Which regions are at risk?
Winter is fast approaching in the Northern Hemisphere, and researchers warn...
"This virus will be in its prime," says David Relman, a microbiologist at Stanford University in California. "There are some pretty sobering and difficult months ahead of us."
Major risk factors
Infections caused by many respiratoryviruses, including influenza and some coronaviruses, increase in winter and decrease in summer. Researchers say it is too early to talk about the COVID-19 pandemic as a seasonal virus. But rising disease statistics suggest that a small seasonal effect is likely to contribute to larger outbreaks in winter. Such conclusions can be drawn based on the already known data on the spread of the virus and the behavior of people in the colder months.
What behavior are we talking about?Mauricio Santillana, a mathematician at Harvard Medical School in Boston, Massachusetts, who models the spread of the disease, says people will be more likely to interact in indoor spaces with poor ventilation, seriously increasing the risk of transmission.
But even if there is a slight seasonal effect,The main factor behind the increase in the spread will be the sheer number of people who are still susceptible to infection, says Rachel Baker, an epidemiologist at Princeton University in New Jersey. Such people need to be extremely careful, the researchers say.
“By far the biggest factor that will affect the size of the COVID-19 outbreak will be control measures such as social distancing and the wearing of masks,” he stressed.
Seasonal trends
The seasonal trends of viral infection are determined by many factors, including human behavior and the properties of the virus - some pathogens do not like hot and humid conditions.
Laboratory experiments show thatThe development of SARS-CoV-2 is favored by cold and dry conditions, especially away from direct sunlight. For example, artificial UV light can inactivate SARS-CoV-2 particles on surfaces and in aerosols, especially at temperatures around 40 ° C. The infectious virus also degrades more quickly on surfaces in warmer, more humid environments. In winter, people tend to heat their homes to around 20 ° C and the air is dry and poorly ventilated, explains Dylan Morris, a mathematician biologist based in Princeton. "Winter conditions indoors are pretty good for viral stability."
To assess whether it increases or decreasesthe number of cases of infection with a particular virus depending on the season, researchers usually study its spread in a particular place several times a year over the years. But without knowing the time, they tried to study the seasonal contribution to the transmission of SARS-CoV-2 by examining the infection rates in different places around the world.
In a study published on October 13,examines the rise in SARS-CoV-2 infections in the first four months of the pandemic, before most countries introduced control measures. It was found that the infection grew fastest in places with less ultraviolet light, and predicted that without any intervention the incidence would decline in the summer and peak in the winter. In winter, “the risk increases, but you can still significantly reduce it by following guidelines,” explains Cory Merow, an ecologist at the University of Connecticut in Storrs and co-author of the study. “Weather is only a small part of the picture.”
But François Cohen, an environmental economist fromThe University of Barcelona in Spain says that testing was also rather limited at the start of the pandemic and remains unreliable. That is why it is now impossible to determine the impact of weather on the spread of the virus. This is still too far away.
Baker tried to identify the impact of climate onseasonal dynamics of cases during a pandemic, using data on moisture sensitivity of another coronavirus. She and her colleagues have modeled the rise and fall in New York City infection rates over several years, with and without climate impacts, and with varying levels of control measures. They found that a small climatic effect could lead to significant outbreaks as the seasons change, if control measures only help contain the virus. The team has published their findings on the medRxiv preprint server, and the authors suggest that stricter controls may be required in winter to reduce the risk of outbreaks.
What's in the future?
“If SARS-CoV-2 can survive better in coldconditions, it is still difficult to separate this contribution from the influence of human behavior,” said Kathleen O'Reilly, a mathematical epidemiologist at the London School of Hygiene and Tropical Medicine. “Influenza has been around for hundreds of years, and the specific mechanism of why flu spikes in winter is still poorly understood.”
And even if the researchers had morereliable data on SARS-CoV-2, they would see only small or negligible seasonal effects at such an early stage in the pandemic when most of the population is still susceptible to the virus.
However, over time, seasonal effects can playa more important role in shaping infection trends as more people develop immunity to the virus. It could take up to five years due to natural contamination, Baker said, or less if people are vaccinated.
But will there ever be a seasonal pattern andhow it looks will depend on many factors that remain to be understood, including how long immunity lasts, how long it will take to recover, and how likely people are to be reinfected.
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