School closures due to Covid lockdowns resulted in much lower rates of the mysterious fever that causes heart disease in kids, according to new research.

Cases of Kawasaki disease fell by 28 percent in 2020, and remained low during the peak pandemic period.

Masking mandates, less air pollution, and reduced circulation of respiratory viruses were thought to be the causes behind the phenomenon.

The numbers shed fresh light on the causes of the most commonly acquired heart disease in children.

Kawasaki disease (KD) affects hundreds of youngsters in the UK each year—about eight in every 100,000—and almost 6,000 children annually in the United States.

Senior author Dr. Jane Burns, director of the Kawasaki Disease Research Center at the University of California-San Diego Medical School (UCSD), said the pandemic provided "an incredible natural experiment" that researchers could take advantage of.

KD continues to puzzle pediatricians, with its triggers and mode of entry into the body yet to be identified.

Unlike Covid-19, it is not contagious. But the discovery that precautions against coronavirus were an effective tool suggests it is inhaled into the upper respiratory tract.

The findings in JAMA Network Open Pediatrics could have a major impact on research and prevention.

When the pandemic began, UCSD was leading a multi-site clinical trial monitoring national KD cases between 2018 and 2020.

Dr. Burns and colleagues combined this effort with additional data from San Diego to track KD incidence as the crisis progressed.

First author and UCSD Professor Jennifer Burney called it a "really interesting story".

"We saw a huge decline in numbers, but unlike other respiratory illnesses during the shelter-in-place period, it didn't disappear entirely, and the dynamics were not the same for all subsets of patients."

Rates of KD are typically higher in male and Asian children, and these groups saw especially large drops in cases during the pandemic.

Another group that saw a disproportionate decrease was children ages one through five. This was notable when compared to infants, who saw no significant change in KD rates during this time period.

The reason is likely because infant behavior was not as significantly impacted by the pandemic. Typical activities and exposures for older children changed more dramatically in 2020.

The patterns suggest social behavior affects exposure to the agents that trigger KD, and are consistent with a respiratory portal of entry.

"Kawasaki disease may be caused by a virus, a pollutant, a microbial aerosol, or all of the above," said Dr. Burns. "The fact the pandemic affected each age group differently supports the idea that there are multiple triggers of KD, and different children develop the disease after exposure to different ones."

KD causes inflammation in the walls of the blood vessels and affects mostly children under five. It can weaken arteries which supply the heart with blood. Symptoms include fever, rash, bloodshot eyes, and redness of the mouth, throat, hands and feet. When untreated, a quarter of patients develop coronary artery aneurysms that can lead to heart attacks, congestive heart failure, or—in three percent of untreated cases—sudden death.

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