This electron microscope photo shows the coronavirus's omicron variant successfully isolated at the National Institute of Infectious Diseases. (Photo courtesy of the National Institute of Infectious Diseases.)

TOKYO -- In the process of mutation, the coronavirus's omicron variant may have become less able to multiply and cause severe symptoms when subjected to heat, according to results published on April 24 by a team of researchers based in Japan and the United States.

Numerous variants have appeared in the three years since the coronavirus first infected humans at the end of 2019. Currently, omicron is the dominant variant. This includes the subvariants BA.5 and BQ1.1. Omicron and these subvariants are known to be less likely to cause serious symptoms compared to the initial strain of the virus and the delta variant that spread in the summer of 2021, but the reason was unclear.

Kyoto University professor Takeshi Noda and others in the research team used induced pluripotent stem cells (iPSCs) to create alveolar epithelial cells, which are found in the lungs. They infected the cells with BA.5, BQ1.1 and delta variants and tested the effect of heat on the virus's growth. At 37 degrees Celsius, the usual temperature inside the lungs, each of the variants replicated efficiently.

However, when subjected to the temperature in the lungs when people have a high fever, at around 40 degrees C, only the delta variant continued to replicate efficiently. The BA.5 subvariant's replication was reduced by roughly 99.9%, while growth of the BQ1.1 subvariant was barely detectable.

The alveoli are the parts of the lungs that take in oxygen and expel carbon dioxide. It's believed that as the coronavirus replicates, these cells are destroyed, making it hard for infected people to breathe and requiring the use of respirators and other devices.

The team's Yoshihiro Kawaoka, project professor at the University of Tokyo's Institute of Medical Science, said, "As the virus has adapted to the environment inside the human body, it is apparently able to replicate in the upper respiratory tract where temperatures are low, but not as well inside the lungs. However, vaccines and medications have also had a large effect on the reduction in severe cases."

The results were published online in The Lancet Microbe, a sister publication of the English journal Lancet.

(Japanese original by Ryo Watanabe, Science & Environment News Department)

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