Heart inflammation may be a rare side effect of the Pfizer vaccine, particularly for young men, Medsafe says, but the Covid-19 vaccine is safer than the alternative.
Drugs and medical devices regulator Medsafe released an alert yesterday warning of an increased risk of myocarditis and pericarditis - inflammation of the heart muscle and the membrane that surrounds the heart - in Pfizer vaccinees.
It appears much more likely to affect men and boys aged under 30, particularly after being given the second dose.
Medsafe said cases so far had generally been mild and not required treatment. Symptoms could include constant tiredness, weakness or cough, chest pain or palpitations, which is the feeling of having a fast-beating or fluttering heart.
While it was not certain whether the vaccine had caused the increased risk, the risk was certainly higher.
However, benefits of taking the vaccine still outweighed the risks.
In the US, the Centers for Disease Control and Prevention (CDC) reported early this month some 1226 reports of the inflammation after being vaccinated with either the Pfizer or Moderna vaccines, out of about 296 million doses given.
Those most affected were aged between 12 and 29, with the median age for the reaction being 26 years, usually within a few days of inoculation.
Most of the reactions - 76 percent - came after the second dose.
In a vaccines safety update a month ago, the CDC reported most myocarditis reactions were in men with 66 percent and 79 percent male for first and second doses respectively.
The highest risk of heart inflammation males aged 12-17, with 66.7 cases reported per million people vaccinated, followed by males aged 18-24 with 56.3 cases per million vaccinated. That compared to 9.1 and 5.5 cases per million for females in the same age brackets.
The CDC report found the benefits - including reduced hospitalisation, ICU admissions and deaths - outweighed the risks in all population groups when considering the risk of inflammation.
"Per million second doses of mRNA Covid-19 vaccine administered to males aged 12-29 years, 11,000 Covid-19 cases, 560 hospitalizations, 138 ICU admissions, and six deaths due to Covid-19 could be prevented, compared with 39-47 expected myocarditis cases after Covid-19 vaccination," the report said.
"The benefits of vaccinating all recommended age groups with mRNA COVID-19 vaccine clearly outweigh the risks of vaccination, including the risk of myocarditis after vaccination."
That was despite the cost-benefit analysis not taking into account other benefits including the prevention of long-term effects from Covid-19 and multisystem inflammatory syndrome in children.
Other vaccines which had not been approved for use in younger populations in the US were not considered.
Medsafe said the most common cause of myocarditis was viral infection, with about 100 people a year discharged from hospital after having it as the main cause of illness.
Director-General of Health Dr Ashley Bloomfield, in the weekly vaccine update yesterday, said the increased risk was associated with an autoimmune response meaning the effect was because of the body fighting against the mRNA vaccine.
He said the alert was to make sure health professionals were aware of the signs and symptoms, and vaccinated individuals should seek immediate medical attention if they experienced chest pain or shortness of breath.