Why does coronavirus spread faster in winter? Increase in Covid spread linked to UV level dip

BOSTON: Natural variations in ultraviolet radiation influences the spread of the novel coronavirus, says a new study which adds that the effect is still modest compared to that of measures like physical distancing, and mask wearing.

The findings, published in the journal PNAS, suggests that the incidence of COVID-19 may have a seasonal pattern in which the virus spreads faster in the winter when it's darker with lower levels of UV radiation than during summer.

"Understanding the potential seasonality of COVID-19 transmission could help inform our response to the pandemic in the coming months," said Jonathan Proctor, a co-author of the study from Harvard University in the US.


In the research, the scientists analysed daily COVID-19 and weather data from over 3,000 administrative regions in more than 170 countries.

They found that the spread of COVID-19 through a population tended to be lower in the weeks following higher UV exposure.

While the reasons for seasonal variations in the spread of COVID-19 remain a mystery, the researchers said there have been some clues that UV could play a role.

They said related species of coronaviruses such as SARS and MERS were found to be sensitive to UV radiation with recent laboratory studies showing that UV inactivates SARS-CoV-2, the virus that causes COVID-19, on surfaces.

But the scientists added that attempts to understand the influence of UV in the real world have been limited by scarce data and the difficulty of isolating climate variables from other factors driving the spread of the virus.

So the scientists compiled and cleaned data from statistical agencies around the world and examined how transmission within a particular population changed according to variations in sunlight, temperature, rainfall, and humidity.

"We basically ask whether daily fluctuations in environmental conditions experienced by a population affect new COVID-19 cases up to two weeks later," explained study co-author Kyle Meng from the University of California Santa Barbara in the US.

The scientists evaluated the relationship between UV and COVID-19 using data from the beginning of the pandemic, and then used that relationship to simulate how seasonal changes might influence the spread of the coronavirus.

They found that changes in UV between winter and summer led to a 7-percentage point decrease in the COVID-19 growth rate on average across the Northern Hemisphere.

According to the researchers, this is about half the average daily growth rate at the beginning of the pandemic.

While the findings of the current study show that COVID-19 exhibits a seasonal pattern due to changes in UV, the scientists said the full seasonality of COVID-19 still remains unclear.

They explained that this is due to the uncertain influences from other environmental factors such as temperature and humidity.

"We are confident of the UV effect, but this is only one piece of the full seasonality picture," said Jules Cornetet, another co-author of the study.

However, the scientists cautioned that UV exposure alone is unlikely to stop the spread of the virus without strong social distancing policies.

"Regardless of the weather, additional measures appear to be necessary to substantially slow the spread," Proctor said.

The researchers added that it is also unclear what mechanism is driving this effect.