Pharmaceutical giant Moderna confirmed Friday that 764,000 doses of its notable two-dose COVID-19 vaccine will be revoked In Europe after a container was found contaminated, Reuters first reported.

The vaccine manufacturer said that only one container was found tainted and that the revocation was merely a safety precaution. Safety concerns were not identified, nor was it specified what exactly was found in the container. Other containers were likely not affected by the contamination, the company clarified.

The Centers for Disease Control announced in February that Moderna’s vaccine was associated with a slightly higher risk of heart inflammation than Pfizer’s, while still stressing that its benefits far outweigh its risks. In Japan, officials halted doses of its vaccine due to the discovery of stainless steel in some of its containers. Moderna later rescinded the shots.

Pfizer and Moderna’s two-dose mRNA shots have proven to be quite successful in preventing severe symptoms from the coronavirus, although various studies that have been published show that their efficacy diminishes after a few months. ​​In a study published last month by the CDC, those who received Johnson & Johnson’s single-dose vaccine were advised to get a booster with one of Pfizer or Moderna's mRNA vaccines.

The Food and Drug Administration believes many vaccinated individuals will need to get boosted again this fall. “We could basically wait until another wave comes along and then try to make another vaccine and try to deal with it,” FDA Center for Biologics Evaluation and Research director Dr. Peter Marks said. “I think we know that doesn't work, because we can't manufacture and deploy vaccine fast enough."