As if allergies were not enough suffering induced by plants mating in our nostrils, a new paper provides evidence high pollen concentrations make a major contribution to COVID-19 transmission.
Confusion about how SARS-CoV-2, and by extension other coronaviruses, spread has been a major impediment to fighting the disease. There are obvious ethical (and practical) problems with putting infectious people in a room with the uninfected and watching how the virus jumps between them. Consequently, a lot of what we know comes from observing patterns of when and where transmission rates were high.
Dr Athanasios Damialis of the Technical University of Munich noted the first widespread cases of COVID-19 in Europe coincided with the seasonal tree pollen peak, and wondered if they were connected. Damialis compared the rate at which the disease has spread with records of pollen abundance for 130 sites in 31 countries. It's a difficult assessment to make, because we know there are so many other factors at play. Changes in social distancing practice or mask wearing can clearly overwhelm any contribution from pollen, and weather is almost certainly important as well. Then there are the still uncertain influences of factors like Vitamin D deficiency, and random, unpredictable superspreader events.
Nevertheless, Damialis and co-authors from half a dozen countries report in Proceedings of the National Academy of Sciences that pollen appears to be a major factor. Infection rates peaked when pollen concentrations were highest in the previous four days. Indeed, the authors write; “We found that pollen, sometimes in synergy with humidity and temperature, explained, on average, 44% of the infection rate variability.” Lockdowns halved the size of the effect, but did not eliminate it. The authors stress, however, the study only included data over a relatively short amount of time, and extending it should give a better sense of the true size of the effect.
Interestingly, the effect stood up even when types of pollen few people are allergic to were included.
Although the size of the effect the paper describes might be surprising, the authors note; “Coexposure to airborne pollen enhances susceptibility to respiratory viral infections, regardless of the allergy status.” That of course is in addition to the fact that pollen will make allergic people cough and sneeze even when COVID-19 isn't affecting them much, probably increase viral load in the air.
Using this information could prove tricky. “Wording should be extremely well considered to avoid misunderstandings and to not cause panic,” the paper notes. Equally, no one should be lulled into thinking pollen-free days are safe.
Nevertheless, if people take extra care when pollen levels are high, either by staying home or wearing masks, we might not only cut down on the spread of COVID-19, but future pandemics as well.
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