Britain's health officials have advised that people with a history of serious allergies not get the Pfizer-BioNTech COVID-19 vaccine. The warning came after two people reported severe reactions on the first day of vaccination.

National Health Service (NHS) medical director Stephen Powis said two NHS workers with a history of allergies were affected. "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," he said.

Powis noted that "both are recovering well" and the Medical and Healthcare Products Regulatory Agency (MHRA) is looking to see if the reactions are linked to the vaccine.

An allergy is a medical condition that causes someone to become sick after eating, touching, or breathing something that is harmless to most people.

For now, MHRA said, "Any person with a history of a significant allergic reaction to a vaccine, medicine or food... should not receive the Pfizer BioNtech vaccine."

The health agency said it would seek further information. Pfizer and BioNTech said they are working with investigators "to better understand each case and its causes.″

Last week, Britain became the first nation in the world to approve the Pfizer-BioNTech vaccine for emergency use. It started its vaccination program yesterday (Tuesday).

Today (Wednesday), the Canadian health agency approved the vaccine for emergency use. And the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) are looking at the data and are expected to approve the vaccine for emergency use shortly.

People with severe allergies excluded from trials

Documents published by Pfizer and BioNTech showed that people with a history of severe allergic reactions were excluded from the trials. The drug-makers also advised doctors to look out for such reactions in trial participants who were not previously known to have severe allergies.

In the United States, the FDA released documents on Tuesday in preparation for an advisory committee meeting on Thursday. The documents say the Pfizer vaccine's efficacy and safety data met expectations for emergency approval and only 0.63 percent of people in the vaccine group reported possible allergic reactions in trials.

Peter Openshaw is a professor of experimental medicine at Imperial College London. He said that was a very small number. "The fact that we know so soon about these two allergic reactions and that the regulator has acted on this to issue precautionary advice shows that this monitoring system is working well," he added.

Sinopharm vaccine has 86 percent efficacy

For several months, Western drug-makers like Pfizer, Moderna and AstraZeneca have released and published information on the vaccine trials. But China and Russia have not released any information on their COVID-19 vaccine candidates.

On Tuesday, the health ministry of the United Arab Emirates (UAE) said in a statement that it found a vaccine developed by China National Pharmaceutical Group, or Sinopharm, to have an 86 percent rate of effectiveness against COVID-19.

UAE said it started Phase 3 trials of the Sinopharm vaccine in July. And the country, home to the well-known cities of Dubai and Abu Dhabi, approved emergency use of the vaccine for some groups.

The health ministry said its study of the vaccine "shows no serious safety concerns." It also said that 31,000 volunteers across 125 nationalities participated in the UAE trial. The volunteers were between 18 and 60 years old and received two shots of the vaccine over 28 days.

The ministry did not report how many volunteers were given the vaccine or a placebo, a shot with an inactive substance. It also did not say if any side effects were identified or how many volunteers became ill.

It was unclear if the announced results included only those taking part in the UAE trials or if they also included results from China and other countries. Both UAE health officials and Sinopharm did not answer requests for comments from international news agencies.

The Sinopharm vaccine uses an inactivated virus to help the body produce antibodies to fight the coronavirus. It is similar to how polio vaccines are made.

The drug-maker is still conducting trials in China and countries like Egypt and Peru. However, the Chinese government has approved the vaccine for emergency use. And almost one million government officials, health care workers and others in China have been injected with the shots.

After the UAE vaccine announcement, city officials in Abu Dhabi said it would restart "all economic, tourism, cultural and entertainment activities in the emirate within two weeks."

I'm Jonathan Evans.

Hai Do adapted this story for Learning English with reporting from the Associated Press and Reuters. Susan Shand was the editor.

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allergy - n. a medical condition that causes someone to become sick after eating, touching, or breathing something that is harmless to most people.

significant - adj. large enough to have an effect or to be noticed

participant - n. a person who is involved in an activity or event

regulator - n. government official who controls a public activity by making and enforcing rules

monitor - v. to watch, observe, listen to and check something for a purpose over a period of time