While a reduction in transmission during the summer in the Northern Hemisphere is possible, it will likely be modest compared to the expected effects of the control measures that have been implemented.

The transmission of many respiratory viruses follows periodic, cyclical, and seasonal patterns, particularly in the temperate region of the Northern Hemisphere, where spikes in incidence are typically observed during the fall and winter. The mechanisms underlying this seasonality involve both the virus and the host. In temperate zones, during the fall and winter, changes in weather conditions can contribute to:

• promote the survival of respiratory viruses in the environment and the airborne transmission of virions, due to lower or moderate levels of temperature, ultraviolet radiation, humidity, precipitation, and wind;

• increase host susceptibility, through reduced immunity, fragility of the respiratory mucosa, changes in mucus, and exacerbation or complications of pre-existing health conditions;

• altering individual behavior by encouraging indoor social activities and interactions that are more conducive to transmission. [2, 4–7].

This review describes the results of experimental studies identifying environmental conditions that influence viral survival; epidemiological studies establishing ecological links between the progression of the epidemic and climatic factors under real-world conditions; and studies projecting the circulation of SARS-CoV-2 based on various scenarios regarding its seasonal behavior and immunity conferred by infection.

Key Takeaways

• Human coronaviruses other than SARS-CoV-2 exhibit seasonal patterns, with reduced transmission during the summer months.

• Increased temperature and humidity experimentally reduce the survival of SARS-CoV-2.

• Several studies show a negative ecological correlation between these climatic variables and the incidence of infection.

• Nevertheless, increases in temperature and relative humidity during the 2020 summer season are expected to have a limited effect on transmission.

• The long-term effect of seasonality will depend primarily on the level of immunity conferred by SARS-CoV-2 infection or by cross-immunity with other coronaviruses.