Trends in serotype distribution and disease severity in adults hospitalised withStreptococcus pneumoniaeinfection in Bristol and Bath: a retrospective cohort study, 2006-2022

BackgroundPaediatric pneumococcal conjugate vaccination (PCV) has reduced adult PCV-serotype disease: PCV7 has greater indirect effects than PCV13. Ongoing surveillance is required to evaluate current vaccine usage and inform future vaccine deployment, particularly with respiratory infection epidemiology changing following SARS-CoV-2 emergence.Methods and FindingsA retrospective cohort study, all adults >16 years admitted to three UK hospitals, 2006-2022, with pneumococcal disease. Medical records were reviewed for each clinical episode and serotype data were obtained from the UK Health Security Agency national reference laboratory. We identified 1,501 (40.3%) cases of invasive pneumococcal disease (IPD) with known serotype, 134 (3.6%) IPD cases without serotype data, and 2,084 (56.0%) non-IPD cases, which are typically missed in national surveillance. Disease incidence increased progressively from 2006-2020, followed by a sudden decline after COVID-19 emergence and then a gradual increase to pre-pandemic levels. Paediatric PCV7 introduction reduced adult PCV7 serotype IPD from 29.4% [24.1-35.4] of IPD in 2006-09 to 7.0% [3.7-12.7] in 2021-22. PCV13 introduction also decreased adult vaccine serotype IPD, but considerable residual adult disease remains, causing 34.3% [28.6-40.4] of IPD in 2006-09 and 21.7% [15.5-29.6] 9 in 2021-22, respectively. Serotype replacement diminished the benefits of PCV introduction: PCV20-13 and non-PCV serotypes represented 27.0% [21.9-32.9] and 9.3% [6.3-13.5] of disease in 2006-2009, and 39.5% [31.5-48.2] and 31.8% [24.4-40.2] in 2021-2022, respectively. Serotype shifts have resulted in increasing disease caused by serotype 3 and 8, and the re-emergence of serotype 19F and 19A. These serotype shifts occurred as clinical disease severity changed, and whilst the COVID-19 pandemic disrupted disease severity trends, these have now largely reverted to previous trajectories. Patient age trended upwards and although CURB65 severity decreased there were increased ICU admission rates. Overall, inpatient mortality decreased and hospitalisation duration remained stable.ConclusionsAfter 17 years of PCV use, residual pneumococcal disease due to the vaccine serotypes among hospitalised adults remains. The sharp decline in pneumococcal disease during the COVID-19 pandemic has now reversed, with increasing cases due to vaccine serotypes, especially serotype 3. Around 68.2% of cases in 2022 were potentially covered by the recently licensed 20-valent PCV.