LONDON (NewsNation Now) — U.K. regulators are warning those with a “significant history” of allergic reactions to avoid the new Pfizer COVID-19 vaccine developed with German firm BioNTech, as the country investigates two adverse reactions.

Britain issued the warning Wednesday, a day after the rollout of its mass vaccination program. The adverse reactions occurred on the first day, officials said.

The United Kingdom’s program marks the start of an unprecedented global immunization effort to help quell a pandemic that has killed more than 1.5 million people around the world.

Britain’s Medical and Healthcare Products Regulatory Agency has said people should not receive the shot if they have had a significant allergic reaction to a vaccine, medicine or food, such as those who have been told to carry an adrenaline shot — such as an EpiPen or other similar devices — or others who have had potentially fatal allergic reactions. The medical regulator also said vaccinations should be carried out only in facilities that have resuscitation equipment.

Such advice isn’t uncommon; several vaccines already on the market carry warnings about allergic reactions, and doctors know to watch for them when people who’ve had reactions to drugs or vaccines in the past are given new products.

The two people who reported reactions were NHS staff members who had a history of significant allergies and carried adrenaline shots. Both had serious reactions but recovered after treatment, the NHS said.

Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said the regulator had done the right thing, but the general public shouldn’t be worried about getting the vaccine.

“For the general population, this does not mean that they would need to be anxious about receiving the vaccination. One has to remember that even things like Marmite can cause unexpected severe allergic reactions,” he said, referring to the food spread that is made from brewer’s yeast.

Detailed data from the vaccine’s trials showed potential allergic reactions in 0.63% of those who received the vaccine, compared with 0.51% of those who received the placebo. Reviewers from the U.S. Food and Drug Administration called this a “slight numerical imbalance.”

Professor Stephen Powis, national medical director for the National Health Service in England, said health authorities Wednesday were acting on a recommendation from the Medical and Healthcare Products Regulatory Agency.

“As is common with new vaccines the MHRA have advised on a precautionary basis that people with a significant history of allergic reactions do not receive this vaccination after two people with a history of significant allergic reactions responded adversely yesterday,’’ Powis said in a statement. “Both are recovering well.”

The MHRA said the advice to health care professionals was “precautionary.”

The comments came as Dr. June Raine, head of the MHRA, told a Parliamentary committee that regulators had received reports of two allergic reactions from the vaccine.

“We know from the very extensive clinical trials that this wasn’t a feature,” she said. “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.

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

Pfizer and BioNTech weren’t immediately available for comment.

All reporting contributed by the Associated Press and Reuters.