Universal pneumococcal vaccination provides marked indirect beneficial effects through herd immunity

Universal pneumococcal vaccinations with the ten-valent pneumococcal vaccine (PCV10) have been offered to Finnish infants under the age of one since 2010, without any catch-up vaccinations for older children. In this issue of Acta Paediatrica 1, Palmu et al. publish their interesting results on the impact of the national programme on the incidence of clinically suspected invasive pneumococcal disease (IPD) in two unvaccinated age-specific cohorts. Their study covered the whole country and comprised unvaccinated children born in 2008–2010 (target population) and unvaccinated children born in 2003–2005 (control population). The groups were followed up for a 4-year period – 2006–2009 and 2011–2014, respectively – using national registers that contained inpatient and outpatient hospital admission and discharge data. In 2011–2014, 93% of Finnish infants received all three PCV10 doses 1. Laboratory-confirmed IPD cases, which were diagnosed by a positive pneumococcal culture in blood or in another normally sterile body fluid, were excluded. This resulted in a case definition of clinically suspected non-laboratory-confirmed IPD or unspecified sepsis. Pneumonia cases were not actively searched and were only included if IPD was clinically suspected and recorded in the registers. The clinical pictures of laboratory-confirmed and non-laboratory-confirmed IPD are indistinguishable, and, in an earlier study, only 22% of clinically suspected IPD cases had positive blood cultures 2. Thus, at the population level, the vaccination-derived decrease in clinically suspected IPD cases is expected to be much more important than the decrease in those with a positive culture. The incidence of non-laboratory-confirmed IPD or unspecified sepsis in unvaccinated children was 39% lower in the target population than in the controls, with a 95% confidence interval (95% CI) of 31 to 46, and the difference was larger, at 68% (95% CI 57–76) when discharge diagnoses alone were included 1. Thus, the national pneumococcal vaccination programme provided herd protection for unvaccinated Finnish children against clinically suspected IPD or unspecified sepsis without microbiological confirmation of the diagnosis. Palmu et al. estimated that the absolute reduction was 10-fold higher than that of laboratory-confirmed disease 1. The case definition of non-laboratory-confirmed IPD was close to our definition of blood culture-negative infection, which clinically resembled occult pneumococcal bacteraemia 3. We compared children hospitalised for acute infections associated with fever and leucocytosis at less than 36 months of age in one hospital district, during a 2-year period before (2008–2009) and after (2012–2013) the implementation of universal pneumococcal vaccinations with PCV10. Hospitalisation rates decreased by 38% if there was an infiltration in the chest radiograph and by 58% if there was no infiltration 3. Previous register-based nationwide studies by the Finnish vaccination research group have documented the substantial impact of the national PCV10 vaccination programme on pneumonia and laboratory-confirmed IPD among vaccinated and nonvaccinated children, and on clinically suspected IPD among young children eligible for vaccination. These were compared to reference populations from the prevaccination era 4-6. The rate (95% CI) of register-based all-cause pneumonia episodes reduced by 23% (18 to 28) 4, and the rate of register-based laboratory-confirmed IPD episodes fell by 80% (72–85) in the children aged <2 years who were eligible for vaccination and by 48% (18–69) in the unvaccinated children aged 2–5 5. The rate of register-based non-laboratory-confirmed IPD episodes reduced by 34% when both admission and discharge diagnoses were included in children eligible for vaccination, depending on the case definition, and fell by 66% when only discharge diagnoses were included 6. A register-based study from Sweden, published in 2018, compared the incidences of acute otitis media, sinusitis and pneumonia and the consumption of antibiotics for upper respiratory tract infections. It covered 2004–2008, before the implementation of universal pneumococcal vaccinations for Swedish infants in 2009, and 2014 which was 5 years after the implementation 7. As expected, the incidence of all four factors decreased substantially in the vaccine-eligible cohort aged 0–4 years. In 2014, 79% of children in that age group were fully vaccinated in the study area and another 14–15% were partly vaccinated, depending on age. Interestingly, the incidence of acute otitis media and sinusitis decreased by 21% and 25%, respectively, in unvaccinated children aged 5–17 years, but there were no significant changes in antibiotic consumption or in the incidence of pneumonia 7. In a previous report from one hospital district in Finland, the prevalence of pneumococcal penicillin resistance in samples obtained from middle ear or other upper airways secretions in children under 5 years of age decreased by 75% and pneumococcal macrolide resistance fell by 46% within 2 years after pneumococcal vaccinations for infants were implemented 8. Decreasing the circulation of invasive pneumococcal strains in the population also protects unvaccinated children and adults from pneumococcal infections through herd immunity, and the level depends on the vaccination rate. There are more than 90 pneumococcal serotypes and those that are not in the vaccine seem to replace those that are in the vaccine over time. The pneumococcal strains that cause invasive infections develop resistance to penicillin and to other antibiotics easily. Thus, decreasing the consumption of antibiotics 7, which further reduces the resistance of circulating pneumococcal strains to antibiotics 8, is of utmost importance for future health care. In conclusion, universal pneumococcal vaccinations seem to have marked indirect beneficial effects through herd immunity, in addition to the primary goal of direct pneumococcal immunity in vaccinated children. Achieving and maintaining the immunity-based beneficial effects of herd immunity require good vaccination programme coverage in children. In addition, exhaustive national registers and meticulous population-based register studies are needed to document these effects. No external funding. Nothing to declare.