TUESDAY, March 9, 2021 (HealthDay News) -- It's that time of year when flowers and trees bloom freely and pollen makes the lives of many miserable. But new research reveals a hidden risk: It could also make you more vulnerable to COVID-19 infection.
COVID-19 infection rates waxed and waned with pollen counts in 2020, according to tracking data gathered across 31 countries in every corner of the globe.
"Airborne pollen can pave the way for viral infections. This is true for allergics and non-allergics," senior researcher Claudia Traidl-Hoffmann, a professor of environmental medicine with the Technical University of Munich in Germany, said during a media briefing Tuesday.
Pollen, sometimes influenced by humidity and temperature, explained 44% of the variability in COVID-19 infection rates between different regions of a country, on average, according to the report published online recently in the Proceedings of the National Academy of Sciences.
The researchers also found that lockdowns during last year's pollen season halved daily infection rates.
"It didn't matter that much how strict the lockdown would be, but actually how early the lockdown would be adopted," said lead researcher Athanasios Damialis, an aerobiologist and chair of environmental medicine with the Technical University of Munich.
Without a lockdown, an increase of 100 pollen grains per cubic meter of air leads to a 4% average increase in COVID-19 infection rates, the study found.
In some German cities, concentrations of up to 500 pollen grains per cubic meter occurred, which led to an overall 20% increase in infection rates, the researchers said.
Why would high pollen counts increase your COVID-19 risk?
Pollen weakens your airway's natural defense against viral infection, an effect already observed with common cold viruses, explained study co-author Stefanie Gilles, a senior scientist with the Technical University of Munich.
Inhaled pollen sticks to the mucous membranes of your airways and secretes substances that hamper the activity of interferon, which are messenger proteins that trigger the body's immune defense, Gilles said.
If you inhale a coronavirus while pollen is dampening your immune response, the virus can replicate more freely and spread to neighboring cells, Gilles explained.
In spring 2020, the pandemic outbreak appeared to coincide with the March tree pollen season in the northern hemisphere, Gilles said.
"There was a stretch of very warm and very dry days all across the northern hemisphere," Gilles said. "Exactly around this period there were high COVID-19 infections, and also a lot of airborne pollen."
For this study, the German researchers gathered pollen counts and infection rates for 130 different regions of 31 countries on all five continents.
Data show that increases in infections happened after an area experienced a pollen count of 250 grains per cubic meter for four days in a row, Damialis said.
It wasn't just that COVID-19 infections increased when pollen counts went up, he noted.
"When we had low pollen, we had the lowest infection rates," Damialis said.
People should keep tabs on pollen counts in their area and wear masks that can filter out pollen particles to protect themselves against both pollen and the new coronavirus, Traidl-Hoffmann said.
However, infectious disease expert Dr. Amesh Adalja isn't convinced that pollen could be a main driver of COVID-19 infections.
"While it is true that pollen exposure in allergic individuals has been shown to down regulate production of interferon -- a molecule key to fighting off viruses like SARS-CoV-2 -- the study only suggests a mild impact of pollen on case counts," said Adalja, a senior scholar at the Johns Hopkins Center for Health Security, in Baltimore.
"This is a hypothesis that needs further study to determine what, if any, added role pollen may play in explaining what is a clearly complex multifactorial process," Adalja concluded.
The American Academy of Allergy, Asthma and Immunology has allergy-tracking data for the United States.
SOURCES: March 9, 2021, media briefing with: Claudia Traidl-Hoffmann, professor, environmental medicine, Technical University of Munich, Athanasios Damialis, aerobiologist and chair, environmental medicine, Technical University of Munich, and Stefanie Gilles, senior scientist, Technical University of Munich; Amesh Adalja, MD, senior scholar, Johns Hopkins Center for Health Security, Baltimore; Proceedings of the National Academy of Sciences, March 8, 2021, online