Age-structured dynamics and susceptibility in the face of infection and vaccination

BackgroundStrikingly low allocation of SARS-CoV-2 vaccine to the African Continent limits its capacity to control transmission. Characterizing the trajectory of vaccination efforts and their impact on the expected burden of SARS-CoV-2 will help planning vaccine delivery strategies, and public health interventions more broadly. As the burden is strongly age-dependent, this requires an understanding of the age-structured dynamics of susceptible individuals, accounting for the combined effects of vaccination and infection induced immunity. Methods and FindingsWe illustrate with projections for diverse African LMIC demographics. To this end, we develop an age-structured mathematical model with vaccination to assess the likely time-horizon to reach target vaccine coverage of high-risk groups, and how susceptibility patterns across age will shift as a result of both infection, and the broadening of vaccination targets from a focus on risk groups to efforts to reach the general population. We base our assessment on the demography, contact patterns and public health capacity of 16 African countries with diverse age pyramids. We identify a considerable divergence in the projected horizon of expanded targeting from prioritized age groups to general vaccination, with longer time among those with higher mean age and lower vaccination capacity. We parameterize the model using realistic demographies and contact patterns to project the changing age profile of susceptibles. We demonstrate that contacts and vaccination jointly drive the early age profile; while immune duration contributes to the transition of age-susceptibility profile in the intermediate future. ConclusionsOur model framework provides a flexible and critical preparedness-tools to inform decision making against future epidemic waves and beyond Covid-19.