The U.K. says people with severe allergies should not take the Pfizer-BioNTech vaccine while two reactions are under investigation.

Thousands of Britons received the first clinically authorized, fully tested coronavirus vaccine on Tuesday, with people reporting minimal side effects.

But two health workers with a history of serious allergies had reactions after being given the vaccine, British drug regulators said on Wednesday. As they investigate what precisely caused the reactions, the regulators warned that people prone to severe allergic reactions should not receive the Pfizer-BioNTech vaccine for the time being.

Scientists said that for the vast majority of people, the new advice should not cause any concerns about receiving the coronavirus vaccine, and that it was difficult to protect against certain rare reactions with any new vaccine.

Dr. Anthony S. Fauci, the nation’s leading expert on infectious diseases, said on Wednesday that the allergic reactions were concerning but most likely rare, the kind of effects that show up when a vaccine moves out of testing and into broader distribution.

“If I were a person that had an underlying allergic tendency, I might want to be prepared that I might get a reaction and therefore be ready to treat it,” Dr. Fauci said, in a webcast moderated by Sanjay Gupta of CNN, sponsored by Harvard and The New England Journal of Medicine.

Dr. Fauci acknowledged that the problem could turn out to affect a lot of people.

“That’s one of the reasons why it’s important to cover the waterfront with different vaccine platforms,” he said, adding, “If in fact we do find out that there is a consistent issue of a certain subset of people like those with allergic reactions, you’ll always have other vaccine platforms that you can use and hopefully you will not see that with those other platforms.”

Because of their severe allergies, the two health workers carried adrenaline pens, the generic term for an EpiPen. The National Health Service said that both workers were recovering well after being treated for symptoms of anaphylactoid reactions that they developed shortly after their shots.

Regulators said the reactions were “associated with administration” of the vaccine, but did not describe the health workers’ symptoms or what kind of treatment they received.

“We know from the very extensive clinical trials that this wasn’t a feature,” Dr. June Raine, the chief executive of Britain’s Medicines and Healthcare products Regulatory Agency, said on Wednesday.

As a precaution, British regulators said, “any person with a history of a significant allergic reaction to a vaccine, medicine or food,” like people who have anaphylactoid reactions or carry an EpiPen, should not receive the Pfizer-BioNTech vaccine. They said that the vaccines should be delivered only at sites with access to resuscitation measures.

Dr. Moncef Slaoui, the head of the Trump administration’s Operation Warp Speed, said that a panel of experts meeting in the United States on Thursday to consider Pfizer’s application for emergency authorization would likely discuss the allergic reactions.

He said he thought that the United States would also advise people with a history of severe allergic reactions not to get the vaccine “until we understand exactly what happened” in the British cases.

After authorizing the Pfizer-BioNTech vaccine for emergency use last week, British regulators said that people with a hypersensitivity to ingredients in the vaccine should not be given the shot. But the regulators’ updated advice on Wednesday expanded that warning to cover anyone with a history of significant allergic reactions.

The regulators are now investigating the two cases from Tuesday, a process that experts said would involve looking at whether the reactions stemmed from the vaccine itself or were incidental. The regulators said they would release updated advice once they do.

Allergic reactions are a very rare response to some vaccines. Anaphylactic reactions, the most serious of those responses, are estimated to occur around once every 100,000 to 1,000,000 doses of the most commonly given vaccines, according to the Institute for Vaccine Safety at the Johns Hopkins Bloomberg School of Public Health.

A 2015 study supported by the Centers for Disease Control and Prevention in the United States turned up 33 confirmed cases of vaccine-triggered anaphylaxis among more than 25 million vaccine doses, translating to a rate of 1.3 cases of such reactions in every million doses of vaccine.

Pfizer’s vaccine trials, which included tens of thousands of participants, excluded people with a history of reacting badly to vaccines, or having severe allergic reactions to any ingredients in the coronavirus vaccine.

Among those who participated in the trials, a very small number of people had allergic reactions. A document published by the Food and Drug Administration in the United States on Tuesday said that 0.63 percent of participants who received the vaccine reported potential allergic reactions, compared to 0.51 percent of people who received a placebo.

In Pfizer’s late-stage clinical trial, one of the 18,801 participants who received the vaccine had an anaphylactic reaction, according to safety data published by the F.D.A. on Tuesday. None in the placebo group did.

Scientists said that there is a very small risk of allergic reactions to any vaccine, just as there is to food and medicines. And anaphylactic reactions to any new vaccine or medicine are particularly difficult to prevent ahead of time.

“Anaphylaxis is almost always very specific, with some known overlaps between exposures, but just because I’m allergic to peanuts doesn’t mean I’m allergic to bee stings,” said Dr. Naor Bar-Zeev, a pediatric infectious diseases specialist and epidemiologist at Johns Hopkins Bloomberg School of Public Health.

He added, “Rare events — and anaphylaxis is among the rarest of all — can never be known from Phase 3 trials or before licensure.”

Dr. Bar-Zeev said the risk of rare side effects from vaccines, especially amid a pandemic, had to be balanced against the risk of contracting the disease itself. But he said that even in the best of circumstances, very rare side effects are only discovered well after clinical trials are conducted and new drugs and vaccines are licensed.

In wealthier countries, he added, regulators have well-established systems for identifying rare side effects, and those events will become evident more quickly in the midst of mass vaccinations.

With any new medicine or vaccine, he said, “Anaphylaxis will occur, will be treatable, and if treated, will be unlikely to cause death. The risk is very, very small indeed.”

Dr. Jesse Goodman, a professor of medicine and infectious diseases at Georgetown University and the former F.D.A. chief scientist, said that it was difficult to fully evaluate the British cases without more details.

But he said that clinical trials typically enroll healthier people than the general population, and that safety issues can come up as the vaccines are administered to the general public.

“The people in clinical trials are different, generally healthier, so as vaccines are used outside of trials things will occur that may or may not be due to the vaccine,” he said. “So to ensure safety and maintain confidence, it becomes really important to sort out quickly whether or not they are related.”

Unlike most new vaccines, which are gradually rolled out over years as people slowly visit their doctors and get new shots, the new coronavirus vaccines will quickly be given to many more people than received it in clinical trials.

“They’re large, well-performed studies, but they’re still of tens of thousands of people,” he said of the clinical trials. “It does mean we need those strong safety monitoring systems.”

People with latex allergies can also, on rare occasions, have severe allergic reactions to vaccines held in vials or syringes with latex.

The changing guidance from British regulators testified to the careful monitoring of vaccines that follows an emergency authorization, like the one issued in Britain last week for the Pfizer-BioNTech vaccine. The speed with which regulators warned people on Wednesday was a sign that the monitoring system was effective, scientists said, and reassurance that people did not need to hesitate to get a coronavirus vaccine.

“For the general population, this does not mean that they would need to be anxious about receiving the vaccination,” said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine.

He added, referring to a famous British love-it-or-hate-it yeast paste: “One has to remember that even things like Marmite can cause unexpected severe allergic reactions.”

Denise Grady, Katie Thomas and Carl Zimmer contributed reporting.

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