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The monovalent Moderna COVID-19 booster vaccine may be associated with an elevated risk for new-onset chronic spontaneous urticaria (CSU), or hives lasting more than 6 weeks, according to a Swiss study.

Among people who got an mRNA COVID vaccine booster and had new-onset hives reported to local allergists, 90% had vaccination precede CSU in the canton of Vaud during the study period, as did 81% of patients in the nationwide cohort, reported Yannick Daniel Muller, MD, PhD, of the University of Lausanne in Switzerland, and coauthors.

Crude incidence rates of CSU following a booster dose in the two cohorts was 1.9-2.1 per 100,000 doses of Pfizer's vaccine and 30.8-43.9 per 100,000 doses of Moderna's product, according to their research letter in .

This difference translated to a relative risk with the Moderna booster dose of 20.8 (95% CI 6.5-66.0) for the Vaud cohort of 80 people, and 16.1 (95% 10.8-24.0) in the Swiss cohort of 782 individuals.

"These data should not discourage patients from being vaccinated," Muller's group concluded. "However, guidelines defining the eligibility and dosing for upcoming mRNA-based boosters are needed for patients with CSU after an mRNA-based COVID-19 vaccine."

The reason behind the possible association are unclear, and warrant further study, according to the study authors.

"As a potential contributing mechanism warranting further investigations, our group previously showed that the Moderna vaccine had a greater association with positive skin and basophil activation tests results compared with the Pfizer-BioNTech vaccine," wrote Muller and coauthors.

"Alternatively, with the Moderna vaccines containing a higher dose of mRNA and being more immunogenic than the Pfizer-BioNTech vaccine, one could speculate that the booster non-specifically triggered CSU in predisposed individuals," they added.

Notably, the current bivalent boosters from Moderna and Pfizer were introduced in late 2022, after the conduct of the present study. After their introduction in the U.S., the original monovalent products were phased out for booster doses.

Just last week, the Vaccines and Related Biological Products Advisory Committee unanimously endorsed the FDA's plan to harmonize COVID-19 primary and booster vaccines to contain a bivalent composition with components targeting BA.4/5 and the original SARS-CoV-2 strain, meaning the original monovalent products will be eliminated from the market altogether going forward.

For their study, the researchers estimated the incidence of vaccine-related CSU locally in Vaud and nationally across Switzerland, where 3,278,808 and 298,813 individuals, respectively, had received a COVID mRNA booster dose from Dec. 1, 2021 to Aug. 31, 2022.

Eligible patients were identified by local allergists and sent an online questionnaire from the Lausanne University Hospital from April to August 2022.

The participating Vaud cohort was 70% women, with an average age of 41 years. The national cohort was 58% women and their average age was 39 years. In Vaud, 14% of participants reported previous experiences of urticaria and 95% reported taking antihistamines, the majority daily.

Limitations of the study include the lack of available Swiss data regarding CSU incidence within the general population. Moreover, confounding factors tied to the Omicron variant of COVID-19 could affect the findings, as 31% of CSU-Vaud cohort members reported infection. Also, incidence rates could not be adjusted, as individual data on vaccine brand tied to age and sex were not unavailable.

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