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Some scientists say they have noticed a pattern of recurring waves of SARS-CoV-2 infections around the world: as pollen levels rose in outdoor air in 31 countries, COVID cases -19 accelerated.
However, other recent studies point in the opposite direction, suggesting that peaks at pollen stations coincide with a drop in the spread of some respiratory viruses, such as COVID-19 and influenza. There is even evidence that pollen can compete with the virus that causes COVID-19 and may even help prevent infection.
So what is it? The answer may still be in the air.
Doctors do not fully understand what causes some viruses, such as those that cause the flu, to circulate according to seasonal patterns.
There are, of course, many theories. These revolve around things like temperature and humidity (viruses usually prefer cooler, drier air), which is believed to help them spread more easily during the winter months. People are exposed to less sunlight during the winter as they spend more time indoors and the earth points to the sun, providing some natural protection. This can play a role both because ultraviolet sunlight acts as a natural disinfectant and can help keep the circulation of viral levels down.
In addition, exposure to sunlight helps the body produce vitamin D, which can help keep our immune responses strong. Extreme temperatures, both cold and hot, also change our behavior, so we spend more time indoors cloistered, where we can cough and sneeze more easily and generally exchange more germs.
Itch in pollen, jump in infections
The new study, published in Proceedings of the National Academy of Sciences, adds a new variable to this mixture: pollen. It is based on data from 248 airborne pollen control sites in 31 countries. The study also considered other effects, such as population density, temperature, humidity, and blocking orders. The study authors found that when pollen in an area increased, so did infections, after an average lag of about 4 days. The study authors say pollen appeared to account for, on average, 44% of the variability in infection rate between countries.
The study’s authors say pollen could be the culprit for respiratory infections, not because viruses attach to pollen grains and travel to the mouth, eyes and nose, but because the pollen appears to be They disrupt our immune defenses, even if a person is not allergic.
“When we inhale pollen, they end up in our nasal mucosa and here they decrease the expression of genes that are important for defense against airborne viruses,” said the study’s author, Stefanie Gilles, PhD, professor of Environmental Medicine from the Technical University of Munich in Germany, said at a press conference.
In a study published last year, Gilles found that mice exposed to pollen produced less interferon and other protective chemical signals to the immune system. Those infected with respiratory syncytial virus had more viruses in the body compared to mice not exposed to pollen. He seemed to see the same effect on human volunteers.
The study authors think that pollen can cause the body to drop its defenses against the airborne virus that also causes COVID-19.
“If you’re in a room full of people and there are other asymptomatic people and you just breathe pollen all day, you’re likely to be more susceptible to the virus,” says Lewis Ziska, PhD, a plant physiologist who studies pollen. Climate change and health at the Mailman School of Public Health at Columbia University in New York City. “Having a mask is obviously very critical in that regard.”
Masks do a great job blocking pollen, so wearing one is even more important when pollen and viruses float around, he says.
However, other researchers say that while the study raises some interesting questions, it cannot show that pollen increases COVID-19 infections.
“Just because two things happen at the same time doesn’t mean one causes the other,” says Martijn Hoogeveen, PhD, a professor of technical science and environment at the Open University in the Netherlands.
Recent study by Hoogeveen, published in Total Environmental Science, found that the arrival of the pollen season in the Netherlands coincides with the end of the flu season, and that COVID-19 infection peaks tend to follow a similar pattern, exactly the opposite of PNAS to study.
Another prepress study, which focused on the Chicago area, found the same thing: as pollen rises, flu cases fall. The researchers behind this study think that pollen can compete with viruses in our airways, helping to prevent them from infecting our cells.
Patterns can be difficult to nail
Why did these studies come to such different conclusions?
Hoogeveen’s article focused on a single country and examined the incidence of influenza infections for four seasons, from 2016 to 2020, while PNAS the study collected data on pollen from January to the first week of April 2020.
He thinks that a single season, or really part of a season, may not be long enough to see significant patterns, especially considering that this new virus for humans is spreading rapidly almost at the same time. He says it will be interesting to follow up on COVID-19 and pollen infections in the coming months and years.
Hoogeveen says that in a comprehensive study covering so many countries it would have been nearly impossible to explain the differences in pandemic control strategies. Some countries adopted the use of masks, orders to stay home, and social distancing, for example, while others adopted less stringent measures to let the virus continue its course in search of herd immunity. .
According to him, limiting the study area to a single country or city helps researchers better understand all the variables that could have been at stake along with pollen.
“There’s still no scientific consensus on what it’s driving, and that’s what makes it such an interesting field,” he says.
Sources
Lewis Ziska, plant physiologist studying pollen, climate change and health at the Mailman School of Public Health in Columbia, New York, New York
Martijn Hoogeveen, PhD, Professor, Technical Sciences and Environment, Open University, Heerlen, The Netherlands
Stefanie Gilles, PhD, Chair of Environmental Medicine, Technical University of Munich, Germany
PNAS, March 23, 2021
MedRxiv, February 16, 2021
MedRxiv, March 2, 2021
Science of the Total Environment, February 10, 20201
News, Technical University of Munich, March 9, 2021
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