Influenza outbreaks are often seasonal – but their seasons vary. Whether Covid-19 has a season is uncertain.
In the northern hemisphere, flu and cold viruses tend to peak in winter months, then die down with warmer weather. “Coronaviruses tend to be associated with winter because of how they’re spread,” explains American epidemiologist Elizabeth McGraw. When the weather is cold, people tend to cluster together, thereby offering viruses contagious opportunity.
Viruses spread via respiratory droplets released when an infected person coughs or sneezes. Such droplets are more likely to spread under certain conditions. “What we know is that the droplets are better at staying afloat when the air is cold and dry,” says McGraw. “When the air is humid and warm, the droplets fall to the ground more quickly, and it makes transmission harder.”
How temperature affects coronavirus transmission and infection mechanics has been studied. At temperatures slightly above freezing, the virus’s lipid envelope solidifies into a gel. The rubbery coat facilitates surviving on surfaces, facilitating person-to-person infection (fomite transmission).
As temperatures approach 15 °C (60 °F), the viral coat gradually thaws, melting into a soupy mix. “Like an M&M in your mouth, the protective covering melts when it enters the respiratory tract,” American cytologist Joshua Zimmerberg said. “It’s only in this liquid phase that the virus is capable of entering a cell to infect it.”
In the tropics, where the air is hot all year round, flu season tends to peak with heavy monsoon rains. That may partly be because people are confined together during monsoon deluges.
“Nobody has a really good theory for what’s going on in the tropics,” says flummoxed American epidemiologist James Tamerius. Different strains of flu virus may spread better under different climatic conditions, Tamerius speculates.
Not every coronavirus hews to the same rules. The camel-to-human MERS coronavirus was not very infectious. The MERS virus “doesn’t have that seasonality because it’s really an animal to human virus and not something causing disease in a seasonal pattern,” explains American epidemiologist Amesh Adalja.
1/3rd of common colds are caused by coronaviruses, of which Covid-19 is the most recently evolved strain. “We’ve seen, basically, explosive spread inside China of person-to-person transmission, so – in that sense – Covid-19 really is behaving like a common-cold causing coronavirus,” says Adalja. “I do think seasonality will play a role. As this outbreak unfolds and we approach spring and summer, I do think we will see some tapering off of cases.”
American epidemiologist Anthony Fauci tentatively agrees but sounds a cautionary note: “we don’t know that about this coronavirus. It doesn’t have a history.”
American epidemiologist Nancy Messionnier concurs that “it’s premature to assume” that Covid-19 will break pace as the season changes. “We haven’t been through even a single year with this pathogen.”
Covid-19 has shown exceptional wiles in its infectiousness, notably its effective spread by suspending symptoms (asymptomatic infection). Covid-19 is a variant of the SARS virus, which was much more lethal but much less infectious. Covid-19 has clearly picked up adaptive advantages which may defy seasonality.
James D. Tamerius et al, “Environmental predictors of seasonal influenza epidemics across temperate and tropical climates,” PLoS Pathogens (7 March 2013).
Allison Aubrey, “Can coronavirus be crushed by warmer weather?,” NPR (12 February 2020).
Nancy Shute, “Flu risk and weather: It’s not the heat, it’s the humidity,” NPR (8 March 2013).
“Why the flu virus is more infectious in cold winter temperatures,” ScienceDaily (1 April 2008).