Severe allergic reactions to COVID-19 vaccines are “exceedingly rare” according to health authorities in the US, where more than 11 million people have now received at least one dose of a COVID-19 vaccine.
According to a report published last week by the Centers for Disease Control and Prevention (CDC), 21 people developed anaphylaxis after getting a COVID-19 jab in the first week and a half of the country’s vaccine rollout.
At that point, 1.9 million people had been vaccinated, putting the rate of severe allergic reactions at 11.1 cases per 1 million doses of vaccine used.
Since the report was finalised, at least eight more cases of vaccine-related anaphylaxis have been confirmed in the US, as millions more Americans receive their first COVID-19 jab.
“Of course, we all would hope that any vaccine would have zero adverse events. But even at 11 cases per million doses administered, it’s a very safe vaccine,” Nancy Messonnier, director of CDC’s National Centre for Immunisation and Respiratory Diseases, said at a press briefing.
Anaphylaxis — the most severe type of allergic reaction — is a known, but very rare, side effect of vaccination.
The CDC’s estimated rate of anaphylaxis after COVID-19 vaccination — 11.1 cases per 1 million doses — is higher than for the flu vaccine, which is 1.3 cases per 1 million doses.
“[It] may seem high compared to flu vaccines, but I want to reassure you this is still a rare outcome,” Dr Messonnier said.
“These are safe and effective vaccines. We have good data to show that.”
What do we know about these allergic reactions?
Of the 21 people who developed anaphylaxis following their first COVID-19 jab, 17 had a history of allergies, including to drugs, medical products, foods and insect stings.
Seven of them had previously suffered anaphylactic reactions, and four were hospitalised.
If left untreated, anaphylaxis can be fatal. None of the 21 people evaluated by the CDC died, and of the 20 people with follow-up information available, all had made a full recovery.
Most of the patients were treated with an emergency injection of epinephrine — the drug used in EpiPens to treat severe allergic reactions — and most developed a reaction within 15 minutes of being vaccinated.
This is why the CDC recommends people wait around at least 15 minutes after they receive their COVID-19 jab (as seen with other immunisations), or 30 minutes if they have a history of anaphylaxis or vaccine allergies.
Paediatrician Nick Wood, from the National Centre for Immunisation Research and Surveillance, said although the rate of anaphylaxis after COVID-19 vaccination appeared slightly higher than after the flu vaccine, the risk remained very low.
“What it means is that the vaccine program and the healthcare workers delivering it need to be prepared, well-trained and have the right equipment to manage an anaphylactic event,” said Dr Wood, an associate professor at the University of Sydney.
“With the right recognition and early administration of adrenaline, it’s very treatable. So you shouldn’t see any severe outcomes.”
Who shouldn’t get vaccinated?
So far, most cases of anaphylaxis in the US have occurred in people who have received the Pfizer vaccine rather than the Moderna one, but it’s not clear whether that’s because more of the Pfizer vaccine has been administered, Dr Messonnier said.
“At this point we think that it is something that is seen with both vaccines and therefore our recommendations apply to both vaccines,” she said.
The CDC recommends that anyone who had an anaphylactic reaction to the first dose of a COVID-19 vaccine should not have the second dose. It also recommends anyone who is allergic to any ingredients in COVID-19 vaccines, or to polysorbate (a type of emulsifier used in some pharmaceuticals), should not get vaccinated.
People with a history of allergies not related to vaccines — for example to foods, pets, insects or oral medications — are still recommended to get a COVID-19 jab.
In the UK, the National Health Service provides similar recommendations, with the addition of previous anaphylactic reactions to any vaccine as a reason not to get vaccinated. In the US, this is listed as a “precaution” but not a definite reason to not get a jab.
In Australia, it will be up to the Therapeutic Goods Administration to provide clinical recommendations for COVID-19 vaccines, which Dr Woods suspects will be similar.
“My guess is that they’re going to have a cautionary no-go for anaphylactic people, and there will be a cautionary note for monitoring people for a minimum of 15, possibly 30 minutes post-vaccine,” he said.
Why do allergic reactions happen?
Allergic reactions can be thought of as the immune system “misfiring”, said Joanna Groom, an immunologist at the Walter and Eliza Hall Institute.
“The idea of a vaccine is that we’re trying to mimic the infection, but your body may see that as an allergen and it just misfires in that process,” Dr Groom said.
Allergic reactions are highly personalised, she said, and having severe allergies doesn’t necessarily put you at increased risk of having an allergic reaction to a vaccine.
As for why a handful of people have experienced anaphylaxis following a COVID-19 jab is still unclear, Dr Wood said.
“It’s not definitively known, but there is this concept that it might be related to the PEG — polyethylene glycol — which is the lipid nanoparticle, the soapy bubble that contains the mRNA,” he said.
PEG, which is a component of both Pfizer and Moderna’s mRNA vaccines, hasn’t been used in vaccines before (these are the first mRNA vaccines approved for use in humans), but it is found in many drugs.
“It might be that that’s the foreign antigen people are reacting to,” Dr Wood said.
He said more information on the likely cause of the allergic reaction would be forthcoming.
“What will happen is that the people who get these [anaphylactic] reactions will undergo a fairly detailed assessment by an allergy specialist, and there are extra tests we can do to try to understand why they might have reacted.”
Dr Messonnier said researchers in the US were now working to find the cause of the allergic reactions, but that it wasn’t yet clear.
The CDC found no geographic clusters of anaphylaxis events, suggesting the allergic reactions were not associated with, or caused by, a single batch of vaccines.
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How do we know the vaccines are safe?
COVID-19 vaccines, like other vaccines, are required to go through large clinical trials and safety checks before being approved for use, Dr Groom said.
“It’s a very rigorous process, and safety is at the forefront of that process,” she said.
Even after vaccines are rolled out in the community, health authorities continue to keep a close eye on them, monitoring safety concerns.
“In the UK and the US, people are getting sent home with a piece of paper saying, ‘please report anything at all’, and actually even with that push … the adverse events are low, so I think that’s really encouraging,” Dr Groom said.