Now, a new population-based study from the National Institutes of Health (NIH) has found evidence of a curious coronavirus advantage for those with allergies.
In an analysis of more than 4,000 people who all lived in households that included minors, researchers noted several curious trends in terms of SARS-CoV-2 infection, including that individuals with a food allergy were only about half as likely to become infected.
Somewhat similarly, the new NIH study found that asthma was not linked to increased risk of SARS-CoV-2 infection, despite asthma being a condition that impacts the respiratory system.
On the other hand, obesity and a high BMI index were factors that increased risk of SARS-CoV-2 infection, as was the age of children and teens sharing the living space.
But the finding with regard to food allergies might be the most remarkable discovery.
"[T]he observed association between food allergy and the risk of infection with SARS-CoV-2, as well as between body-mass index and this risk, merit further investigation," says Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases.
Researchers aren't sure why food allergies seem to make people less vulnerable to SARS-CoV-2, but there are a few possible explanations.
Half of all the participants in the study claimed they had been diagnosed with a food allergy, asthma, eczema, or allergic rhinitis. These self-reports were backed up by a subset of blood tests, which revealed antibodies linked to allergic disease.
Researchers then tracked the spread of SARS-CoV-2 in participant households from May 2020 to February 2021.
People with eczema and asthma didn't show extra vulnerability to the virus, but they also didn't seem to be any more protected.
Those with food allergies, meanwhile, were at a 50 percent lower risk of SARS-CoV-2 infection.
Not all forms of asthma are atopic (aka highly allergic), and previous studies have shown that only those with atopic asthma express lower airway levels of the ACE2 receptor, which is what SARS-CoV-2 attaches to.
This suggests that the virus does not have as many ways to invade cells in the lungs of those with respiratory allergies.
Something similar could be occurring among people with food allergies, although the authors only looked at SARS-CoV-2 infection, and not the severity of the infection.
"It is not known whether this is also the case in food allergic individuals, but it is tempting to speculate that type 2 inflammation, a characteristic of food allergy, may reduce airway ACE2 levels and thus the risk of infection," the researchers write.
"Supporting this possibility, we found significantly greater levels of general atopy among those with self-reported food allergy, relative to both those without food allergy, and even those with asthma."
Interestingly, while some studies suggest allergic asthma protects from severe cases of COVID-19, the current study found the condition does not protect from the initial contraction of the virus.
What's more, when a participant with asthma or food allergies did contract the novel coronavirus, they were no more likely to be asymptomatic.
Further research is needed to tease apart the mechanisms behind the new findings, but the authors are hopeful that their research can offer new avenues for COVID-19 prevention.
The study was published in the Journal of Allergy and Clinical Immunology.