People with pre-existing allergic conditions are more vulnerable to experiencing long-term symptoms associated with COVID-19, or long COVID, according to a study.

Although the data as a whole from the studies suggested that individuals with asthma or rhinitis might be at increased risk of long-term COVID-19 after SARS-CoV-2 infection, the evidence for these associations was very uncertain.

Therefore, more robust epidemiological research is needed to clarify the role of allergy in the development of long COVID, said researchers.

The new findings, published in Clinical & Experimental Allergy, were based analysis of 13 studies, published between January 1, 2020 and January 19, 2023.

The analysis included a total of 9,967 participants of all ages with confirmed SARS-CoV-2 infection who were followed for at least 12 months.

Four of the 13 studies provided estimates on the association between pre-existing allergic conditions such as asthma or hay fever and long COVID.

Pre-existing asthma was associated with an increased risk of long COVID, but the evidence was very uncertain, the authors said.

The odds ratio (OR) for preexisting asthma and long COVID was 1.94.

In an analysis of three studies that included data on preexisting allergic rhinitis, the condition was significantly associated with an increased risk of long-term COVID-19.

Only 3 of the 13 studies measured long COVID with physical examinations and comprehensive questionnaires, the authors cautioned.

Moreover, among the 13 studies, the researchers found a high risk of bias due to patient selection (mostly hospitalised patients) and loss of follow-up.

"We need a better, harmonised 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 corresponding author Christian Apfelbacher, from the Institute of Social Medicine and Health Systems Research, in Germany.