The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused the ongoing coronavirus disease 2019 (COVID-19) pandemic, which has claimed more than 5.66 million lives worldwide. The successive waves of infection have made it clear that the virus will eventually transition to an endemic disease. Therefore, projections of its seasonality are crucial to inform public health policy. This is not an easy task owing to the short duration of the pandemic, heterogeneous implementation of preventive measures, and a host of other factors.

Study: Projecting the seasonality of endemic COVID-19. Image Credit: Glowing Graphics/Shutterstock

A new study has been published on the medRxiv* preprint server that uses long-term data on the incidence of circulating coronaviruses and estimates the endemic seasonality of SARS-CoV-2 in global population centers.

Background

The annual variation in the transmission of other respiratory viruses (including coronaviruses) could be used to gauge the seasonality of SARS-CoV-2. However, this method could lack rigorous quantification. Alternatively, data on certain relatives of SARS-CoV-2 could be analyzed through a quantitative comparative phylogenetic approach. It must be noted that these methods have already yielded critical estimates of the durability of immunity against reinfection by SARS-CoV-2.

A new study

Scientists applied comparative phylogenetics to extensive long-term incidence data on HCoV-OC43, HCoV-NL63, HCoV-HKU1, and HCoV-229E across major population centers to estimate the seasonal force of infection. This method is independent of isolation of interventions or identification of underlying mechanisms, and the projections are important for anticipating and optimizing public policy for high-risk periods.

Key findings

Researchers studied a broad range of predominantly temperate localities across North America, Europe, and Asia. They stated that SARS-CoV-2 could be expected to transition to a seasonal pattern of high incidence across the Northern Hemisphere in the late fall and winter months. This result should provide fundamental insights for determining local public health policies in peak infection times.

A major advancement of the current study, over previous analyses, is that it is based on multi-year endemic coronavirus incidence data. This implies that it is free from the biases introduced by the emergence of the pandemic and large-scale public health interventions. Overall, the results are consistent with the seasonal incidence trends of common human-infecting respiratory viruses in the northern hemisphere.

The results on the seasonality of SARS-CoV-2 shed light on the incidence trends under endemic conditions. Researchers stated that seasonality could be further amplified by an overwhelmed and lagging public health response. They further stated that less seasonality for SARS-CoV-2 during pandemic spread would be expected than in its eventual endemic incidence, with a smaller forcing factor amplified by pathogen population dynamics.

The results obtained in the current study project a seasonal rhythm of SARS-CoV-2. This rhythm is similar to the trends observed among many other human-infecting respiratory viruses. The greater respiratory incidence in the winter has been attributed to several factors, such as temperature, humidity, ultraviolet radiation, and host behavior. However, this trend is reversed in the Southern Hemisphere and muted in the tropics.

The Southern Hemisphere was undersampled and more studies in this area would help further the understanding of coronavirus seasonality and facilitate preparedness. Denser sampling could also help in developing more precise regional estimates. Sampling in the tropics would also enable testing of the muted seasonality, which should be studied more.

The projections could be influenced by public health interventions and evolutionary change impact. The emergence of novel variants, such as the Delta or Omicron variants, can hinder public health efforts and impact seasonal trends. The results obtained in this study suggest that rises of novel variants could frequently coincide with surges in other seasonal respiratory viruses, potentially overwhelming healthcare facilities.

Conclusion

Prior knowledge of seasonality is crucial as it could aid in informed and advanced public health messaging regarding seasons of high concern. Researchers have stated that even with extensive vaccination, SARS-CoV-2 will most likely join HCoV-229E, HCoV-NL63, HCoV-OC43, and HCoV-HKU1 as a circulating endemic coronavirus. The approach developed in this study guides a wide array of public health decisions until the pandemic phase of SARS-CoV-2 spread has passed and long-term data on COVID-19 incidence is collected.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.