LONDON (AP) — U.K. regulators have had two reports of possible allergic reactions from people who took part in the first day of Britain's mass coronavirus vaccination program against COVID-19.

Dr. June Raine, head of the U.K.'s medical regulatory agency, reported those reactions as she testified Wednesday to a Parliamentary committee. The U.K. began vaccinating elderly people and medical workers with a vaccine developed by American drugmaker Pfizer and Germany's BioNTech on Tuesday, the world's first rollout of the vaccine.

“We're looking at two case reports of allergic reactions,’’ she said. “We know from the very extensive clinical trials that this wasn’t a feature.”

“But If we need to strengthen our advice, now that we have had this experience with the vulnerable populations, the groups who have been selected as a priority, we get that advice to the field immediately," she said.

Raine’s comments came as part of a general discussion of how her agency will continue to monitor people who receive the Pfizer vaccine, which waas authorized for emergency use last week.