US Centers for Disease Control and Prevention says that six people have developed severe allergic reactions to the Pfizer coronavirus vaccine.
The CDC held an experts meeting on the vaccination on Saturday.
It says that more than 270,000 people in the country have received the first vaccine shot jointly developed by Pfizer and BioNTech between Monday and Saturday.
It also says that it received a report that six people who had been given the shot by Friday showed anaphylactic reactions.
Other than the one who had a history of vaccine allergic reactions, none of them had experienced such reactions in the past.
An official of the US Food and Drug Administration said on Friday that they suspect a chemical called polyethylene glycol could be the component causing the reaction.
Meanwhile the FDA approved the emergency use of another vaccine developed by Moderna and a CDC advisory panel endorsed its use in people aged 18 and older.