years old (100%, 95% CI: 61.0–100) with a history of laboratory-confirmed A (H1N1) 2009 infection exhibited HI titers of 1:80 or above, but only 11 of 23 children in this age group (47.8%, 95% CI: 33.5–66.5) who reported vaccination with PandemrixH had HI titers $1:10, and 21.7% had HI titers $1:40
Comparison of seroprevalence against influenza A (H1N1) 2009 in pre- and post-pandemic sera indicates that about a quarter of the unvaccinated preschool (1–4 years) and almost half of the unvaccinated school age children (5–17 years) were infected with influenza A (H1N1) 2009 during the pandemic in Germany
Summary
In Germany, a total of approximately 223,000 laboratoryconfirmed symptomatic cases of pandemic influenza A (H1N1) 2009 infections have been notified from April 1st 2009 until March 31st 2010, accounting for a cumulative incidence of 272 per 100,000. The main wave of the pandemic occurred in weeks 43 to 51 of 2009 (unpublished data, Robert Koch-Institute, Berlin, Germany). For adults and adolescents $14 years, ultimate vaccination coverage has been estimated at ,8% on the basis of population-wide telephone surveys [1]. Data on vaccination coverage for ,14 year old children are not available. 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
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