Scientists demonstrated that it is safe for children and adolescents to get a COVID-19 vaccine after having multisystem inflammatory syndrome (MIS-C).

The researchers found that there were no reports of serious complications including myocarditis or MIS-C reoccurrence. About half of participants experienced mild and typical reactions, including arm soreness and fatigue, they said.

The multicentre, observational study, the largest of its kind to examine COVID vaccination in this group, helps resolve a lingering question about whether the COVID vaccine can increase the risk of health problems in young people who have had MIS-C. The study appears in the Journal of the American Medical Association (JAMA) Network Open.

MIS-C is a poorly understood condition that affects 1 in about 3,000 to 4,000 children and adolescents who had COVID-19, according to the Centres for Disease Control and Prevention (CDC). It occurs a few weeks after COVID infection and can lead to organ failure.

MIS-C symptoms can range from stomach pain, fever, and rash to inflammation of the heart muscle, a serious condition called myocarditis. The exact causes of MIS-C are unknown, but medications can be given to decrease the inflammation that can damage organs.

Some families and healthcare professionals have questioned whether COVID vaccines could lead to more serious adverse reactions in those with a history of MIS-C, including a recurrence of the disease, but data on this topic were lacking.

The cross-sectional study, funded by the National Institutes of Health (NIH), US, included 22 medical centres, 21 in the US and 1 in Canada, participating in the National Heart, Lung, and Blood Institute's (NHLBI's) Long-Term Outcomes After the Multisystem Inflammatory Syndrome in Children (MUSIC) study, the study said.

The study enrolled 385 patients aged 5 years or older with prior MIS-C who were eligible for COVID-19 vaccination. Of this group, 185 (48.1 per cent) received at least one vaccine dose. The median age was 12.2 years and 73.5 per cent were male, the study said.

The participants were racially diverse – 24.3 per cent were Black, 31.9 per cent were Hispanic, and 28.6 per cent were white. The median length of time from their MIS-C diagnosis to their first vaccine dose was 9 months, the study said.

Of those who received a COVID vaccination following MIS-C, mild adverse reactions - mostly arm soreness and fatigue - occurred in 49 per cent of them, similar to the general population. There were no reports of serious complications, including myocarditis or recurrence of MIS-C, the researchers said.

"We are very reassured by the results and this safety data should be comforting to families and healthcare professionals when considering and recommending vaccination," said study co-leader Matthew D. Elias, University of Pennsylvania, Philadelphia, US.

The researchers have routinely treated children with MIS-C throughout the pandemic, said the study.

Audrey Dionne, from Harvard Medical School, Boston and another co-leader of the study added that the findings provide support for the CDC's recommendation that patients with a history of MIS-C receive a COVID vaccine at least 90 days after diagnosis and that it is safe to do so.

"In light of the acute and long-term consequences of COVID-19 it is vital to continue the development, testing, and deployment of preventive as well as therapeutic agents in at-risk groups as well as the general population," said Gary H. Gibbons, director of NHLBI, part of NIH.

To date, more than 9,000 patients have been diagnosed with MIS-C in the United States, and 74 have died, according to data from the CDC. However, the disease appears to be on the decline, according to studies by others, the study said.

"A big part of that decline is that COVID vaccination has been protective against this rare condition in those who have received it," Dionne said.

While many patients with MIS-C make a full clinical recovery, some studies suggest chronic symptoms linger after MIS-C, which is why long-term outcome studies will be beneficial, the researchers said.

The MUSIC study is part of an NIH collaborative research effort called CARING for Children with COVID, which aims to better understand how COVID affects children, who account for roughly 13 per cent of the total cases in the United States.

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