Exploring how COVID-19 pandemic trajectory might unfold in the future

Researchers who adapted standard epidemiological models to explore how the COVID-19 pandemic trajectory might unfold in the next five years report diverse scenarios ranging from recurring severe epidemics to elimination. Their work emphasizes how dependent the future course of the pandemic is on the nature of the adaptive immune response to this virus and on the efficacy of future potential vaccines. The duration and severity of future COVID-19 cases will depend on the strength of both natural and vaccine-derived immunity against the SARS-CoV-2 pathogen.

However, there is still uncertainty about both. Here, to explore how variations in immune responses could impact the magnitude and timing of COVID-19 cases in the next five years, Chadi Saad-Roy and colleagues adapted a series of standard epidemiologic models of the spread of infection (the SIR and SIRS models, respectively). They used these adapted models to evaluate results of four future scenarios that consider different outcomes for the nature of the adaptive immune response to SARS-CoV-2 infection, the effect of transmission rate seasonality, the adoption of non-pharmaceutical interventions, and the availability and effectiveness of a vaccine.

Depending on these variations, dramatically different immunity landscapes and burdens of critically severe cases could emerge, they say, ranging from sustained epidemics to near elimination. The results underscore that understanding the immunology of secondary infection - which impacts the number of those susceptible to the virus - is critical. They also reveal how the pandemic trajectory will be substantially altered by mass deployment of vaccines, though, this is strongly dependent on vaccine efficacy.

Their work also shows that relying on the status of infection of an individual as the main "observable" during an ongoing epidemic is not sufficient to characterize the complex immune landscape generated by the pandemic.

Regular testing of antibody presence and correlates of protection such as T cell immunity are critical, to accurately characterize population-level natural and vaccinal immunity to this pathogen. The authors note that in order to focus on immune dynamics, they made several simplifying assumptions.