It’s too soon to say whether COVID is seasonal like the flu — but where clusters aren’t under control, infections will continue to swell.

Smriti Mallapaty

Changing seasons might affect the transmission of SARS-CoV-2, but researchers say it’s too early to tell.Credit: Beata Zawrzel/NurPhoto/Getty

Winter is fast approaching in the Northern Hemisphere, and researchers warn that COVID-19 outbreaks are likely to get worse, especially in regions that don’t have the virus’s spread under control.

“This virus is going to have a heyday,” says David Relman, a microbiologist at Stanford University in California. “We are looking at some pretty sobering and difficult months ahead.”

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Infections caused by many respiratory viruses, including influenza and some coronaviruses, swell in winter and drop in summer. Researchers say it’s too early in the COVID-19 pandemic to say whether SARS-CoV-2 will become a seasonal virus. But growing evidence suggests that a small seasonal effect will probably contribute to bigger outbreaks in winter, on the basis of what is known about how the virus spreads and how people behave in colder months.

People will be interacting more often indoors in places with poor ventilation, which will increase the risk of transmission, says Mauricio Santillana, a mathematician at Harvard Medical School in Boston, Massachusetts, who models disease spread.

But even if there is a small seasonal effect, the main driver of increased spread will be the vast number of people who are still susceptible to infection, says Rachel Baker, an epidemiologist at Princeton University in New Jersey. That means people in places that are going into summer shouldn’t be complacent either, say researchers.

“By far the biggest factor that will affect the size of an outbreak will be control measures such as social distancing and mask wearing,” says Baker.

Evidence so far

Seasonal trends in viral infection are driven by multiple factors, including people’s behaviour and the properties of the virus — some don’t like hot, humid conditions.

Laboratory experiments reveal that SARS-CoV-2 favours cold, dry conditions, particularly out of direct sunlight. For instance, artificial ultraviolet radiation can inactivate SARS-CoV-2 particles on surfaces1 and in aerosols2, especially in temperatures of around 40 °C. Infectious virus also degrades faster on surfaces in warmer and more humid environments3. In winter, people tend to heat their houses to around 20 °C, and the air is dry and not well ventilated, says Dylan Morris, a mathematical biologist at Princeton. “Indoor conditions in the winter are pretty favourable to viral stability.” 

To assess whether infections with a particular virus rise and fall with the seasons, researchers typically study its spread in a specific location, multiple times a year, over many years. But without the benefit of time, they have tried to study the seasonal contribution to SARS-CoV-2 transmission by looking at infection rates in various places worldwide.

A study4 published on 13 October looked at the growth in SARS-CoV-2 infections in the first four months of the pandemic, before most countries introduced controls. It found that infections rose fastest in places with less UV light, and predicted that, without any interventions, cases would dip in summer and peak in winter. In winter, “the risk goes up, but you can still dramatically reduce your risk by good personal behaviour”, says Cory Merow, an ecologist at the University of Connecticut in Storrs, and a co-author of the study. “The weather is a small drop in the pan.”

But Francois Cohen, an environmental economist at the University of Barcelona in Spain, says that testing was also quite limited early in the pandemic, and continues to be unreliable, so it is impossible to determine the effect of weather on the spread of the virus so far.

Baker has tried to tease apart the effect of climate on the seasonal pattern of cases during the course of a pandemic, using data about the humidity sensitivity of another coronavirus. She and her colleagues modelled5 the rise and fall in infection rates over several years for New York City with and without a climate effect, and with different levels of control measures. They found that a small climate effect can result in substantial outbreaks when the seasons change if control measures are only just managing to contain the virus. “That could be a location where climate might nudge you over,” Baker says. The team posted its results on the preprint server medRxiv on 10 September; the authors suggest that stricter control measures might be needed during winter to reduce the risk of outbreaks.

In the future

If SARS-CoV-2 can survive better in cold conditions, it’s still difficult to disentangle that contribution from the effect of people’s behaviour, says Kathleen O’Reilly, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine. “Flu has been around for hundreds of years and the specific mechanism as to why you have peaks of flu in the winter is still poorly understood,” says O’Reilly.

And even if researchers had more reliable data for SARS-CoV-2, they would see only small or negligible seasonal effects so early in the pandemic, when much of the population is still susceptible, says Relman.

Over time, however, seasonal effects could play a more important part in driving infection trends, as more people build up immunity to the virus. This could take up to five years through natural infection, or less if people are vaccinated, says Baker.

But whether a seasonal pattern emerges at all, and what it will look like, will depend on many factors that are yet to be understood, including how long immunity lasts, how long recovery takes and how likely it is that people can be reinfected, says Colin Carlson, a biologist who studies emerging diseases at Georgetown University in Washington DC.