The European Medicines Agency (EMA) is recommending Novavax's Covid-19 vaccine carry a warning of two inflammatory conditions of the heart, based on a small number of reported cases.
The conditions - myocarditis and pericarditis - should be listed as new side effects in the product information for the vaccine, Nuvaxovid, the EMA said on Wednesday.
The agency has also requested that the US drugmaker provide additional data on the risk of these side effects occurring.
Last month, the EMA 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 - which are based on the groundbreaking messenger RNA (mRNA) technology - 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 around 250,000 doses of Nuvaxovid have been administered in Europe so far since its launch in December, according to the European Centre for Disease Prevention and Control.