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When frightening reports began to emerge in the spring of British and U.S. children falling severely ill with an inflammatory syndrome linked to COVID-19, Montreal’s Sainte-Justine Hospital was the first to report a cluster of similar cases in Canada.

At the time, pediatricians couldn’t say for certain that the novel coronavirus was to blame. The new illness, multisystem inflammatory syndrome in children (MIS-C), was thought to follow a COVID-19 infection, and blood tests for antibodies that would indicate a past coronavirus case were not yet available in Canada.

Now results are in for the children treated at Sainte-Justine: Of 21 suspected cases, only four had antibodies to SARS-CoV-2, the virus that causes COVID-19. One child tested positive for an active infection.

“It’s very, very interesting for us, because we continue to think that this is related to SARS-CoV-2,” said Elie Haddad, the head of pediatric immunology at Sainte-Justine. “When they told us that only four patients were positive in serology, we were surprised.”

The Sainte-Justine patients are only a small sample, but the results suggest MIS-C is perhaps rarer in Canada than it first appeared.

In making sure no patients were missed, pediatricians in Canada may have included cases with similar symptoms but no connection to the coronavirus. In British Columbia, none of the six patients investigated for possible MIS-C since the beginning of the pandemic tested positive for antibodies, according to the province’s Ministry of Health.

“This is a very, very rare condition,” said Charlotte Moore Hepburn, director of medical affairs for the Canadian Paediatric Surveillance Program (CPSP), which is tracking suspected cases of MIS-C and serious COVID-19 complications in children. “The numbers across Canada – and we have comprehensive numbers from every Canadian jurisdiction – are very small.”

MIS-C, which the Canadian Paediatric Society sometimes calls PIMS-TS, is generally marked by fever, intestinal distress, elevated inflammation levels on laboratory tests and the overlapping symptoms of toxic-shock syndrome and Kawasaki disease, a rare childhood illness that inflames the blood vessels, most dangerously in the heart.

When the syndrome was first reported in places hit hard by early coronavirus outbreaks – including London, New York and Italy, Spain and France – tests showed that in a majority of cases, children either tested positive for an active infection or had antibodies for the virus.

Those results struck fear in parents who had taken comfort in how otherwise gentle the new virus is on children.

A few children have died of MIS-C. In the United States, 186 cases were identified in 26 states between mid-March and mid-May, according to a study published last month in the New England Journal of Medicine. Of those children, 148 required intensive care, 37 received mechanical ventilation and four died.

But in Canada, no children are known to have died of MIS-C, and only a handful of those suspected of having it were in intensive care, pediatric hospitals and provincial health officials say.

Serological tests on other suspected cases of MIS-C in Canada are pending. Alberta is investigating four, according to Tom McMillan, a spokesman for Alberta Health. None of those children tested positive for active coronavirus infections in nasal swabs, he said.

Toronto’s Hospital for Sick Children is investigating 50 suspected cases of MIS-C. Four of those patients tested positive for an active infection. SickKids expects testing for antibodies in blood samples to begin next week, Jessamine Luck, a spokeswoman for the hospital, said in an e-mail.

CHEO, formerly known as the Children’s Hospital of Eastern Ontario, is investigating more than 20 possible cases, all of them negative for an active infection, spokesman Paddy Moore said.

Montreal Children’s Hospital is waiting for serology testing to be available for 10 suspected cases, said Rosie Scuccimarri, a pediatric rheumatologist. None of those children tested positive for an active COVID-19 infection. One was admitted to the ICU, and that child’s case had greater similarity to toxic shock than the others, Dr. Scuccimarri added.

“I think that a lot of these may be Kawasaki disease patients rather than true MIS-C, but the definition is broad,” she said, adding that such a new disease still has many unknowns.

MIS-C is so new that its definition differs from country to country, but most places require some evidence of a current or past coronavirus infection, or exposure to someone who tested positive for the virus.

That has posed a problem in Canada, where blood tests for coronavirus antibodies are not routinely available, even though Health Canada has approved several.

In Ontario, a spokeswoman for Health Minister Christine Elliott said by e-mail that antibody tests will be available “in the near future,” once Public Health Ontario and other laboratories have finished evaluating them. Concerns about the accuracy of antibody tests have been raised in other countries.

Dr. Haddad of Sainte-Justine said he is confident the tests on his hospital’s suspected MIS-C cases were accurate. They were run twice: Once in a research lab at the University of Montreal and once at Sainte-Justine’s microbiology lab, he said.

However, Dr. Haddad said he still thinks the unusual spike of patients with symptoms of Kawasaki and toxic shock this spring is connected to the coronavirus.

“The only thing I have to argue is the chronological aspect,” he said. “The fact is that all over the world in countries where there were outbreaks of coronavirus, we observed much higher frequency than in the previous years of [Kawasaki-like symptoms].”

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