Japanese scientists have found that the replicative capacity of two SARS-CoV-2 Omicron variants significantly decreases at 40 degrees Celsius, compared with that at 37 degrees Celsius, the normal human body temperature.

It may be one of the possible reasons why Omicron variants are less pathogenic than the Delta ones, the study said.

Led by the University of Tokyo, the researchers compared the replicative capacity of one Delta variant B.1.617.2 and two Omicron variants BA.5 and BQ.1.1 in the presence of a fever, one of the most common symptoms in people who are infected with SARS-CoV-2.

They infected human alveolar epithelial cells (AECs) generated from induced pluripotent stem cells with the three variants, then incubated them at two different temperatures – 37 degrees Celsius, a normal body temperature, and 40 degrees Celsius, elevated human body temperature during illness.

"All three variants had similar growth kinetics on human AECs at 37°C," said the researchers in their study published in the Lancet Microbe journal.

"Notably, although the viral titres of B.1.617.2 at 2 dpi were 10 times lower at 40°C than at 37°C, viral titres of BA.5 were 1,000 times lower at 40°C than at 37°C, and BQ.1.1 was unable to replicate at the higher temperature in human AECs," read the study.

They said higher body temperature might substantially restrict the replication of the two Omicron variants in the lungs, which could have an important role in limiting disease severity.

Further study is needed to find the determinants responsible for the temperature sensitivity of SARS-CoV-2 variants, which could lead to a better understanding of viral pathogenesis, they added.