Modelling immunity gaps to quantify infection resurgences.

When COVID-19 restrictions were removed, many countries observed infection surges in respiratory pathogens like respiratory syncytial virus (RSV) and influenza. This has been postulated to have been caused by reduced immunity in populations due to non-pharmaceutical interventions that reduced transmission of these pathogens. This pandemic-related phenomenon has been termed 'immunity debt' or 'immunity gap'. We propose a simple extension of the classic susceptible-immune-susceptible model to explore this phenomenon. The model is parametrized using RSV antibody data derived from healthy adults in Aotearoa, New Zealand. We consider a case study based on the prolonged stringent public health measures during the border closure years of 2020-2022 and compare these findings to observed hospitalization trends in Aotearoa, New Zealand. Our model predicts that diseases with very fast waning immunity are less likely to see increased infection rates after prolonged periods of stringent public health measures. However, diseases that wane moderately fast, such as RSV, are more likely to see a strong resurgence of cases when restrictions ease. Our results can be used to predict disease characteristics most likely to lead to strong resurgences after periods of prolonged restrictions and thus inform future public health responses.