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The mRNA vaccines for COVID-19 were not associated with appendicitis in a Danish population-based study.

Among over 4 million vaccinated individuals, those who received the Pfizer-BioNTech or Moderna vaccines did not have any significant risk of developing appendicitis within 21 days after the first (adjusted risk ratio [aRR] 0.93, 95% CI 0.79-1.11) or second dose (aRR 0.99, 95% CI 0.84-1.18) compared with those who remained unvaccinated.

And the "null association" remained stable regardless of gender, age, and which mRNA vaccine was received, reported Helene Kildegaard, MD, of the University of Southern Denmark in Odense, and colleagues in a research letter.

Risk of appendicitis also wasn't significant within 21 days of infection with SARS-CoV-2, compared with the unvaccinated reference group (aRR 1.25, 95% CI 0.79-1.99).

A safety signal for appendicitis was first seen in Pfizer-BioNTech's phase III trial, where a higher number of cases were observed in the vaccinated, and real-world data from Israel showed an "excess risk of appendicitis of 5.0 episodes per 100,000 individuals after vaccination," Kildegaard's group noted.

"However, an found no association," the group wrote.

For this study, Kildegaard and colleagues examined national Danish registry data on 4,048,883 participants who received COVID-19 mRNA vaccination and individuals who had a PCR-confirmed SARS-CoV-2 infection during Dec. 27, 2020, to Nov. 30, 2021. Most received the Pfizer-BioNTech vaccine for both doses.

The unvaccinated reference group comprised the 89% of the Danish population who were ages 12 and up and had also been tested for SARS-CoV-2 infection. Those who had a previous case of appendicitis, an appendectomy, or received non-mRNA vaccination for COVID-19 were excluded.

The main outcome was based on a composite of having "a first ever hospital contact" with a diagnostic code for appendicitis or an appendectomy.

Propensity score-morbidity weighting was used to adjust for factors including demographics, municipality, history of diabetes or inflammatory bowel disease, and antibiotic prescription refills during the last 2 years.

Half of the cohort was female, and the median age was 49 to 51 years. The 159,115 (4%) infected with SARS-CoV-2 tended to be younger, with a median age of 31 to 32.

Within 21 days of follow-up, 330 episodes of appendicitis occurred out of the 4,048,883 individuals who received the first dose (8.1 episodes per 100,000 vaccinated individuals). The rate of appendicitis after the second vaccine dose was slightly greater, with 340 cases among the 3,944,408 individuals who received it (8.6 episodes per 100,000 vaccinated individuals).

"Further studies from different settings will be needed to fully eliminate appendicitis as an mRNA COVID-19 vaccination safety concern," they concluded.

Limitations of the study included the use of registry data and its accuracy in identifying appendicitis cases, the authors acknowledged. Long-term risks of appendicitis, beyond 21 days, were not reported.

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