ABSTRACT
Background There are concerns that current pneumococcal conjugate vaccine (PCV) schedules in sub-Saharan Africa sub-optimally interrupt vaccine-serotype (VT) carriage and transmission, thus limiting indirect protection. We assessed pneumococcal carriage in vaccinated children and unvaccinated populations targeted for indirect vaccine protection, between 4 and 6 years after the 2011 introduction of a 13-valent PCV (PCV13) 3+0 schedule in Malawi.
Methods We conducted four sequential prospective nasopharyngeal carriage surveys in urban Blantyre, from June, 2015, to April, 2017. We recruited healthy PCV13-vaccinated children 3-6 years old, children 5-10 years old born before PCV13 introduction, and HIV-infected adults 18-40 years old on antiretroviral therapy. Carriage risk by age was analysed by non-linear regression.
Findings We sampled 1382 PCV13-vaccinated children, 889 PCV13-unvaccinated children, and 985 adults. VT carriage prevalence declined from 23% to 17% among vaccinated children (adjusted prevalence ratio [aPR] 0.75, 95% CI 0.56-1.01; p=0.062) and 27% to 15% among unvaccinated children (aPR 0.65, 95% CI 0.44-0.98; p=0.039). Adult prevalence remained 14% (aPR 0.92, 95% CI 0.59-1.44; p=0.72). VT carriage probability declined with age, with a decay half-life of 5.3 years (95% CI 3.2-9.0).
Interpretation The PCV13 3+0 schedule in Malawi has not achieved optimal reduction in pneumococcal carriage prevalence, compared to high-income settings. This is likely due to recolonisation of vaccinated children with waning vaccine-induced immunity and suboptimal indirect protection of unvaccinated populations. Rigorous evaluation of strategies to augment vaccine-induced control, including alternative schedules and catch-up campaigns among children under 5 years old is required.
Funding Bill & Melinda Gates Foundation, Wellcome Trust UK, Medical Research Council.