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Health Canada is adding information about heart inflammation to the product labels of Pfizer and Moderna COVID-19 vaccines, after rare cases of the potential side effect have been reported, particularly among young men.

However, the agency says the vaccines are safe and effective for protecting people against COVID-19, and continues to encourage Canadians to get fully vaccinated as soon as they’re eligible. Health Canada has approved the Pfizer vaccine for individuals aged 12 and up, and the Moderna vaccine for those 18 and older.

In an advisory published online, the health authority said it is updating product labels for these vaccines to inform Canadians and health care professionals of possible side effects of myocarditis and pericarditis after vaccination. Myocarditis is an inflammation of the heart muscle and pericarditis is an inflammation of the lining around the heart.

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Heath Canada data show that as of June 18, there have been 65 cases of myocarditis or pericarditis reported in individuals in Canada after receiving COVID-19 vaccines, though unlike international reports of a higher incidence of these conditions among young men, there is no clear pattern among the cases recorded in this country.

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Last week, officials with the U.S. Centers for Disease Control and Prevention’s (CDC) Advisory Committee on Immunization Practices said the heart conditions appear to be triggered by mRNA COVID-19 vaccines, which are produced by Pfizer and Moderna. However, they said the benefits of vaccination currently still outweighed the risks of COVID-19 in teenagers and young adults.

Their data showed that after around 300 million doses of mRNA vaccines, a total of 1,226 cases of myocarditis and periocarditis were reported in the U.S. Most occurred after a second vaccine dose, and there were more cases in males than females. The median age of those who had these conditions after a second dose was 24.

Similar reports of rare cases of myocarditis have also emerged from Israel, mostly in young men and usually after the second dose of an mRNA vaccine.

In its advisory, Health Canada says individuals who have received the Pfizer and Moderna vaccines should seek medical attention immediately if they experience chest pain, shortness of breath or sensations of having a fast-beating, fluttering or pounding heart within several days of vaccination.

Of the 65 reported cases in the Health Canada data, 50 had received a Pfizer vaccine, 10 received a Moderna vaccine and five received an AstraZeneca vaccine or its India-manufactured counterpart, Covishield. Neither AstraZeneca nor Covishield vaccines are mRNA vaccines. Among those who received a Pfizer vaccine, 28 were females, with a median age of 51; and 22 were males, with a median age of 38.

Experts say the benefits of COVID-19 immunization outweigh the risk of rare cases of the heart condition.

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“It’s a small risk,” said Gavin Oudit, a professor in the department of medicine at the University of Alberta and clinician-scientist at the Mazankowski Alberta Heart Institute. “The benefit from the vaccination is tremendous.”

Pericarditis is fairly benign, while myocarditis tends to be more serious, Dr. Oudit said. But in most of the reported cases after COVID-19 vaccination, these conditions were mild and resolved on their own, or after treatment with anti-inflammatory drugs, he said. Symptoms can include chest pain, fever, fatigue, muscle aches and shortness of breath.

Abdu Sharkawy, an infectious-diseases specialist at the University Health Network in Toronto, said the cases he and his colleagues have encountered seem to have been in patients who followed a relatively short vaccine-dosing interval, waiting as little as five weeks between shots. He suggested that waiting a minimum of eight weeks between doses may reduce the risk of myocarditis and pericarditis among young people. Extending the dosing interval may allow one’s immune response to “settle down a bit” before boosting it with a second dose, Dr. Sharkawy said.

Myocarditis and pericarditis can also be triggered by infections, including COVID-19, he said, saying it’s important to note that these conditions reported after vaccination tend to be more easily treated and short-lived than viral-induced cases.

“If you get the same thing from COVID, you may not be as lucky,” he said.

CDC advisors last week said individuals with a history of pericarditis or myocarditis can still receive their COVID-19 vaccines. However, they recommended that those who developed myocarditis after a first dose should defer their second doses until additional vaccine safety data are available.

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