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Updated: Jan 2, 2026
Experimental Human Pneumococcal Carriage
Published on: February 15, 2013
Determinants of high residual post-PCV13 pneumococcal vaccine-type carriage in Blantyre, Malawi: a modelling study
J Lourenço1, U Obolski2,3, T D Swarthout4,5
1Department of Zoology, University of Oxford, Oxford, UK. jose.lourenco@zoo.ox.ac.uk.
Insights
Malawi
Area of Science:
- Public Health
- Vaccinology
- Epidemiology
Background:
- Malawi introduced the 13-valent pneumococcal conjugate vaccine (PCV13) in 2011.
- Nasopharyngeal carriage surveys were conducted in Blantyre between 2015-2018.
- Vaccine serotype (VT) carriage remained high, with varied impact across age groups.
Purpose of the Study:
- To investigate the determinants of age-specific VT carriage post-PCV13 introduction.
- To understand factors influencing pneumococcal conjugate vaccine effectiveness in Malawi.
Main Methods:
- A dynamic transmission model was employed.
- Bayesian Markov-chain Monte Carlo methods were used to fit the model to survey data.
Main Results:
- Age-related immunity and transmission potential were crucial for observed VT carriage patterns.
- VT carriage reduction varied by age, occurring earlier in younger individuals.
- Estimated vaccine efficacy against carriage was 66.87%; projected 10-year impact in children was 76.23%.
Conclusions:
- Both vaccine and host factors influence pneumococcal VT transmission.
- The local force of infection significantly impacts vaccine effectiveness.
- Findings are relevant for Sub-Saharan African countries and future vaccination strategies.
Background:
In November 2011, Malawi introduced the 13-valent pneumococcal conjugate vaccine (PCV13) into the routine infant schedule. Four to 7 years after introduction (2015-2018), rolling prospective nasopharyngeal carriage surveys were performed in the city of Blantyre. Carriage of Streptococcus pneumoniae vaccine serotypes (VT) remained higher than reported in high-income countries, and impact was asymmetric across age groups.
Methods:
A dynamic transmission model was fit to survey data using a Bayesian Markov-chain Monte Carlo approach, to obtain insights into the determinants of post-PCV13 age-specific VT carriage.
Results:
Accumulation of naturally acquired immunity with age and age-specific transmission potential were both key to reproducing the observed data. VT carriage reduction peaked sequentially over time, earlier in younger and later in older age groups. Estimated vaccine efficacy (protection against carriage) was 66.87% (95% CI 50.49-82.26%), similar to previous estimates. Ten-year projected vaccine impact (VT carriage reduction) among 0-9 years old was lower than observed in other settings, at 76.23% (CI 95% 68.02-81.96%), with sensitivity analyses demonstrating this to be mainly driven by a high local force of infection.
Conclusions:
There are both vaccine-related and host-related determinants of post-PCV13 pneumococcal VT transmission in Blantyre with vaccine impact determined by an age-specific, local force of infection. These findings are likely to be generalisable to other Sub-Saharan African countries in which PCV impact on carriage (and therefore herd protection) has been lower than desired, and have implications for the interpretation of post-PCV carriage studies and future vaccination programs.
