Transmission electron microscope image of SARS-CoV-2 virus particles emerging from the surface of a cell cultured in the lab. Image captured and colorized at Rocky Mountain Laboratories in Hamilton, Montana. Credit: NIAID

In results that surprised even the study authors, NIH-funded researchers have found people with food allergies are less likely to become infected with SARS-CoV-2 compared with people without allergies. Additionally, the study provides some of the strongest evidence to date that asthma is not a risk factor for SARS-CoV-2 infection, symptoms, higher viral loads, or transmission events.

Published in the Journal of Allergy and Clinical Immunology, the study included 4,142 participants in 1,394 households across 12 U.S. cities who contributed at least one nasal swab between May 15, 2020 to Feb. 1, 2021. Participants were recruited from existing, NIH-funded studies focused on allergic diseases. Thus, roughly half of the participants had self-reported food allergy, asthma, eczema or allergic rhinitis (hay fever).

Every two weeks, participants submitted nasal swabs to test for SARS-CoV-2 and filled out weekly surveys. If a member of the household developed symptoms consistent with COVID-19, additional nasal swabs were taken. Blood samples also were collected periodically and after a family’s first reported illness, if there was one. In total, 10.5% of the households experienced one or more SARS-CoV-2 infections.

During the analysis, researchers unexpectedly found that food allergy was associated with lower risk of SARS-CoV-2 infection and household transmission. In fact, participants—regardless of age—reporting food allergy were at 50% lower risk of infection. However, asthma and hay fever were not associated with reduced infection risk.

The investigators speculate that that type 2 inflammation, a characteristic of allergic conditions, may reduce levels of the ACE2 receptor on the surface of airway cells. SARS-CoV-2 uses this receptor to enter cells, so its scarcity could limit the virus’s ability to infect them.

“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,” the researchers write in their paper.

Meanwhile, the researchers found that participants with asthma, eczema, and upper respiratory allergy were not at increased risk for SARS-CoV-2 infection; nor were they more likely to have symptomatic infection or higher SARS-CoV-2 viral loads. Additionally, the investigators say infected households with asthmatic individuals were not at increased risk of transmission.

This rings true for obese asthmatic participants, as well. Contrary to previous studies, this HEROS (Human Epidemiology and Response to SARS-CoV-2) study showed obese, asthmatic individuals were not at an increased risk for SARS-CoV-2 infection.

However, the researchers found the opposite was true for non-asthmatic obese individuals. The HEROS study confirmed previous findings that indicate obesity is a risk factor, recording a strong, linear relationship between BMI and risk of infection. According to the results, every 10-point increase in BMI percentile raised the risk of infection by 9%. Participants who were overweight or obese had a 41% greater risk of infection than those who were not.

“Potential biological mechanisms underlying this effect include increased ACE2 expression in obese subjects, or neutrophilic airway inflammation also described in obese individuals, which has been associated with increased viral replication for several respiratory viruses,” write the investigators.

Overall, the researchers were surprised to rule out asthma as a risk factor for SARS-CoV-2 infection, but rule in food allergy. The study was conducted before any variants of concern emerged, so future research could focus on the effect of different strains in the HEROS study population.

“Different types of systemic and airway inflammation may contribute to the variable infection risk and understanding the mechanisms explaining these observations may offer new pathways for disease prevention,” conclude the investigators.

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