Long-term effect of COVID-19 social distancing measures on pertussis transmission in England; a mathematical modelling study

Background. In 2024 England, in common with many other countries, experienced a pertussis resurgence the cause of which is unclear. We used a pertussis transmission model, previously developed to investigate the cause of the pertussis resurgence in England in 2012, to explore potential factors contributing to the increase in pertussis cases observed in England in 2024. Methods An age-stratified dynamic transmission model fitted to pertussis notification data from England between 1953 and 2013 was run until 2054 with and without changes in social mixing as estimated from Google mobility and school attendance data during the COVID-19 pandemic. The model assumes vaccination protects better against disease than pertussis infection, and that an infection results in more durable immunity than vaccination. Counterfactual scenarios were also run to explore the effect of reductions in vaccine coverage during the pandemic and of the addition in 2014 of boosters in the 2nd year of life and in adolescence. Results A resurgence was only generated with reduced social mixing and could not be explained by short-term reductions in vaccine coverage. Additional boosters at 18 months and 14 years from 2014 would not have prevented a resurgence though would have reduced its magnitude. Peaks of increased pertussis incidence are predicted over the next decade. The parameter sets that generated a resurgence in 2024 had the shortest duration of acellular vaccine protection, median 5 years with 90% protection against infection. Conclusion This modelling study implicates reduced mixing in England during the COVID-19 pandemic as the cause of the pertussis resurgence in 2024 together with the short duration of protection from acellular vaccine. Interruption of the background rate of natural boosting during the pandemic increased the pool of susceptible individuals resulting in increased transmission post-pandemic with clinical cases in those with waned vaccine–induced protection and the unvaccinated, including infants of unvaccinated mothers. In countries using acellular pertussis vaccines, infection continues to play an important role in maintaining population immunity around an endemic equilibrium. Improved pertussis vaccines that provide more complete and more durable protection against infection are needed to improve pertussis control.