Multiple countries across Europe and parts of Asia suspended the use of AstraZeneca’s Covid-19 vaccine over fears the medication is leading to the development of blood clots in some patients who have received the medication.

The United States must stick to a two-dose strategy for the Pfizer-BioNTech and Moderna Covid-19 vaccines, top U.S. infectious disease official Anthony Fauci told the Washington Post newspaper.

Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), noted that it would take time – possibly months – before people 18 years of age and younger could get a Covid-19 vaccine because clinical trials were either underway or had not yet begun.

The U.S. Food and Drug Administration is expected to issue a new and more stringent guidance regarding a potential Emergency Use Authorization (EUA) for a Covid–19 vaccine.