The new JN.1 coronavirus strain now spreading rapidly in Japan is better at evading the human immune system and is more infectious than prior strains, researchers have found.

This evasion, known as “immune escape,” means the immune system cannot respond to the infection, and researchers including Kei Sato, a professor at the Institute of Medical Science, University of Tokyo, say the new variant has the potential to become an epidemic strain going forward.

The JN.1 is a mutation of the omicron BA.2 variant, which became mainstream during the pandemic in 2022. JN.1 has been spreading worldwide since around November 2023, and the World Health Organization (WHO) designated it a “variant of interest (VOI)” in December.

According to Japan’s National Institute of Infectious Diseases, the percentage of JN.1 variant detected among COVID-19 patients increased from just over 10% in the first week of December 2023 to a little over 30% about three weeks later — and is believed to be significantly increasing. But until now, the detailed characteristics of the new strain were unknown.

The team used data from epidemiological studies of viral genomes from the U.K., France and Spain, and used cultured cells for their research. The results showed that JN.1’s “effective reproductive number,” indicating how many people one infected patient can spread the virus to, is around 1.2 to 1.4 times that of the currently prevalent strain.

Experiments using cultured cells revealed that JN.1 may be about twice as infectious as the BA-2-86 omicron subvariant, commonly referred to as pirola, which spread worldwide and was observed for the first time in Japan in the summer of 2023.

The new variant’s immune escape ability is 3.6 to 4.5 times that of the pirola strain for antibodies created in the body following vaccination, and 3.8 times for antibodies developed after being infected with the COVID-19 virus.

The research team said there “is a concern that JN.1 may spread across the globe and become mainstream in the pandemic going forward,” and that effective infection control measures must be properly implemented.

The study results were published in the sister journal of British medical journal “The Lancet.”


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