Natural immune boosting can cause synchrony in susceptibility and outbreaks of respiratory infections with rapidly waning immunity

Natural immune boosting is a mechanism in which re-exposure to a pathogen while immunity is waning can prolong protection against reinfection. Its role in viral respiratory-tract infections with rapidly waning immunity has not been studied in mathematical models. Here we use a deterministic compartmental model to investigate the effect of immune boosting on such infections, and how the reduction in transmission due to non-pharmaceutical interventions during the covid-19 pandemic could affect immune waning and boosting. We find that immune boosting can introduce unstable equilibria into the model, and we show in simulations that this can amplify, or even cause, oscillations in infections and disease outbreaks. After periods of reduced transmissibility, representing non-pharmaceutical interventions, simulations with more immune boosting exhibit larger rebound outbreaks that occurred sooner. Observed incidence of respiratory syncytial virus infection in Scotland between 2016 and 2023 can be equally explained by models with high levels of immune boosting, and models without natural immune boosting. To produce the same incidence, models with more immune boosting require a greater mean transmissibility, suggesting that models underestimating natural immune boosting will also underestimate transmissibility.