The United States is reviewing the need for a third Covid-19 booster shot among residents who have already been vaccinated but needs to see more data to know if additional shots could raise people's risk of serious side effects, a US health official has said.

The official said the second dose for two-shot Covid-19 vaccine regimens was associated with higher rates of side effects, suggesting a third dose could potentially come with even greater risks.

“We're keenly interested in knowing whether or not a third dose may be associated with any higher risk of adverse reactions, particularly some of those more severe — although very rare — side effects,” said Jay Butler, deputy director at the US Centres for Disease Control and Prevention, during a media briefing.