Projecting the transmission dynamics of COVID-19 pandemic

As the COVID-19 pandemic continues, a major unanswered question is how SARS-CoV-2 will persist in the human population after its initial pandemic stage. A new modeling study suggests that the total incidence of the virus through 2025 will depend crucially on the duration of human immunity - of which scientists know little now. Longitudinal serological studies are thus urgently required, the authors say, to determine the extent of population immunity, whether immunity wanes, and at what rate.

Based on their modeled projections of future virus transmission scenarios, the authors also suggest that while one-time social distancing may suppress critical cases to within hospital capacity (a key metric of success for social distancing), infection will resurge once these measures are lifted, overwhelming hospitals, such that social distancing may need to be maintained intermittently into 2022. Increasingly, public health authorities consider it unlikely that SARS-CoV-2 will follow its closest cousin, SARS-CoV-1, and be eradicated by intensive public health measures after causing a brief pandemic. Instead, the transmission could resemble that of pandemic influenza by circulating seasonally. Knowing the likelihood of this scenario is key for mounting an effective public health response. Here, using data on seasonality from known human coronaviruses and assuming some cross-immunity between SARS-CoV-2 and other coronaviruses, Stephen M. Kissler and colleagues built a model of multi-year interactions.

They used this model to investigate for what duration social distancing measures need to stay in place to maintain control of SARS-CoV-2, projecting the potential dynamics of COVID-19 over the next 5 years. Based on their simulations, they say the key factor modulating virus incidence in coming years is the rate at which virus immunity wanes - which is yet to be determined. They also report that under all scenarios simulated, including one-time and intermittent social distancing, infections resurge when the simulated social distancing measures are lifted. When social distancing is relaxed when virus transmissibility is heightened in the fall, an intense winter outbreak may occur, overlapping with flu season and exceeding capacity of hospitals. Another modeled scenario shows a resurgence in SARS-CoV-2 could occur as far in the future as 2025. New therapeutics could alleviate the need for stringent social distancing but in the absence of these, surveillance and intermittent distancing may need to be maintained into 2022, write Kissler and colleagues. This would give hospitals time to increase critical care capacity while allowing population immunity to accumulate. The authors conclude the study by addressing its limitations. "Our goal in modeling such policies is not to endorse them but to identify likely trajectories of the epidemic under alternative approaches," they write.