Epidemiology of Pneumococcal Meningitis Before and After the Introduction of 10-valent Pneumococcal Conjugate Vaccine (PCV10) in Poland and Finland

Streptococcus pneumoniae, or pneumococcus, can cause severe infections such as pneumonia, sepsis, and meningitis. Pneumococcal meningitis is a leading cause of bacterial meningitis worldwide. About 100 distinct S. pneumoniae serotypes have been identified. The most important serotypes are included in the available vaccines. In Finland, the 10-valent conjugate vaccine (PCV10) was introduced into the national immunization program in September 2010. Poland introduced PCV10 in January 2017.

For this study, we used national surveillance data to determine the contribution of S. pneumoniae to the total burden of bacterial meningitis and evaluate the long-term trends in pneumococcal meningitis in Finland. We also assessed the population-based impact of PCV10 on the epidemiology of pneumococcal meningitis. In Poland, national population-based surveillance data were analyzed to obtain information on the baseline epidemiology of pneumococcal meningitis before the introduction of the first childhood pneumococcal conjugate vaccine, with a particular focus on incidence, serotype distribution, and antimicrobial susceptibility.

During 1998–2014, pneumococcus was the most common cause of bacterial meningitis in Finland. It was also the leading cause of meningitis deaths in adults. The incidence rate of pneumococcal meningitis substantially decreased after the introduction of the conjugate vaccine. Seven years after PCV10 introduction, the overall incidence of pneumococcal meningitis decreased by 27% (95% CI = 12%, 39%), while the incidence of PCV10 serotypes decreased by 68% (95% CI = 57%, 77%). In age groups 0–4 and 50–64 years, overall incidence decreased by 64% and 34%, respectively. In adults 65 years of age, an 157% (95% CI = 56%, 342%) increase in non-PCV10 serotypes offset a 69% reduction in PCV10 serotypes. Regardless, the overall mortality rate decreased by 42% (95% CI = 4%, 65%). The overall case fatality proportion was 16% in the pre-PCV10 period and 12% in the PCV10 period (p = .41); among persons 50–64 years of age, the case fatality proportion decreased from 25% to 10% (p = .04).

In Poland, the annual incidence rate of overall pneumococcal meningitis ranged from 0.21 cases per 100,000 person-years in 2005 to 0.47 in 2015. Substantial regional differences in reported rates were observed. Overall incidence increased by 7% annually (95% CI = 6%, 8%) on average, primarily due to an increase of 3% (95% CI = 2%, 5%) among persons 15–49 years of age, 12% (95% CI = 10%, 13%) among those 50–64 years of age, 18% (95% CI = 16%, 19%) among those 65–74 years of age, and 9% (95% CI = 7%, 10%) among those 75 years of age. Overall case fatality proportion ranged from 4% in children <1 year to 31% in adults 65 years. PCV10 serotypes were responsible for 46% of all pneumococcal meningitis cases and 75% among children <5 years. However, the proportion of PCV10 serotypes decreased from 52% in 2008–2011 to 41% in 2012–2015. The decline in children <5 years (from 87% to 57%) was more pronounced. Over a quarter of all isolates and half of the isolates in children <5 years were resistant to penicillin. Isolates with decreased susceptibility to cefotaxime constituted 13% of all isolates.

S. pneumoniae was the main contributor to bacterial meningitis in Finland. A substantial reduction in the morbidity and mortality due to pneumococcal meningitis was seen seven years after vaccine introduction. Nevertheless, an important disease burden remains in older adults because of an increase in pneumococcal meningitis caused by non-PCV10 serotypes.

Although pneumococcal meningitis incidence rates were increasing in Poland, the overall and age-specific rates were considerably lower than those reported from other European countries before PCV10 introduction. The proportion of serotypes covered by the PCV10 vaccine during the baseline period was comparable with other European countries before PCV introduction. The low observed rates, considerable regional differences, and high case fatality proportion among the reported cases suggest substantial under ascertainment and underreporting of meningitis cases in Poland. Addressing these limitations and implementing improvements in the surveillance system should be considered when future studies to assess the impact of PCV10 are planned and conducted.