The Seasonality of Human Coronaviruses and Future Implications for SARS-CoV-2
Author(s): Alan T. Evangelista, Atif Abdalla
The seasonality of influenza viruses and endemic human coronaviruses was tracked over an 8-year period (2013-2020) to assess key epidemiologic reduction points in disease incidence for an urban area in the northeast United States. Patients admitted to a pediatric hospital with worsening respiratory symptoms were tested using a multiplex PCR assay from nasopharyngeal swabs. The additive seasonal effects of outdoor temperatures and indoor relative humidity (RH) were evaluated. The 8-year average peak activity of human coronaviruses occurred in the first week of January, when droplet, aerosol, and contact transmission was enabled by the low indoor RH of 20-30%. Previous studies have shown that an increase in RH from 30% to 50% has been associated with markedly reduced viability and transmission of influenza virus and animal coronaviruses. As disease incidence was reduced by 50% in early March, to 75% in early April, to greater than 99% at the end of April, a relationship was observed from colder outdoor temperatures in January with a low indoor RH to a gradual increase in outdoor temperatures in April with an indoor RH up to 50%. As a lipid-bound, enveloped virus with similar size characteristics to endemic human coronaviruses, SARS-CoV-2 should be subject to the same dynamics of reduced viability and transmission with the seasonal increase in indoor relative humidity. Major factors in the reduction of community SARS-CoV-2 can be attributed to vacci-nation, acquired natural immunity post-infection, indoor mask-wearing, social distancing, and contact tracing. In addition to these factors, the seasonal effect of the transitioning from lower to higher indoor RH with increasing outdoor temperatures could contribute to the future seasonality of SARS-CoV-2 cases. Over the 8-year period of this study, human coronavirus activity displayed a greater than 99% incidence reduction in the months of June through September, and the future implication would be that SARS-Cov-2 may follow a similar pattern.