When hayfever season comes and your eyes water and nose runs, you can feel a small compensation for the suffering: There is a negative correlation between susceptibility to a range of allergies and the chance of catching COVID-19, at least in the UK population, according to one study. The reasons behind this result could turn out to be rather trivial, or tell us something important about the working of our immune systems that might help us combat allergies, infectious diseases, or both.

For many people, it's intuitive that if their immune system can be derailed by some pollen, a virus that has killed more than five million people is likely to be particularly threatening. However, an analysis of data on those infected in the UK between May 2020 and February 2021, published in the journal Thorax, reports the opposite, among some more expected findings.

Professor Adrian Marineau of Queen Mary University of London and co-authors tested a range of demographic factors, investigating their relationship with COVID-19 infection for over 15,000 UK residents, the majority of whom also responded to at least some of the monthly follow-ups. Almost 3 percent of those participating got infected during the period.

Some of the results were in line with other studies using different methods. Living in crowded houses, socializing indoors with those outside their household, being obese or of Asian ethnicity all carried higher chances of testing positive for COVID-19 over the period. Certain jobs were also risk factors.

In other ways, the study contradicted other research – for example, finding men were no more likely to contract the disease than women. Old age wasn't a risk factor for infection – the opposite in fact, even though it definitely is for death or hospitalization. “There is limited overlap between risk factors for developing COVID-19 versus those for intensive care unit admission and death as reported in hospitalised cohorts,” the paper notes.

The study also explored some factors others haven't looked into, including diet and supplement intake. Many of these didn't turn out to be significant either way. However, those who experience atopic diseases (triggered by allergens) such as eczema and hayfever were 23 percent less likely to test positive for COVID-19 during the period, after controlling for other factors. People who had both an atopic disease and asthma had a 38 percent lower risk than the rest of society.

As an observational study, this research can't explain the associations, only raise questions for others to explore. However, a clue may lie in the fact that people taking immunosuppressants actually had a 53 percent lower chance of catching COVID-19 than the national average. This seems even more counter-intuitive until you consider that people who not only have weakened immune systems, but know this to be the case, were probably a lot more careful to not get infected through the study period.

Whether through social distancing or more consistent mask-wearing, people who knew the virus was more likely to be a death sentence probably took a lot fewer risks. It's reasonable to assume that applied to people with asthma as well, and possibly also people with hayfever.

It's also possible, however, that something about an overly zealous immune system, often inherited from Neanderthal or Denisovan ancestors, makes for a stronger first line of defense against the virus.

“This may reflect decreased expression of ACE2, the gene encoding the SARS-CoV-2 receptor, which has been reported in people with both high levels of allergic sensitisation and asthma,” the paper notes. If so, it's not a big enough effect that people with eczema should decide they don't need vaccination, but it might be a useful pointer for future research.