How risk of side effects could change with Covid-19 vaccine boosters


Aadditional doses of Covid-19 vaccine are probably deploying in the United States later this year. This begs the question: What will the side effects of a booster injection look like? Is there a higher or lower risk of an adverse event compared to previous regimens?

Overall, Covid-19 vaccines are extremely safe and remarkably effective. But as gunfire entered millions of guns from the end of last year, researchers discovered a handful of sometimes serious side effects, which were so extremely rare that clinical trials that have led to the authorizations of the shots – even with tens of thousands of participants – may not have captured them. Researchers don’t fully understand the root causes of some of the side effects, but they also have reported that the risk of some of them, including some heart and blood clotting problems, is much higher after a Covid-19 infection itself than after receiving a Covid-19 vaccine.

STAT asked experts what the adverse event landscape might look like after another dose, and they stressed they were speculating. Because the additional doses have not yet been delivered to many arms, the amount of data is limited, so it is impossible to predict what will happen. (Whether boosters are necessary from a protection standpoint is another issue. Many experts support there is no evidence to support adults in high need of boosters, although some groups of people, such as those with weakened immune systems, do.)


“We read tea leaves,” said Mark Slifka of Oregon Health and Science University, an expert in immunology, viruses and vaccines.

Better answers could come soon. Israel and some European countries are ahead of the United States in their version of the stimulus packages. A study Looking at the early days of Israel’s recall campaign, giving another dose of Pfizer-BioNTech vaccine to people 60 years of age or older, did not appear to reveal any new safety concerns. And statements from the companies about their small studies of boosters indicated similar safety features to previous doses.


Since most problems with vaccination arise soon after the injection is given, “if there was a dramatic increase in risk in terms of the safety profile, we would start to see it soon,” said Slifka.

Below, STAT describes some of the side effects associated with different vaccines and what experts are considering at this point.

The most common and least worrying problem: feeling sick after an injection

With the two-dose mRNA vaccines from Pfizer and Moderna, the second shot turned out to be a real doozy for some who have rolled up their sleeves. It wasn’t universal, but a lot of people have had redness, fever, and aches for a day or two, usually after their second injection.

The culprit was the immune system which really got stronger. If the first hit smacked the body’s defenses with a key protein from the SARS-CoV-2 virus, then the immune system was ready to leap when it saw that spike protein again with hit # 2. It was in the name of building a durable and sturdy blockade in the body, but it meant some people got hangovers after the second dose.

“The immune system says ‘I know what it is’ and attacks it more vigorously,” said Kawsar Talaat, Johns Hopkins infectious disease physician and vaccine researcher.

Talaat said it’s possible that, because the immune system is already ready to recognize and target the spike protein, some people may experience something similar after a third dose.

But Slifka came up with another hypothesis, one where maybe the third hit won’t be as bad, or affect so many people. In the United States, the second doses were given three to four weeks after the first dose, so the immune system was always in an enhanced state from the first injection. Maybe, Slifka said, if people don’t get a booster for at least eight months later, their immune systems will have calmed down, and the third hit won’t come with the same kick as the second hit.


Shortly after the authorization of mRNA vaccines last year, it became evident that some people had severe allergic reactions. The discovery led the United States and other countries to advise that people should be monitored for 15 minutes after receiving their injections – 30 minutes for people with a history of severe allergies – and that providers should be equipped to give epinephrine, which can treat anaphylaxis.

The initial estimate for the Pfizer vaccine was up to 11 cases of anaphylaxis per 1 million doses administered, but more recent data suggests that the rate for the Pfizer and Moderna vaccinations is approximately 5 per million doses administered. At a meeting of the Centers for Disease Control and Prevention advisory committee last month, experts noted that most cases have occurred in women after the first dose, although they added that this may have been at least in part because people who had such a reaction on the first shot did not received their second dose – a move from the CDC recommended.

Always, further research showed that people who appeared to have some form of allergic reaction after the first dose could be safely given a second dose.

As for people who have not had an allergic reaction to the vaccines, they are probably at low risk for such a response to a third dose, Talaat said. “If you have tolerated two doses of a vaccine, you are much less likely to have an anaphylactic reaction to a third dose.”

