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A man enters a COVID-19 vaccination clinic in Montreal on May 27, 2021.Paul Chiasson/The Canadian Press

Canadian health authorities say they are monitoring reports of heart inflammation following mRNA COVID-19 vaccines, which are produced by Pfizer and Moderna, after U.S. officials reported a higher-than-expected number of cases in those who had received second doses.

Health Canada said a small number of these cases have been reported in Canada, but it is not yet known whether there is any link between them and mRNA vaccines. It added that it encouraged people to get immunized with any of the authorized and available vaccines.

Even so, the reports add a new complication for parents and teens trying to weigh the potential risks of vaccine complications against the risk of COVID-19.

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On Thursday, the U.S. Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) presented data that showed heart conditions were rare. But they were more frequent than the two agencies would normally expect among individuals aged 16 to 24. The cases were more common among males, especially after a second dose.

Researchers have observed two types of heart conditions. One, called myocarditis, is an inflammation of the heart muscle. Another, pericarditis, is an inflammation of the membrane around the heart.

These findings echoed similar observations in Israel of heart inflammation, particularly among young men, after COVID-19 vaccination.

In the U.S., among all age groups, a total of 216 cases of myocarditis and pericarditis were reported after first doses, and 573 cases were reported after second doses.

The CDC and FDA said they would have normally expected to see between two and 19 cases among the age 16 to 17 age group. Instead, they recorded 79 cases. And while they expected eight to 83 cases in people aged 18 to 24, they recorded 196.

Fifteen patients were still in hospital, including three in intensive care – two of whom had other health issues. Out of 221 patients whose recovery status was known, 81 per cent recovered fully from their symptoms. The other 19 per cent included patients with ongoing signs or symptoms.

In an e-mail, Health Canada said the reported number of cases of myocarditis and pericarditis following an mRNA vaccine in Canada is much lower than would be expected in the general population. Over the past 30 days, it said, 23 cases were reported in people after receiving a Pfizer or Moderna vaccine, which it said is 10 times lower than the baseline expected in the general population.

In separate statements, Moderna and Pfizer said they were aware of reports of heart inflammation following mRNA vaccines, and that no causal link has been determined.

Kelly Grindrod, a pharmacist and associate professor at the University of Waterloo School of Pharmacy, said experts must now figure out whether these reported cases are, in fact, caused by the vaccine or whether they are a coincidence.

But, she said, “It’s looking more and more like it’s very possible it is the vaccine.”

Questions that still need to be answered include how often these cases occur, when they occur, and how serious they are, she said.

Meanwhile, she said, as more serious COVID-19 variants emerge, it is important to consider the risk of serious infection.

“We’re always weighing that risk-benefit. Is there a greater risk from a vaccine versus from an illness? COVID is very serious,” she said, adding that while many children do not to get very ill from COVID-19, some do.

According to the most recent government data, 11 people ages 0 to 19 have died of COVID-19 since the start of the pandemic, and 1,281 have been hospitalized.

Dr. Grindrod added that children receive very good protection from the COVID-19 vaccine. (Currently, only the Pfizer vaccine is authorized in Canada for children 12 and older. The Moderna vaccine is approved for those 18 and older.)

Anna Banerji, a pediatric infectious-disease expert and associate professor at the University of Toronto’s faculty of medicine, said it’s likely that myocarditis and pericarditis are side effects of the vaccines, and it’s important for parents and young people to know that this may be the case.

“But I still think the best thing is to get the kids vaccinated – not just for COVID, but also so they can get back to school, for their mental health and so we can open things up,” Dr. Banerji said.

David Juurlink, a drug safety researcher and head of the division of clinical pharmacology at the University of Toronto, also said he believed the heart inflammation cases were side effects of the vaccines. But, he said, navigating the risks and benefits of vaccines is “a thorny issue.”

Although these vaccines are very safe, he said, they will have rare side effects, and so it makes sense to give vaccines to those who have the highest likelihood of benefitting from them and the lowest risk of being harmed – in other words, older age groups.

While he was eager for his own children, who range in age from 15 to 22, to get their first doses of the vaccine, Dr. Juurlink said he is inclined to wait for more data before deciding what to do about second doses.

“I would like to have more information about the true incidence of this complication before the second dose is given,” he said. “And I think that their second dose should, for now, be given to people who are at higher risk of getting sick.”

Editor’s note: An earlier version of this article incorrectly referred to the rate of myocarditis and pericarditis per 1 million doses of COVID-19 vaccines.

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