Mu COVID Variant, Which Sparked Fears Over Vaccine Resistance, Eradicated in U.S.

The "B.1.621" or "Mu" variant of COVID-19 that sparked fears of vaccine resistance and increased infectiousness, appears to have been eradicated in the U.S., according to virus tracking website

According to the most recent data on the website, for September 20, the Mu variant accounted for zero percent of the cases of COVID-19 in the U.S., and there were no confirmed cases of the variant found.

The Mu strain had previously been found in nearly every U.S. state. The Mu variant peaked in the U.S. on June 19, when it accounted for three percent of the country's total cases of COVID-19.

The strain was first detected in Colombia in January, and since been found in at least 40 countries, but is thought to currently be responsible for less than 0.5 percent of global infections, according to

As of September 26, some 8,557 cases of Mu have been detected since it was first seen, the website said.

The WHO and the Centers for Disease Control and Prevention (CDC) have three categories of COVID-19 variants that make up their watch lists: variants of interest (VOI), variants of concern—which includes Alpha, Beta, Delta and Gamma—and variants of high consequence.

The WHO officially labeled the Mu variant a VOI on August 30, meaning the variant has genetic differences to the other known variants and is causing infections in multiple countries, meaning it might present a particular threat to public health.

However, the numbers globally are low at present, meaning it is not a variant of concern.

When Professor Anna Durbin, Director, Center for Immunization Research, Johns Hopkins Bloomberg School of Public Health, was asked if Mu had been out-dominated by Delta, she replied "this is most likely explanation."

"Virus strains are competing with one another and it is definitely survival of the fittest, essentially the virus that can infect more people faster. It is likely that Mu was not able to out-compete delta," she added.

"I also think vaccines contributed, as they provide protection to more people making fewer susceptible hosts.

"The more viruses replicate, the more they are able to mutate and adapt. Vaccines provide protection and reduce the amount of infectious virus and reduce the amount of time viruses can replicate. This means the virus has much less opportunity to mutate and adapt."

The news comes as researchers in Japan released a study on Monday showing that the Pfizer-BioNTech COVID-19 vaccine seems to be adequately effective against the B.1.621 (Mu) variant.

A team of scientists from Yokohama City University School of Medicine and Yokohama City University Hospital found that the vaccine is 76 percent effective in neutralising the variant. A dual monoclonal antibody cocktail containing casirivimab and imdevimab was also found to be effective.

Delta variant remains the dominant strain in the U.S., accounting for nearly 100 percent of all sequenced samples.

However, cases have been declining for several days, suggesting the country might have seen the peak of the summer wave of the variant. But the outlook for the winter months remain uncertain, experts told Newsweek on Monday.

According to data from the CDC, the country's seven-day moving average of cases has been declining since September 14, when the daily average of new cases was just under 150,000.

The daily average was 95,228 for September 26.

Coronavirus particles
An artist's illustration of the SARS-CoV-2, which causes COVID-19. The Mu variant of COVID-19 that sparked fears of vaccine resistance and increased infectiousness, appears to have been eradicated in the United States, according to virus tracking website iStock