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

Abstract

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.

Keywords: Adult IPD; PCV13; PCV20; PCV21; Serotype 3; Serotype 8.

Fig. 1

Serotypes of isolates expressing vaccine serotypes causing invasive pneumococcal disease in adult patients (≥ 18 years) in Portugal, 2018–2023. The number of isolates expressing each serotype in each of the age groups considered is indicated. Isolates recovered from patients 18–49 years are indicated by black triangles, from patients 50–64 years by black circles and from patients ≥ 65 years by open squares. Isolates presenting both erythromycin resistance and penicillin non-susceptibility (EPNSP) are represented by yellow bars. Penicillin non-susceptible isolates (PNSP, MIC > 0.06 mg/L) are indicated by green bars. Erythromycin resistant pneumococci (ERP) are indicated by orange bars. Isolates susceptible to both penicillin and erythromycin are represented by blue bars. In one case in the 18–49 years group, the antimicrobial susceptibility was unknown because the diagnosis was made by molecular methods and is not represented in Fig. 1. The age of one patient was not available. The serotypes included in the PCV13, PCV15, PCV20 and PPV23 are indicated by the arrows. AddPPV23, additional serotypes included in PPV23. The orange line (all PPV23 serotypes) was interrupted under 6A, as this serotype in not included in PPV23. In the smaller chart are represented serotypes exclusively included in PCV21, and the dashed arrow in both charts represents the total number of isolates expressing PCV21 serotypes.

Fig. 2

Proportion of isolates expressing serotypes included in approved pneumococcal vaccines causing invasive pneumococcal disease in adult patients (≥ 18 years) in Portugal, 2014–2023. The data up to 2018 were presented previously. See text for the serotypes included in each of the vaccines.