Also Covid-19 spreads more easily in the winter months
Winter epidemics of respiratory infectious disease were documented as early as 400 B.C. by Hippocrates in the "Book of Epidemics". Since then, many respiratory viruses have been identified as etiological agents of epidemics as well as the cause of seasonal infections alike has been elucidated.
Despite great preventive efforts, epidemics of viral respiratory tract infections are still widespread and can have fatal consequences in susceptible people. In the United States alone, the estimated costs per year of the common cold amount to $40 billion and to over $87 billion for influenza respectively. In addition, new types of viral respiratory epidemics occur during winter months, such as the severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002-2003 and currently SARS-CoV-2 (Covid-19).
Studies indicate that possible seasonal factors of the winter environment may favor the spread of a variety of respiratory virus infections. These include changes in temperature, absolute humidity, sunlight, vitamin status and host behavior of the virus during the winter months.
In addition, environmental factors influence host susceptibility by modulating the respiratory defense mechanisms and promote the viability and transmission of respiratory viruses. Furthermore, human behavior patterns influence the contact rates between infected and susceptible individuals - people stay more often in closed, heated rooms with very low humidity which strongly promotes virus transmission.
Seasonal environmental factors such as temperature, humidity, sunlight, etc. can hardly be changed, while social behavior and preventive measures in the winter months can. The question is, what additional possibilities will there be soon for the prevention of viral respiratory infections such as SARS-CoV-2 in addition to well-known measures such as social distancing, masks, vaccinations, etc.
Resource: Annual Review of Virology, Miyu Moriyama, Walter J. Hugentobler and Akiko Iwasaki