Study population and setting
Authors used a simple (S)usceptible-(I)nfected-(R)ecovered-(S)usceptible compartmental model to explore potential transmission dynamics of SARS-CoV-2 over the first five years of the pandemic under different scenarios. They first explored the impact of non-pharmaceutical interventions (NPIs) leading to a 40% reduction in transmission intensity, assuming climatic conditions similar to New York, over a 5-year period with differing durations of natural immunity. Assuming a weekly vaccination rate of 0-1%, authors then explored the impact of vaccine introduction 18 months into the pandemic assuming different durations of vaccine-induced immunity and reduction in susceptibility to secondary infections. Finally, authors estimated the potential impact of vaccine hesitancy and some sectors of the population having intrinsically higher contact rates.
Summary of Main Findings
Authors found that reduced susceptibility to secondary infections could increase the time to secondary peaks, but that these peaks may be higher due to the accumulation of susceptible individuals. This could be compounded by implementation of NPIs. With increasingly imperfect immunity and especially with short vaccine-induced immunity, high vaccination rates were required to achieve elimination. Thus, both high rates of vaccination and relatively long-lived immunity are required to substantially reduce the burden of secondary infections. These qualitative projections of immunity on medium- and long-term dynamics were not affected by moderate variability in transmission within the population. Vaccine hesitancy increased the vaccination threshold required to achieve herd immunity particularly if these individuals were less adherent to NPIs and therefore had increased contact rates compared to non-refusers. Notably in this scenario, vaccination alone was not enough to prevent outbreaks.
Authors examined a wide range of plausible scenarios for natural or vaccine-induced immunity based on existing knowledge of SARS-CoV-2 and other seasonal human coronaviruses. They also considered how NPIs, climate, heterogeneity in transmission, and vaccine hesitancy could impact medium- and long-term SARS-CoV-2 transmission dynamics.
Authors make a number of simplifying assumptions including that SARS-CoV-2 transmission will become seasonal similarly to Beta-CoV HCoV-HKU1. In all scenarios vaccination is assumed to be transmission blocking rather than reducing disease severity. They also simplify other important factors such as age, clinical severity, and transmissibility which can lead to superspreading events known to be important for SARS-CoV-2 transmission.
This study provides insights into the potential medium- and long-term dynamics of SARS-CoV-2 transmission beyond the current pandemic and quantifies the potential effect that vaccination could have. Authors also provide an interactive site to explore different strategies required to optimise vaccination and NPI implementation (https://grenfelllab.shinyapps.io/sarscov2/).
This review was posted on: 5 April 2021