The resilience of vaccine serotypes in adult invasive pneumococcal disease in Portugal, 2018-2023.

In Portugal, after a decline in the proportion of adult invasive pneumococcal disease (IPD) associated with serotypes included in PCV7 and PCV13 attributed to their use in children, we observed a stabilization in the number of IPD episodes caused by serotypes included in conjugate vaccines, suggesting that no further benefit could be expected from pediatric vaccination. Worldwide, the COVID-19 pandemic and the measures adopted to contain viral transmission had major impacts on other respiratory transmissible pathogens, including pneumococcal infections. Many countries reported a decline in IPD cases, followed by an increase, coinciding with the relaxation of those measures. In Portugal, between 2018 and 2023, we found a pronounced decrease in the number of IPD samples in 2019-2021 (76% reduction in 2020-2021 relative to 2018-2019). Although we observed an increase in the following years, the annual number of IPD cases did not exceed the pre-pandemic levels, contrary to what was already reported for children in Portugal. Despite the fluctuations in the number of IPD cases, there were no major changes in serotype distribution, when compared to the pre-pandemic period. Serotypes 8 and 3 continue to be the dominant serotypes, together accounting for approximately 40% of all cases, although there was an increase in serotype 3, and a decrease in serotype 8 samples. An increase in serotype 4 IPD was found, in line with other countries. Despite more than two decades of conjugate vaccine use, a considerable proportion of IPD cases could have been potentially prevented by vaccination. Because higher valency conjugate vaccines such as PCV20 or PCV21 showed a very high coverage in Portugal (74% and 80% respectively), the direct vaccination of adults with these vaccines could have a profound effect on the burden of disease and serotype distribution.