Effectiveness of pneumococcal conjugate vaccines against invasive pneumococcal disease in Vietnamese children prior to national introduction: A matched case-control study

Background : Evidence on the effectiveness of pneumococcal conjugate vaccines (PCVs) in Vietnam remains limited, despite the availability of 10-valent (PCV10) and 13-valent (PCV13) conjugate vaccines in the private sector since 2014 and 2019. We evaluated the effectiveness of PCVs against invasive pneumococcal disease (IPD) among Vietnamese children prior to national introduction. Methods : We conducted a matched case-control study between February 2022 and January 2025 in southern Vietnam. Cases were children aged 2–59 months hospitalized with culture-confirmed IPD from normally sterile sites at three tertiary pediatric hospitals. Four age- and neighborhood-matched community controls were enrolled per case. Vaccine effectiveness (VE) was estimated using conditional logistic regression as (1 − adjusted odds ratio) × 100%. Results : We enrolled 72 IPD cases and 288 matched controls; 37.5% of cases had received ≥1 PCV dose. The most frequent clinical syndromes were meningitis (36.1%) and pneumonia-associated sepsis (30.6%). Serotype 19A predominated (26.4%), followed by 6A (13.9%) and 19F (11.1%); 77.8% of cases were caused by serotypes included in PCV13. For vaccine-type IPD (VT-IPD), adjusted VE of PCV10 against PCV10-type IPD was 86.3% (95% CI, 13.3–97.9) for ≥1 dose and 89.5% (95% CI, 16.1–98.7) for ≥2 doses; VE of PCV13 against PCV13-type IPD was similarly high. Against all-serotype IPD, adjusted VE was 65.1% (95% CI, 21.8–84.4) for ≥1 PCV10 dose and 84.4% (95% CI, 20.0–97.0) for ≥1 PCV13 dose. No significant effectiveness was observed against non-PCV10-type IPD. VE was highest against serotype 6 A and declined with increasing time since vaccination. Conclusions : This first Vietnamese evidence demonstrates that PCV10 and PCV13 provide substantial protection against VT-IPD and overall IPD in young children despite low vaccine coverage. The predominance of serotype 19A and waning protection over time emphasize the importance of selecting 19A-containing formulations and booster schedules to inform timely national PCV introduction in Vietnam.