What You Need to Know About Allergic Reactions to Covid Vaccines
A resident receives the Moderna Covid-19 vaccine at the King’s Point retirement home in Delray Beach, Florida, U.S. (Photographer: Saul Martinez/Bloomberg)

What You Need to Know About Allergic Reactions to Covid Vaccines

Like all new drugs, the vaccines that have been authorized in Western countries to protect against Covid-19 come with some safety concerns and side effects. Many people who’ve received the first two shots deployed, one from Pfizer Inc. and BioNTech SE and another from Moderna Inc., have experienced fever, headache and pain at the site of the injection. These side effects generally disappear quickly. At least a couple of dozen have had a serious allergic reaction, called anaphylaxis, to the vaccines, which were about 95% effective in preventing Covid in clinical trials. No fatalities have been reported, and health care workers are monitoring vaccine recipients to treat the reactions that arise.

1. What is anaphylaxis?

The body fights foreign invaders through a variety of mechanisms that include making protective proteins called antibodies, releasing toxins that kill microbes, and marshaling guardian cells to battle the infection. As in any conflict, sometimes the effort to repel an infection can itself be damaging. In rare cases, it can produce runaway inflammation and swelling of tissues in a serious allergic reaction called anaphylaxis. As much as 5% of people in the U.S. have had such a reaction to various substances. It can be fatal if, for example, the person’s airway swells shut, though deaths are rare. Allergies to insect stings and foods can provoke it, though drug reactions are the most common cause of anaphylaxis fatalities in the U.S. and U.K.

2. Where have Covid vaccines triggered cases?

According to a Jan. 6 report by the U.S. Centers for Disease Control and Prevention, 21 cases of anaphylaxis associated with the Pfizer-BioNTech vaccine had been confirmed in the country as of Dec. 23. Of those people, 17 had a documented history of allergies, and seven had a history of anaphylaxis. A Dec. 19 presentation from the CDC referenced two cases in the U.K. associated with the same vaccine, and later in the month, in Israel, a man suffered anaphylactic shock an hour after receiving it, according to the Jerusalem Post. He said he’d had earlier reactions to penicillin, the paper reported. CDC officials say they’ve also seen the reactions in recipients of the Moderna shot and are compiling data on them.

3. Has anaphylaxis been connected to vaccines before?

Yes. They occur about 1.3 times per million doses of flu vaccine administered, and with other vaccines have been seen at rates of 12 to 25 per million doses, though the studies were small. For the Pfizer-BioNTech Covid vaccine, according to the CDC, the rate as of Dec. 23 is 11.1 per million doses, which is very low. The agency said the risk surrounding the vaccine is less than the risk of getting a severe case of Covid-19.

4. How long does the risk last?

Usually not long. Anaphylactic reactions normally occur within minutes to hours of exposure to a specific substance, said Michael Kinch, a drug development expert and associate vice chancellor at Washington University in St. Louis. In the U.S. so far, the time lag ranged from two to 150 minutes, with the median being 13 minutes, according to the CDC.

5. What’s being done about the risk?

The U.K. and U.S. have advised people who have allergies to any component of a Covid vaccine not to receive it. Anaphylaxis can be quickly countered with antihistamines and adrenaline injectors like Mylan NV’s Epi-Pen that slow or halt immune reactions, and health workers giving the vaccine are keeping such items at the ready. These treatments don’t cancel out the beneficial effects of vaccines. In the U.S., health workers are observing everyone who receives the vaccine for at least 15 minutes post-injection to watch for signs of a reaction; those with a worrying history of allergic reaction are monitored for twice as long. People who have had reactions to a first dose of vaccine shouldn’t receive a second, according to the CDC.

6. Do we know what in the shots is causing the reactions?

That isn’t clear. The two leading candidates are polyethylene glycol -- a chemical found in many foods, cosmetics and medications -- and lipid nanoparticles that encapsulate the messenger RNA, a genetic component in the vaccines, according to Eric Topol, a clinical trials expert and director of the Scripps Research Translational Institute. Polyethylene glycol has been previously linked to a handful of anaphylaxis cases. Once a cause has been narrowed down, it may be possible to make Covid vaccines even safer than they are now, Topol said.

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