Here’s what we know about the mu variant

A coronavirus variant known as “mu” or “B.1.621” was designated by the World Health Organization as a “variant of interest” earlier this week and will be monitored by the global health body as cases continue to emerge across parts of the world. It is the fifth variant of interest currently being monitored by the WHO.

Q: Where was it first detected and where is it now?

A: The variant was first detected in Colombia in January 2021, where cases continue to rise. It has since been identified in more than 39 countries, according to the WHO, among them the United States, South Korea, Japan, Ecuador, Canada and parts of Europe.

Q: How widespread is mu in the United States?

A: About 2,000 mu cases have been identified in the United States, so far, according to the Global Initiative on Sharing All Influenza Data (GISAID), the largest database of novel coronavirus genome sequences in the world. Most cases have been recorded in California, Florida, Texas and New York among others.

However, mu is not an “immediate threat right now” within the United States, top infectious-disease expert Anthony Fauci told a press briefing on Thursday. He said that while the government was “keeping a very close eye on it,” the variant was “not at all even close to being dominant” as the delta variant remains the cause of over 99% of cases in the country.

[U.S. COVID death toll hits 1,500 a day]


Mu has yet to be designated a variant of interest or concern by the Centers for Disease Control and Prevention, which did not immediately respond to a request for comment from The Washington Post.

The California Department of Public Health said in a statement to The Post on Thursday, that 348 cases associated with mu had been reported in the state so far and that it would continue “to monitor all variants circulating in the state.”

Q: Will my coronavirus vaccine work against mu?

A: It’s unclear how much protection the vaccines offer against this variant. “The Mu variant has a constellation of mutations that indicate potential properties of immune escape,” the WHO said in a statement Tuesday, raising concerns that it may be more resistant to coronavirus vaccines than other variants. “But this needs to be confirmed by further studies,” it added.

Fauci said that while laboratory data had shown that the mu variant can evade certain antibodies - among them those induced by vaccine shots - there is currently a lack of clinical data and other research involving people, showing this. He underscored that in general, vaccines remain effective and the best protection against the coronavirus.

Vaccine maker Pfizer told The Post in an email that it was studying the mu variant and expected to share data soon with a peer-reviewed journal.

“To date, we are encouraged by both the real-world data and laboratory studies of the vaccine and see no evidence that the virus or circulating variants of concern regularly escape protection,” said Pfizer’s spokesperson Kit Longley.

Representatives from other coronavirus vaccine makers Moderna, Johnson & Johnson and AstraZeneca have yet to reply to requests for comment.

Q: Is the mu variant more transmissible?

A: Paúl Cárdenas, a professor of infectious diseases and genomics at Universidad San Francisco de Quito in Ecuador, has studied mu and told The Post that current evidence showed that it was likely “more transmissible” than the original coronavirus strain. Mu has “been able to outcompete gamma and alpha in most parts of Ecuador and Colombia,” he said.

However, there was no sign yet that people should be more worried, Cárdenas added.

“People should know that these variants emerge all the time and it is important that they are characterized in order to be tracked,” he said.

Q: What next?

A: Mu is the fifth “variant of interest” being monitored by the WHO. It has not yet been moved up to the WHO’s list of “variants of concern” which currently includes the delta variant ravaging the United States, along with the alpha, beta and gamma variants that are deemed more transmissible or virulent by the public health body.

Most viruses change over time, and although some mutations have little to no impact on the virus’s properties, others can change how it spreads, its severity and the effectiveness of vaccines or other medicines.

For now, the WHO says more studies are needed to understand the characteristics of the mu variant - and that it will monitor how it may interact, in particular, with the more common delta variant.