The European Medicines Agency is recommending Novavax's COVID-19 vaccine carry a warning of the possibility of two types of heart inflammation, an added burden for a shot that has so far failed to win wide uptake.
The heart conditions - myocarditis and pericarditis - should be listed as new side effects in the product information for the vaccine, Nuvaxovid, based on a small number of reported cases, the EMA said on Wednesday.
Novavax said no concerns about heart inflammations were raised during the clinical trials of Nuvaxovid and that more data would be gathered, adding that the most common cause of myocarditis is viral infections.
"We will work with the relevant regulators to assure our product information is consistent with our common interpretation of the incoming data," US vaccine developer Novavax added.
In June, the US Food and Drug Administration flagged a risk of heart inflammation from the Novavax vaccine.
Myocarditis and pericarditis were previously identified as rare side effects, mostly seen in young men, from groundbreaking messenger RNA (mRNA) vaccines made by Moderna and the Pfizer and BioNTech alliance, with the vast majority of those affected recovering fully.
The EMA said on Wednesday it had asked Novavax to provide additional data on the risk of these side effects.
Last month, the European Union agency identified severe allergic reactions as potential side effects of the vaccine.
Novavax was hoping that people who have opted not to take Pfizer and Moderna's vaccines would favour its shot because it relies on technology that has been used for decades to combat diseases including hepatitis B and influenza.
However, only about 250,000 doses of Nuvaxovid have been administered across Europe since its launch in December, according to the European Centre for Disease Prevention and Control.