After almost two years of bad news, a potential hope is finally on the horizon: COVID-19 may be a seasonal illness.
Since the new coronavirus first emerged in late 2019, scientists and policy makers have debated whether it’s seasonal — like flu, with increased transmission during the winter — or equally contagious throughout the year. A paper published last week in Nature Computational Science provides strong evidence that COVID-19 is indeed a seasonal low-temperature infection, and this has important implications for how we can learn to live with the virus.
Xavier Rodó, director of the ISGlobal Climate and Health program and coordinator of the study, said: “Our findings support the view that COVID-19 is a true seasonal low-temperature infection similar to influenza and the more benign circulating coronaviruses. makes it necessary to put emphasis on ‘air hygiene’ through space ventilation.”
As explained in the article, low humidity conditions cause aerosol droplets to slightly evaporate in the air, reducing their size. This could make the droplets of water in our breath be small enough to bypass the immune defenses in our noses, potentially making diseases, including COVID-19, more contagious. For this reason, the researchers say, more emphasis should be placed on good ventilation, as well as possible meteorological situations, in public policy and medical interventions.
Nor is this research the first to conclude that COVID-19 is likely seasonal. A study published in May in the journal GeoHealth examined the link between environmental conditions and COVID-19 transmission rates in five countries, concluding that the virus followed a seasonal pattern similar to the flu. This week’s study confirms this on a much larger scale: The team studied the spread of COVID-19 in more than 160 countries on five continents, before policy interventions were implemented, then during the first, second and third waves.
Without input from public health policies, the team found a negative correlation between the rate of transmission of COVID-19 and temperature and humidity levels worldwide. In other words, the lower the temperature or humidity, the higher the contamination rate. But as the epidemic progressed, the pattern continued. The first wave ended as temperatures and humidity levels increased, followed by the second wave as we entered autumn. This model was valid worldwide, at the country level, at the regional and even at the city level, with only one exception: summer 2020.
“[This] can be explained by a variety of factors, including youth gatherings, tourism, and air conditioning,” says lead author Alejandro Fontal for summer 2020. As the research explains, air conditioning may be the answer to some questions: “In the rich countries of the Northern Hemisphere, the indoor microclimate that is cooled during the [hot] months, as in many outbreaks observed in meat processing plants in the United States and Europe, where refrigerated air is mechanically re-ventilated. can host the transmission.”
While the team believes the results are convincing, they note that the study has several limitations. First, COVID-19 is a new virus, so the epidemiological record is somewhat limited. They also point out that their results show a correlation, not a causation, so while the virus waves are likely to be linked to temperature and humidity, the possibility that a hitherto unknown factor is at work cannot be ruled out.
Still, the team hopes their work can help inform future health policies and treatments, and allow for “more curated and tailored climate services and early warning systems for COVID-19.” They say measures such as the curfew can be evaluated in terms of limiting climate impacts on the disease, and most importantly, the team says their method will continue even if the disease “becomes endemic, and therefore defines the annual schedule for the vaccine.”
“The question of whether COVID-19 is a true seasonal disease is becoming more and more central, with implications for determining effective response measures,” Rodó says. It supports the view that there is a true seasonal low-temperature infection similar to coronaviruses.”
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