Seasonality and its impact on COVID-19

Publication Summary

• A combination of factors are likely to combine to exacerbate the epidemic of COVID-19 during the winter months. These factors include continued susceptibility of the population, the direct effect of environmental variables (such as temperature and UV light) the indirect effect of poor weather leading to people spending more time indoors and other seasonal changes in contact rates due to school opening, seasonal festivals, etc. There are other effects that may exacerbate the severity of COVID-19 disease during the winter. This paper examines the evidence of these effects. • The direct effect of winter environmental conditions on transmission is likely to be small. Winter conditions will increase viral persistence on outdoor surfaces due to reduced temperatures and UV levels, in unheated indoor environments due to lower temperatures and in day-time outdoor aerosols due to reduced UV levels (high confidence). However, the outdoor environment is not dominant in SARS-CoV-2 transmission, and indoor environmental conditions (where the vast majority of transmission is likely to occur) are more constant. • Changes in behaviour are expected to occur. Patterns of school opening are likely to affect transmission but the evidence is inconsistent across available studies, resulting in low confidence of the assessment. In addition, there is limited data to suggest that contacts increase in the winter months (low confidence) and that social contacts may increase towards the end of the year and then fall again in January (low confidence). Individuals appear to spend longer indoors during the winter, when ventilation rates in buildings are lower (medium confidence). All studies are based on data from previous time periods, and physical distancing measures are likely to influence observed patterns this winter, further reducing uncertainty of the assessment. • Over the next 6-12 months changes in SARS-CoV-2 susceptibility (unrelated to seasonal factors) are likely to dominate the epidemiology. Susceptibility is likely to have a much bigger impact on transmission than environmental factors (high confidence). • It seems likely that co-infection with influenza viruses worsens the clinical course (medium confidence). There is no evidence, at present, to suggest that other physiological changes that may occur over winter will affect the severity of disease.