# Pneumococcal carriage and disease in adults hospitalised with community-acquired pneumonia in Mongolia: prospective pneumonia surveillance program (2019–2022)

## TL;DR

This study examines pneumococcal carriage and disease in adults hospitalized with pneumonia in Mongolia, finding a significant decline during the pandemic and a shift toward non-PCV13 serotypes.

## Contribution

The study provides novel data on pneumococcal serotype distribution and carriage in adults from Mongolia, highlighting the impact of the pandemic and gaps in adult vaccination.

## Key findings

- Pneumococcal carriage prevalence dropped by 66% in the late-COVID-19 period compared to pre-pandemic.
- Non-PCV13 serotypes were more prevalent in adult carriage and disease despite high childhood PCV13 coverage.
- Pneumococcal pneumonia cases decreased by 82% during the late-COVID-19 period.

## Abstract

Streptococcus pneumoniae is an important cause of pneumonia in older adults, however, serotyping and indirect impact information from low and middle-income countries is lacking. Mongolia has a childhood 13-valent pneumococcal conjugate vaccine (PCV13) program, but no adult pneumococcal vaccination program. We describe pneumococcal carriage rates, disease and serotype distribution among adults hospitalised with pneumonia, and explore changes over the COVID-19 pandemic period.

Adults (≥ 18 years) hospitalised with clinical pneumonia were enrolled over 3 years (March 2019-February 2022) into a prospective pneumonia surveillance program. Nasopharyngeal swabs were tested to detect pneumococci using lytA qPCR and molecular serotyping by DNA microarray and metagenomics. Pneumococcal pneumonia was identified using serotype-specific urinary antigen detection and BinaxNOW® assays. Pneumococcal carriage and pneumonia prevalence were assessed over the COVID-19 period with log-binomial regression used to estimate prevalence and adjusted prevalence ratios (pre- versus early- and late-COVID-19 periods).

Of 3,178 pneumonia cases, S. pneumoniae was identified in 12.1% (333/2,759) of swabs and 8.6% (253/2,925) of urine samples. PCV13 serotype carriage prevalence was 3.1% (82/2,663) and non-PCV13 serotype carriage prevalence 5.7% (152/2,663). In the late-COVID-19 period, pneumococcal carriage prevalence was reduced by 66% (aPR 0.34, 95%CI 0.25–0.46) and pneumococcal pneumonia by 82% (aPR 0.18, 95%CI 0.12–0.27) compared with the pre-COVID-19 transmission period.

Despite paediatric vaccination with high coverage, we identified some residual PCV13 serotypes with predominance of non-PCV13 serotypes carried and causing disease in adults. Direct adult vaccination which targets these serotypes will potentially reduce disease in adults in Mongolia.

The online version contains supplementary material available at 10.1186/s41479-025-00184-w.