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Pre-existing asthma and rhinitis were associated with an increased risk of long-term COVID symptoms in a meta-analysis, though the link was questionable.

Low-certainly evidence indicated that asthma was associated with an increased risk of developing long COVID in hospital-based populations (OR 1.94, 95% CI 1.08-3.50), reported Christian Apfelbacher, PhD, of the University of Magdeburg in Germany, and coauthors.

Similarly, pre-existing rhinitis was also associated with a greater risk of long COVID development (OR 1.96, 95% CI 1.61-2.39).

"The evidence for these associations is very uncertain, therefore more robust epidemiological research is needed," authors of the systematic review concluded in .

One issue to overcome, and a subject of much debate, is how to specifically define long COVID -- often the development of symptoms such as dyspnea and fatigue after recovery from acute COVID illness. These symptoms can persist in patients for upwards of a year, appearing in patients regardless of the severity of their initial COVID-19 infection.

"We need a better, harmonized definition of what is considered long COVID for epidemiological studies of this sort. Regardless, we will be updating our analysis once further studies have been published in the next few months," said Apfelbacher in a .

Fernando Carnavali, MD, of Mount Sinai in New York City, also remarked on the lack of a universal definition of long COVID and urged caution when interpreting the present study results.

"From the long-COVID perspective, we welcome any efforts to try to have a better understanding of what the risk factors are," he told MedPage Today, but noted that in reality the evidence is of low certainty. "We need more data."

Most likely, Carnavali said, multiple risk factors and mechanisms are at play to account for the different symptoms associated with long COVID.

Sayantani Sindher, MD, of Stanford Medicine Healthcare in Palo Alto, California, commented that symptom overlap between long COVID and allergic conditions -- such as difficulty breathing, coughing, and symptoms that worsen after exercise -- makes analysis all the more difficult.

Sindher told MedPage Today via email that research indicating stronger associations between allergic conditions and long-COVID risk would be needed before changes can be made to clinical practices.

Based on a survey, long COVID was to impact approximately 6% of all U.S. adults or 11% of people infected with COVID-19.

Apfelbacher's team explained that there are several mechanisms that could explain the possible association between allergic conditions and long COVID. One hypothesis is that the type-2 T-helper lymphocyte allergic immune response and elevated eosinophils observed to be protective against severe acute infection may actually increase the risk of long COVID.

Sindher also proposed another pathway, stating that "it is possible that the respiratory system in long-COVID individuals recovers slower than normal due to the frequent burdens placed upon it from asthma or allergic rhinitis. This could lower the threshold upon which symptoms are noticeable."

For their meta-analysis, Apfelbacher and colleagues pooled 13 prospective studies with a total of 9,967 participants who had confirmed SARS-CoV-2 infection. The proportion of women ranged from 18.4% to 66.1%, while the median long COVID prevalence was at 53.3%.

Four patient subsets were included in the meta-analysis -- pre-existing asthma in hospital-based populations, asthma in the general population, allergic rhinitis, and allergies.

Risk of bias for the studies included was ultimately determined to be high, largely due to the use of highly selective populations, such as previously hospitalized patients, and a high loss to follow-up.

Limitations included that four of the studies used in the analysis lacked sufficient data to calculate odds ratios, and that studies were analyzed even if there was no differentiation between asthma and allergic asthma.

Apfelbacher and colleagues stressed the importance of further research to understand long COVID and any factors that may impact development of the condition.

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