March 10, 2021 -- Scientists trying to understand the recurring waves of coronavirus infections around the globe say they've noticed a pattern: As pollen levels increased in outdoor air in 31 countries, COVID-19 cases accelerated.
Yet other recent studies point in the opposite direction, suggesting that peaks in pollen seasons coincide with a fall-off in the spread of some respiratory viruses, like COVID-19 and influenza. There's even some evidence that pollen may compete with the virus that causes COVID-19 and may help prevent infection.
So which is it? The answer may still be up in the air.
Doctors don't fully understand what makes some viruses — like the ones that cause the flu — to circulate in seasonal patterns.
There are, of course, many theories. These revolve around things like temperature and humidity — viruses tend to prefer colder, drier air — something that's thought to help them spread more easily in the winter months. People are exposed to less sunlight during the winter, as they spend more time indoors, and the earth points away from the sun, providing some natural shielding. That may play a role because ultraviolet light from the sun acts like a natural disinfectant and may help keep circulating viral levels down, and sunlight helps the body make vitamin D, which may help keep our immune responses strong.
Extreme temperatures — both cold and hot — also change our behavior, so that we spend more time cloistered indoors, where we can more easily cough and sneeze on each other and generally swap more germs.
Spike in Pollen, Jump in Infections
The new study, published in the Proceedings of the National Academy of Sciences, adds a new variable to this mix — pollen. It relies on data from 248 airborne-pollen-monitoring sites in 31 countries. The study also took into account other effects, such as population density, temperature, humidity, and lockdown orders. The study authors found that when pollen in an area spiked, so did infections, after an average lag of about 4 days. The study authors say pollen seemed to account for, on average, 44% of the infection rate difference between countries.
They say pollen could be a culprit in respiratory infections, not because the viruses hitch a ride on pollen grains and travel into our mouth, eyes, and nose, but because pollen seems to perturb our immune system, even if a person isn't allergic to it.
"When we inhale pollen, they end up on our nasal mucosa and here, they diminish the expression of genes that are important for the defense against airborne viruses," study author Stefanie Gilles, PhD, chair of environmental medicine at the Technical University of Munich in Germany, said in a press conference.
In a study published last year, Gilles found that mice exposed to pollen made less interferon and other protective chemical signals to the immune system. Those then infected with a respiratory virus had more virus in their bodies compared with mice not exposed to pollen. She seemed to see the same effect in human volunteers.
The study authors think pollen may cause the body to drop its defenses against the airborne virus that causes COVID-19, too.
"If you're in a crowded room and other people are there that are asymptomatic, and you've just been breathing in pollen all day long, chances are that you're going to be more susceptible to the virus," says study author Lewis Ziska, PhD, a plant physiologist who studies pollen, climate change, and health at Columbia University's Mailman School of Public Health in New York City. "Having a mask is obviously really critical in that regard."
Masks do a great job of blocking pollen, so wearing one is even more important when pollen and viruses are floating around, he says.
Other researchers, however, say that, while the study raises some interesting questions, it can't prove that pollen is increasing COVID-19 infections.
"Just because two things happen at the same time doesn't mean that one causes the other," says Martijn Hoogeveen, PhD, a professor of technical sciences and environment at The Open University in the Netherlands.
Hoogeveen's recent study, published in Science of the Total Environment, found that the arrival of pollen season in the Netherlands coincides with the end of flu season, and that COVID-19 infection peaks tend to follow a similar pattern — exactly the opposite of the PNAS study.
Another preprint study, which focused on the Chicago area, found the same thing — as pollen climbs, flu cases drop. The researchers behind that study think pollen may actually compete with viruses in our airways, helping to block them from infecting our cells.
Patterns May Be Hard to Nail Down
Why did these studies reach such different conclusions?
Hoogeveen's paper focused on a single country and looked at the incidence of flu infections over 4 seasons, from 2016 to 2020, while the PNAS study collected data on pollen from January through 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 meaningful patterns, especially considering that this new-to-humans virus was spreading quickly at nearly the same time. He says it will be interesting to follow what happens with COVID-19 infections and pollen in the coming months and years.
Hoogeveen says that in a large study spanning so many countries it would have been nearly impossible to account for differences in pandemic control strategies. Some countries embraced the use of masks, stay-at-home orders, and social distancing, for example, while others took less-stringent measures in order to let the virus run its course in pursuit of herd immunity.
Limiting the study area to a single country or city, he says, helps researchers better understand all the variables that might have been in play along with pollen.
"There is no scientific consensus yet, about what it is driving, and that's what makes it such an interesting field," he says.