Though those 18 and younger are the least likely age group to contract COVID-19 in Canada (they represent just 6.3 per cent of known cases), there has been an increase in instances of a rare inflammatory disease among children that health officials are linking to the novel coronavirus.

Doctors in Ontario, Quebec and Alberta have been seeing cases of  children presenting with symptoms consistent with those of Kawasaki disease and Toxic Shock Syndrome. Health officials worldwide are now diagnosing them with multisystem inflammatory syndrome in children (MIS-C).

The illness is a dysregulated immune response that can lead to long-term and life-threatening heart issues, as well as acute inflammation of other internal organs. 

Affected children are admitted to intensive care with symptoms such as rashes, high fever, gastrointestinal problems like pain and vomiting, low blood oxygen levels, shortness of breath, bloodshot eyes, and others.

UPDATE: There are now at least 350 children in the United States with Kawasaki-like inflammatory syndrome MIS-C, including 7 in Florida👇. https://t.co/coJmKVoi9y

— Matt McCarthy (@DrMattMcCarthy) May 26, 2020

The Centers for Disease Control and Prevention in the U.S. issued a health advisory on May 14 associating MIS-C with COVID-19, while countries such as the U.K. and Italy started reporting the phenomenon back in April.

The U.S. has now seen more than 350 cases of the unusual disease, which itself is not contagious.

Hospitals such as SickKids in Toronto and Sainte-Justine in Montreal have seen dozens of cases, while B.C. has seen at least six, provincial Health Officer Dr. Bonnie Henry said in a press briefing on May 25.

One potential patient was also just identified in Alberta on May 29, though exact numbers are not yet known.

NEW:

One new case of potential Multisystem Inflammatory Syndrome (akak MIS-C) is being investigated in Alberta. #yeg #yyc #ableg

— Courtney Theriault (@cspotweet) May 27, 2020

"Reports from Europe and North America have described clusters of children and adolescents requiring admission to intensive care units with a multisystem inflammatory condition," the World Health Organization (WHO) stated in a briefing on the subject on May 15.

"Initial hypotheses are that this syndrome may be related to COVID-19 based on initial laboratory testing showing positive serology in a majority of patients."

Though MIS-C is being widely considered a symptom or complication of COVID-19 in children, patients may have been asymptomatic before the rapid MIS-C onset and may even test negative for the novel coronavirus in nasopharyngeal and/or serological testing.

Know the signs of MIS-C in children exposed to #COVID19 https://t.co/SM2sHfsU1l.

Conozca los síntomas del síndrome inflamatorio multisistémico (MIS-C) en niños expuestos al #COVID19.

Rekonèt siy MIS-C sou timoun ki ekspoze a #COVID19 la. pic.twitter.com/6P6ZR2BMn1

— Miami-Dade County (@MiamiDadeCounty) May 29, 2020

Some experts, like Vice Chair of the U.S. National Pediatric Disaster Coalition Pat Frost, believe that those with MIS-C may have been exposed to COVID-19 around four to six weeks prior.

"COVID-19 itself is a pathological inflammatory disease, it's not just an infection or a virus," Frost said in a webcast on the condition hosted by the Paramedic Chiefs of Canada and the International Academies of Emergency Dispatch.

"It has the potential to trigger this huge host inflammatory process...that is what is happening in this condition."

ATTENTION ALL PARENTS
Doctors have warned parents to watch for and report possible cases of multisystem inflammatory syndrome (MIS-C) in children infected with COVID-19 virus. Read more on the condition and how to talk to your children about COVID-19.https://t.co/uO3AvXtP5X pic.twitter.com/FPN3HLUzYd

— Salus Hearing Centre (@SalusHearing) May 29, 2020

The uptick in MIS-C cases is particularly interesting given that, as WHO notes, COVID-19 has been not only less prevalent but also less severe among children and adolescents (which scientists believe may be a result of a higher concentration of ACE2 receptors among youth).

"Limited data describe clinical manifestations of COVID-19 that are generally milder in children compared with adults," WHO says, adding that "relatively few cases of infants confirmed to have COVID-19 have been reported [and] those who are infected have experienced mild illness."

So, though the demographic is less likely to contract and suffer severe illness from COVID-19, MIS-C provides a whole new risk to worry about if they are exposed.