An international team of scientists has found a correlation between high levels of airborne pollen and COVID-19 infection rates, according to a study published in the US scientific journal Proceedings of the National Academy of Sciences (PNAS).

"When we have pollen counts higher than 250 pollen grains per cubic meter of air over several days, that is a large pollen peak," Stefanie Gilles, a study co-author from the Technical University of Munich, said at a press conference, and this could affect infection rates. The scientists found this was the case for people with or without pollen allergies.

While the scientists observed that COVID-19 infection rates increased when pollen levels were higher, they do not yet know if high levels of pollen alone cause increased COVID-19 infection risk. Health professionals have responded to the study, saying many factors can influence infection rates.

The Ärzteverband Deutscher Allergologen (AeDA), a medical association of German allergists, said in a press release that although the study used the best data available, exact correlations between cause and effect can never be clarified in such an epidemiological study.

"The study says that environmental factors, including pollen, may be responsible for part of the change in infection numbers in spring 2020," Karl-Christian Bergmann, an AeDA member and allergist at the Charité University Hospital in Berlin, told DW, quoting the study authors' fact check. "From that, you can already see how cautious the formulation is here."

What the study found

From January to April 8, 2020, the international research team collected data from 130 pollen stations in 31 countries. They then compared this pollen data with weather data and the number of infections from around the start of March to early April, when the first coronavirus wave first hit Europe and the United States. Factors like population density and lockdowns were considered in the study.

Of the 31 countries involved in the study, the majority of which were in Europe, 14 were found to have a positive and significant association between high pollen levels and COVID-19 infections. Seven countries didn't have enough pollen to be studied for the pollen effect.

The pollen effect alone could increase the rate of COVID-19 infections by up to 20%, and this increased to 44% when combined with other environmental factors like temperature and humidity, Gilles said.

Lockdown was found to halve infection rates under similar pollen levels, according to the study, but this is also influenced by other factors, such as people having less contact during this time.

Without lockdown, an increase of 100 pollen per cubic meter correlated with a 4% average increase of infection rates, according to the study.

"We show by this study that not only personal risk factors are important but also environmental factors have to be taken into consideration," said Traidl-Hoffmann.

Pollen alone is not able to cause a COVD-19 infection, Traidl-Hoffmann stressed, "but airborne pollen can pave the way for viral infections."

Are additional protective measures needed?

If you go outside on a high pollen day you are at an increased risk for the following three days, said Claudia Traidl-Hoffmann, director of the Institute for Environmental Medicine at Helmholtz Zentrum in Munich.

Gilles said that previous research had shown that the interferon response, a part of the immune response,  can generally be weaker in people with allergies and asthma compared to healthy individuals, but there is no direct evidence that COVID-19 is more frequent in people with these health conditions, especially during the pollen season. 

During the high-pollen season, people in COVID-19 risk groups should keep themselves informed about pollen levels, adapt their outdoor activities and wear particle filtering masks, the scientists said. 

But the Charité's Bergmann said that until more research and clearer results were available, there was no justification for additional protective measures. "Everyone can continue to go outside and can run, can go for a walk, whether allergy sufferers or not. Nobody has to stay inside now," he said.

Pollen may weaken immune system

The scientists hypothesize that when the coronavirus particles enter the body with pollen grains, the pollen interferes with the body’s immune response, allowing the virus to replicate more easily.

This line of thought follows a study co-authored by Gilles in 2020, which found that when there was a lot of pollen in the air and it was breathed in, it weakened the airways’ immune response to respiratory viruses that cause the common cold, such as rhinoviruses.

When these viruses are breathed in, a cell infected with the virus will send out signaling proteins called interferons, which tell other nearby cells to put up their defense mechanisms.

But if pollen grains are present when the virus particles enter the body, the pollen releases substances that directly interfere with the interferon immune response of the cells, allowing the virus to replicate more freely and spread to neighboring cells, said Gilles. 

The scientists are planning to do a similar study for SARS-CoV-2 to determine whether the same weakening of the immune system occurs when pollen is present with the coronavirus.

Should people with allergies be concerned?

While the scientists do not exactly know which of the pollen’s substances is responsible for weakening the immune system, it is different to ones that cause allergies, said Gilles.

This means medication like antihistamines would likely not be recommended to counter the pollen's effect on the immune response.

"To speculate, I would say antihistamines and allergies, that's a different story," said Gilles. "It doesn’t have [anything] to do with COVID-19 and with the interferon response, at least based on our current knowledge."

The scientists' end goal is to develop a smartphone application that would alert people when there is pollen in the air that might affect them.