Seroprevalence and Severity of 2009 Pandemic Influenza A H1N1 in Taiwan

Abstract

BackgroundThis study is to determine the seroprevalence of the pandemic influenza A H1N1 virus (pH1N1) in Taiwan before and after the 2009 pandemic, and to estimate the relative severity of pH1N1 infections among different age groups.Methodology/Principal FindingsA total of 1544 and 1558 random serum samples were collected from the general population in Taiwan in 2007 and 2010, respectively. Seropositivity was defined by a hemagglutination inhibition titer to pH1N1 (A/Taiwan/126/09) ≥1:40. The seropositivity rate of pH1N1 among the unvaccinated subjects and national surveillance data were used to compare the proportion of infections that led to severe diseases and fatalities among different age groups. The overall seroprevalence of pH1N1 was 0.91% (95% confidence interval [CI] 0.43–1.38) in 2007 and significantly increased to 29.9% (95% CI 27.6–32.2) in 2010 (p<0.0001), with the peak attack rate (55.4%) in 10–17 year-old adolescents, the lowest in elderly ≥65 years (14.1%). The overall attack rates were 20.6% (188/912) in unvaccinated subjects. Among the unvaccinated but infected populations, the estimated attack rates of severe cases per 100,000 infections were significantly higher in children aged 0–5 years (54.9 cases, odds ratio [OR] 4.23, 95% CI 3.04–5.90) and elderly ≥ 65years (22.4 cases, OR 2.76, 95% CI 1.99–3.83) compared to adolescents aged 10–17 years (13.0 cases). The overall case-fatality rate was 0.98 per 100,000 infections without a significant difference in different age groups.Conclusions/SignificancePre-existing immunity against pH1N1 was rarely identified in Taiwanese at any age in 2007. Young children and elderly – the two most lower seroprotection groups showed the greatest vulnerability to clinical severity after the pH1N1 infections. These results imply that both age groups should have higher priority for immunization in the coming flu season.

Epidemiology of seasonal influenza in Bangkok between 2009 and 2012

“…In this study, cases of pH1N1 were detected in At the emergence of pandemic (H1N1) in 2009, the mean age of infected patients was 19 years, which correlates with other studies that have reported that pH1N1, in contrast to seasonal influenza, specifically targets younger age groups [11,12]. This may be explained by the fact that older adults have developed some level of protective antibody against pH1N1 due to previous infection with the related H1N1 virus [13].…”

Post-Pandemic Seroprevalence of 2009 Pandemic Influenza A (H1N1) Virus in Shandong Province, China

“…These results are consistent with those reported in Ontario (33.3z) (22) and Pittsburgh (40z) (28), which showed a higher seropositive rate in the AE60 years age group at the end of the influenza season and October 2010. However, the results were discordant with those of reports from Taiwan (11.8z) (18) and Greece (14.4z) (23) that were carried out in September and June 2010. Overall, the study showed that the highest percentage of individuals with antibodies to A(H1N1)pdm09 were children.…”

Effect of Climatic Conditions on Epidemic Patterns of Influenza in Okinawa, Japan, during the Pandemic of 2009: Surveillance of Rapid Antigen Test Results

“…Taiwan also had 2 intense epidemics during the 2009 pandemic in August-September and in December (26). The epidemic curves for Okinawa were more similar to those for Taiwan than to those for mainland Japan.…”