Myocarditis and pericarditis

Myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the tissue around the heart) have also been linked to mRNA vaccines, with higher rates in men under 40 compared to older men and women. in general, and the greatest risk in boys. 12 to 17 years old, health authorities said. Most cases occurred soon after the second dose.

The vast majority of cases studied so far have been mild compared to cases of myocarditis linked to other causes, including viral infections. And a recent study Israel discovered that there was a higher risk of myocarditis associated with Covid-19 infection than with a Covid-19 vaccine.

For some experts, the clustering of cases soon after the second injection suggests that they are the result of the immune response which, after being initiated by the first dose, goes too far and causes inflammation. One assumption for people who develop myocarditis or pericarditis after the first stroke is that some had an initial infection with Covid-19, so the first stroke was the equivalent of a second exposure, similar to the second dose in patients. people who had not had Covid-19.

David Juurlink, head of the division of clinical pharmacology at the University of Toronto, is concerned that if it is the immune response that triggers the inflammation, it could occur at higher rates after the third dose. Beyond that, he also believes the cases may be more serious.

“I’m concerned that for young men who have been given two doses and are getting a third, the risk may be higher,” Juurlink said.

In granting full approval to Pfizer’s shot last month, the Food and Drug Administration noted Myocarditis and pericarditis rates in vaccinated boys aged 16 to 17 could be as high as 1 in 5,000, but the benefits of the injections always outweighed the risks.

But the possibility of a third dose could reignite the debate around the risk-benefit profile, in particular for boys and young men. If the third doses end up increasing the risk of myocarditis even more in these groups – and it is not clear that one more injection is all that is needed to protect them from Covid-19 – should they be widely given boosters? ? Or should it just be the elderly, who are at lower risk of developing myocarditis after injections and whose immunity to Covid-19 could weaken more worryingly than that of young people?

The data that could help address these issues do not yet exist. But boosters won’t be available to everyone at the same time. They will begin to reach out to people who received their initial regimen first, including the elderly and health workers. If researchers detect an increase in myocarditis among them after their third injection, Juurlink said, it could indicate that health officials should take this high risk into account when considering boosters for younger men.

On the other hand, Slifka raised the possibility that if the third injections are given long enough after the second injection, the immune system may have had a chance to stabilize and there will be no further increase in blood pressure. risk of myocarditis or pericarditis. .

The question of coagulation

While U.S. officials are first advancing mRNA vaccines in booster programs, a Johnson & Johnson booster could eventually be made available as well. The injection of J&J was allowed as a single dose regimen, so the booster would be a second injection.

The most concerning adverse event related to the injection of J&J is what has been termed thrombosis with thrombocytopenia syndrome, or vaccine-induced immune thrombocytopenia and thrombosis (VITT), which is characterized by an unusual combination of extensive blood clots. and low platelet counts. It seems extremely rare, but has also caused serious health crises and some deaths. (The disease, which could be the result of an aberrant immune response, has also been linked to the Oxford-AstraZeneca vaccine, which uses a similar design to the J&J vaccine. The AZ vaccine has not been licensed in the United States. .)

Experts said they were unsure whether the risk of this particular side effect might change with a second injection of the J&J vaccine. The AZ vaccine is given in two doses, so it could shed some light on what might happen with a second dose of the J&J vaccine, but analyzes cases of VITT usually detected them after the first dose. The risk appears to be higher in young adults than in older adults, according to a study published last month on 170 certain and 50 probable cases of VITT in the UK occurring after the first dose of AZ vaccine. Initially, VITT seemed to occur more frequently in women, although the recent British study found no difference based on gender.

One possibility is that a tiny fraction of people have some sort of genetic factor that makes them vulnerable to a disease like developing VITT. If this is the case, people who have received their first dose of J&J safely may not be at great risk to receive a second.

J & J’s stroke has also been linked in rare cases to Guillain Barre syndrome, which involves muscle weakness and sometimes temporary paralysis. Experts told STAT they were unsure if or how the risk might change with a second dose of the injection.

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