Abstract
Background In November 2011, Malawi introduced 13-valent pneumococcal conjugate vaccine (PCV13) into the routine infant schedule. Four to six years after introduction, rolling prospective nasopharyngeal carriage surveys were performed in the city of Blantyre. Carriage of Streptococcus pneumoniae vaccine serotypes (VT) remained higher than reported in developed countries, and VT impact was surprisingly asymmetric across age-groups with older individuals experiencing higher carriage reduction (35% for unvaccinated 8-9 years old, versus 26% for vaccinated 3-5 years old).
Methods Bayesian Markov-chain Monte Carlo was used to fit a transmission model to age-specific VT carriage data. Simulations were used to reconstruct past and project future carriage dynamics, as well as to explore determinants of PCV13 impact across ages.
Results Accumulation of naturally acquired immunity in age and age-specific transmission potentials with dominance of individuals younger than 5 years of age were key to reproduce observed post-PCV13 VT carriage. Age-groups experienced periods of faster VT carriage reduction sequentially in time, from younger to older groups. Individual-level protection against carriage (vaccine efficacy) was estimated as 68.8% (95% CI 41.6-88.1%). Population-level reduction in VT carriage (vaccine impact) over the first 10 years and among children aged 0-9 years was estimated at 70% (CI 95% 41-82%), much lower than observed elsewhere.
Conclusions In Blantyre, estimated vaccine efficacy has been similar to other regions, but vaccine impact is being offset by a high, age-heterogeneous local force of infection. Such age-dependent profiles strongly determine regional PCV impact and need to be better characterised if we are to maximize intervention impact in high transmission settings.
- Abbreviations
- VT
- vaccine type
- NVT
- non-vaccine type
- PCV
- pneumococcal conjugate vaccine
- CI
- confidence interval
- bMCMC
- Bayesian Markov-chain Monte Carlo
- ODE
- ordinary-differential equations
- FOI
- force of infection
- dVP
- duration of vaccine-induced protection