The Mu variant of the Covid-19 virus, also known as B.1.621, is more resistant to antibodies than any other variants, a study showed.

Japanese researchers, including those at the University of Tokyo, released the results of their joint study in the New England Journal of Medicine on Wednesday.

The Mu variant was first reported in Colombia in January. The country had a spike in Covid-19 cases from March to July. The Mu variant drove up the spread of Covid-19 infections, especially since May.

The research team evaluated the sensitivity of the Mu variant to antibodies induced by natural Covid-19 infection or vaccination. To analyze the sensitivity, the researchers used serum samples from 13 people who had recovered from Covid-19 who were infected between April 2020 and September 2020 and serum samples from 14 recipients of the Pfizer vaccine.

The results showed that the Mu variant was 10.6 times as resistant to antibodies as the parental virus, and it was also resistant to antibodies of vaccinated people.

The Mu variant was twice as resistant to neutralization by convalescent serum and 1.5 times as resistant to neutralization by vaccine serum as the Beta variant, the research team found.

So far, the Beta variant was thought to be the most resistant variant.

“The Mu variant shows a pronounced resistance to antibodies elicited by natural SARS-CoV-2 (Covid-19) infection and by the BNT162b2 (Pfizer) mRNA vaccine,” the research team said. “Because breakthrough infections are a major threat of newly emerging SARS-CoV-2 variants, we suggest that further characterization and monitoring of this variant of interest is warranted.”