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Abstract Limited data from Asia are available on long-term effects of pneumococcal conjugate vaccine introduction on pneumococcal carriage. Here we assess the impact of 13-valent pneumococcal conjugate vaccine (PCV13) introduction on nasopharyngeal pneumococcal carriage prevalence, density and antimicrobial resistance. Cross-sectional carriage surveys were conducted pre-PCV13 (2015) and post-PCV13 introduction (2017 and 2022). Pneumococci were detected and quantified by real-time PCR from nasopharyngeal swabs. DNA microarray was used for molecular serotyping and to infer genetic lineage (Global Pneumococcal Sequence Cluster). The study included 1461 infants (5–8 weeks old) and 1489 toddlers (12–23 months old) enrolled from family health clinics. We show a reduction in PCV13 serotype carriage (with non-PCV13 serotype replacement) and a reduction in the proportion of samples containing resistance genes in toddlers six years post-PCV13 introduction. We observed an increase in pneumococcal nasopharyngeal density. Serotype 15 A, the most prevalent non-vaccine-serotype in 2022, was comprised predominantly of GPSC904;9. Reductions in PCV13 serotype carriage will likely result in pneumococcal disease reduction. It is important for ongoing surveillance to monitor serotype changes to potentially inform new vaccine development.

Starting in June 2016, the 13-valent pneumococcal conjugate vaccine (PCV13) was introduced into the routine immunization program of Mongolia by using a 2+1 dosing schedule, phased by district. We used prospective hospital surveillance to evaluate the vaccine’s effect on pneumonia incidence rates among children 2–59 months of age over a 6-year period. Of 17,607 children with pneumonia, overall adjusted incidence rate ratios showed decreased primary endpoint pneumonia, very severe pneumonia, and probable pneumococcal pneumonia until June 2021. Results excluding and including the COVID-19 pandemic period were similar. Pneumonia declined in 3 districts that introduced PCV13 with catch-up campaigns but not in the 1 district that did not. After PCV13 introduction, vaccine-type pneumococcal carriage prevalence decreased by 44% and nonvaccine-type carriage increased by 49%. After PCV13 introduction in Mongolia, the incidence of more specific pneumonia endpoints declined in children 2–59 months of age; additional benefits were conferred by catch-up campaigns.

Summary: Background: Community-acquired pneumonia is a leading cause of morbidity and mortality among children and adults worldwide. Adult pneumonia surveillance remains limited in many low- and middle-income settings, resulting in the disease burden being largely unknown. Methods: A retrospective cohort study was conducted by reviewing medical charts for respiratory admissions at four district hospitals in Ulaanbaatar during January 2015–February 2019. Characteristics of community-acquired pneumonia cases were summarized by disease severity and age. To explore factors associated with severe pneumonia, we ran univariable and age-adjusted logistic regression models. Incidence rates were calculated using population denominators. Results: In total, 4290 respiratory admissions met the case definition for clinical pneumonia, including 430 admissions of severe pneumonia. The highest proportion of severe pneumonia admissions occurred in adults >65 years (37.4%). After adjusting for age, there were increased odds of severe pneumonia in males (adjusted odds ratio [aOR]: 1.63; 95% confidence interval [CI]: 1.33-2.00) and those with ≥1 underlying medical condition (aOR: 1.46; 95% CI: 1.14-1.87). The incidence of hospitalized pneumonia in adults ≥18 years increased from 13.49 (95% CI: 12.58-14.44) in 2015 to 17.65 (95% CI: 16.63-18.71) in 2018 per 10,000 population. The incidence of severe pneumonia was highest in adults >65 years, ranging from 9.29 (95% CI: 6.17-13.43) in 2015 to 12.69 (95% CI: 9.22-17.04) in 2018 per 10,000 population. Interpretations: Vaccination and other strategies to reduce the risk of pneumonia, particularly among older adults and those with underlying medical conditions, should be prioritized. Funding: Pfizer clinical research collaboration agreement (contract number: WI236621).