Impact of the 10-valent pneumococcal conjugate vaccine (PCV10) on pneumococcal carriage in healthy children and children with acute otitis media and pneumonia: emergence of serotypes 3, 6C and 19A in Croatia.

BACKGROUND

In 2019, the 10-valent pneumococcal conjugate vaccine (PCV10) was introduced in the Croatian immunization programme, a first for this European PCV-naïve country. This study aimed to evaluate the impact of PCV10 on pneumococcal serotype distribution among asymptomatic children and children with pneumonia and/or acute otitis media.

METHODS

Cross-sectional studies were conducted before and after the PCV10 introduction, with nasopharyngeal swabs collected from 1500 healthy children under 48 months of age. An additional 324 children under 18 years with pneumonia and/or acute otitis media, from whom Streptococcus pneumoniae was isolated, were also included. Isolates were identified by conventional methods, serotyped by Quellung reaction, and tested for antimicrobial susceptibility using disk diffusion and gradient test methods. We report prevalence, absolute risk (prevalence) difference (RD) and relative risk (prevalence) ratio (RR) differences between exposed and control children.

RESULTS

Carriage prevalence among healthy children increased from 19.9% to 28.7%, primarily due to a rise in non-vaccine serotypes (NVT). Adjusted probabilities for serotypes 6C (RR 3.18; 95% CI, 1.43-7.06), 11A (RR 2.8; 95% CI, 1.22-6.39), 19A (RR 4.18; 95% CI, 1.18-14.9) and 23A (RR 3.93; 95% CI, 1.87-8.24) were significantly higher in healthy exposed children. Prevalences of these serotypes were also higher in exposed children with pneumonia/acute otitis media. In this cohort, serotype 3 increased (RR 4.6; 95% CI, 2.02-10.3), becoming the leading post-PCV10 isolate in the overall studied population. Serotypes 3 and 19A were almost entirely responsible for complicated pneumonia cases for which the probability increased by 21-fold. Antimicrobial susceptibility remained similar across periods.

CONCLUSIONS

In the early post-vaccine period significant increase of PCV10 vaccine-related serotypes (6C, 19A) was observed. Continued monitoring is also essential due to concerning rise of serotype 3 in patients with mucosal infections and a higher risk for complicated pneumonia.