National surveillance of invasive pneumococcal diseases in Taiwan, 2008-2012: Differential temporal emergence of serotype 19A

Introduction: Serotype replacement after the introduction of seven-valent pneumococcal conjugate vaccine (PCV7) and the future availability of multivalent PCVs prompted the listing of invasive pneumococcal disease (IPD) as a notifiable disease in Taiwan in October 2007. Here, we report the national surveillance results. Methods: The study population comprised the whole nation of Taiwan from 2008 to 2012. Restricting to cases with viable isolates, we calculated the incidence, case fatality ratio, prevalence of serotype 19A, and percentage of vaccine preventable IPD. Results: 3659 cases of IPD were identified yielding an incidence of 3.2 per 100,000 population; the highest incidence was among children aged 2-4 years (21.1 per 100,000 population). The case fatality ratio was 9.2% and the highest ratio was among adults aged ≥75 years (19.0%). The percentage of PCV7 preventable IPD decreased for all age groups, especially sharply among children aged 2-4 years, from 65.8% in 2008 to 12.9% in 2012. The prevalence of serotype 19A increased from 5.5% in 2008 to 25.3% in 2012 among all Streptococcus pneumoniae, displaying a differential temporal emergence among different age groups. Serotype 19A became the most prevalent serotype among children aged <2 years in 2009, children aged 2-4 and 5-17 years in 2010, and adults aged 18-49 years in 2012. Conclusions: The incidence of IPD fluctuated during the study period, with ongoing decrease due to PCV7 vaccine serotypes and increase due to non-vaccine serotypes. Serotype 19A became the most prevalent serotype in 2010 among all S. pneumoniae.