Use of surveillance data to estimate the effectiveness of the 7-valent conjugate pneumococcal vaccine in children less than 5 years of age over a 9 year period

Background: Case-control studies evaluating post-licensure effectiveness of conjugate vaccines can be laborious and costly. We applied an indirect cohort method to evaluate the effectiveness of seven-valent pneumococcal conjugate vaccine (PCV7) against invasive pneumococcal disease (IPD) and compared the results to the effectiveness measured using a standard case-control study conducted during the same time period. Methods: IPD cases among children 2-59 months old were identified through the Active Bacterial Core surveillance system during 2001-2009. We used logistic regression to calculate the odds ratio of vaccination (versus no vaccination) among cases (PCV7-type IPD cases) and non-cases (non-PCV7-type IPD cases), controlling for the presence of underlying conditions. Vaccine effectiveness (VE) was calculated as one minus the adjusted odds ratio. Results: Among 4225 IPD cases reported during 2001-2009,2680 (63%) had serotype information and vaccine history. Effectiveness of >= 1 dose of PCV7 against PCV7-types was 88% (95% confidence interval (Cl) 78-94%) among children with comorbid conditions and 97% (95% Cl 92-98%) among healthy children. Among healthy children, VE was higher in 2001-2003 (97%, 95% Cl 95-98%) compared to 2004-2009 (81%, 95% Cl 64-90%). The annual estimates of VE in 2004-2009 showed great variability and wide confidence intervals due to the small number of PCV7-type cases. Conclusions: An indirect cohort design using IPD surveillance data confirms the findings of the case-control study and, therefore, appears suitable for estimating PCV7effectiveness. This method would be most useful shortly after vaccine introduction, and less useful in a setting of very high vaccine coverage and fewer vaccine-type cases. Published by Elsevier Ltd.