It’s important to know whether transmission of the novel coronavirus SARS-CoV-2 will change seasonally, as it does with other respiratory infections like influenza, for planning public health responses to COVID-19.
Scientists studied the transmission dynamics of various human coronaviruses as well as the effect of weather on COVID-19 in an attempt to answer the topic of coronavirus seasonality. Although definitive results cannot yet be drawn, preliminary research suggests that SARS-CoV-2 transmission is more likely to occur during the winter months.
Is there a season for coronavirus?
Infectious diseases that create outbreaks at specific times of the year are sometimes referred to as “seasonal infections.”
[1] Temperature and humidity, as well as human behaviour during the summer and winter months, can interact with varying degrees of population immunity to affect how common a virus infection is in the community at different seasons of the year. Some respiratory viruses, such as influenza, have a seasonal pattern of transmission, with epidemics typically occurring in the winter.
Although there are exceptions, this seasonality is most prominent in temperate regions of the planet, with more intricate trends noted in the tropics. During the 2009 H1N1 influenza pandemic, for example, there was a summer wave of infections as the virus travelled among highly susceptible northern hemisphere populations, which were disproportionately affected.
Because SARS-CoV-2 was only recently identified as a human pathogen, it is still unclear if transmission will vary seasonally in different parts of the world. We do know, however, that it is spread between humans by direct and indirect contact with respiratory droplets, just like other respiratory viruses like the four human coronaviruses that have been circulating in the human population for decades. In temperate countries, these coronaviruses, which are normally associated with moderate cold symptoms, tend to circulate alongside influenza during the winter.
What can we learn from other human coronavirus infections that occur during the year?
HKU1, HCoV-229E, HCoV-NL63, and HCoV-OC43 are four human coronaviruses that are regularly encountered in human populations.
[2] These viruses are most prevalent in the UK and other temperate regions of the world during the winter months, peaking between January and March. Between 2005 and 2017, a systematic investigation of their prevalence and peak infection period was undertaken in Scotland. Patients who reported with respiratory sickness to their doctor’s office or a hospital were tested for a panel of viruses that cause respiratory infections, including seasonal human coronaviruses.
The incidence of these coronaviruses varied, with HKU1 having such a low prevalence that screening for it was halted in 2012; the greatest prevalence in this Scottish community was HCoV-OC43. The study also discovered that coronavirus distribution increased throughout the winter, but that the spread of each variety of coronavirus differed from year to year. HCoV-229E infections peaked every second year during the 13-year study period, while HCoV-OC43 and HCoV-NL61 infections peaked every year, though the patterns were not always consistent.
Is it possible that the weather has an impact on the spread of SARS-CoV-2?
A number of research have looked into the possibility of a link between weather and the frequency of COVID-19 infections, but there has yet to be a clear consensus. Many of these studies are now available via pre-print sites and have not yet been peer-reviewed, thus they should be regarded with caution and scepticism. One method is to compare the average temperature and humidity across different geographical areas with the rate of viral transmission. [4] [5][6]
Is it possible for coronavirus to survive in hot weather?
There was no link between temperature or humidity in a study that estimated the basic case reproduction number (R0, a measure of the rate of infection) across Chinese provinces.
[4] This finding is in line with another study that looked at 224 cities across China and found no obvious link between ambient temperature and new infection rates. [5] A separate study of 429 Chinese cities, on the other hand, discovered a negative link between the virus’s rate of transmission and local temperature. [6]
So yet, there is only a sliver of evidence supporting seasonal variation in SARS-CoV-2 transmission, with the possibility of a larger spread in temperate locations over the winter months. More research is needed to validate the seasonal variation and determine if it is caused by differences in virus survival outside of the human host or seasonal patterns in human social behaviour (such as spending more time indoors during the winter).