Despite low absolute risk, there is an increased relative risk of carditis -- a type of heart inflammation -- associated with Covid vaccine developed by US drugmaker Pfizer and China's CoronaVac, researchers have found.
The findings, published in Annals of Internal Medicine, show a significant increased risk in adolescents after the second dose.
Patients who received Pfizer were three times more likely to experience carditis than unvaccinated patients.
On the other hand, patients who received CoronaVac had a similar chance as unvaccinated patients to experience carditis.
Researchers from the University of Hong Kong also observed that risk increase associated with Pfizer's Covid vaccine was predominant in males and was more likely to be seen after the second dose.
"The vaccination strategies may need to continuously consider the risks and benefits for different sub-populations, rather than taking a aone-size-fits-all' approach," they suggested.
Carditis is a rare inflammation of the heart often caused by bacterial, viral, and parasitic infections. Common subtypes of carditis include myocarditis, an inflammation of the heart muscle, and pericarditis, an inflammation of the outer lining of the heart.
Case reports of carditis after PfizerA vaccine vaccination have accrued globally.
The team studied 160 case patients (with carditis) and 1,533 control patients (without carditis) to examine the potential risk of carditis associated with vaccination with Pfizer vaccine or CoronaVac.
After conducting analyses, the team found 20 cases of carditis associated with Pfizer and seven associated with CoronaVac vaccination.
Cumulative incidence of carditis after vaccination was 0.57 per 100,000 doses of Pfizer and 0.31 per 100,000 doses of CoronaVac, demonstrating a very low absolute risk of carditis after vaccination.
However, none of the 20 case patients with carditis after Pfizer vaccination were admitted to the ICU or died within the observation period, compared with 14 of 133 unvaccinated patients admitted to the ICU and 12 deaths, the researchers said.