Oct. 6, 2021 -- Amid the Delta variant summer surge in COVID-19 cases, hospitalizations, and deaths, some experts warned that the next potential threat was starting to emerge: the Mu variant of the coronavirus.
Mu made headlines because its specific mutation meant it could evade vaccine immunity — spiking a new fear among the immunized.
But then an interesting thing happened — the prevalence of Mu in circulation in the U.S. dropped off quickly over a matter of weeks.
The robust transmissibility of the Delta variant allowed it to remain the dominant coronavirus strain, despite the risk from Mu, some experts believe.
"In a competition between Delta and Mu, even though Mu is likely more vaccine-resistant than Delta, the sheer transmission advantage could easily explain the disappearance of Mu," says Jesse Erasmus, PhD, director of virology at the pharmaceutical company HDT Bio, and acting assistant professor at the University of Washington School of Medicine in Seattle.
Still, we should not let our guard down, says Pedro Piedra, MD, professor of molecular virology and microbiology at Baylor College of Medicine in Houston.
"In the United States we did not see the Mu variant taking off like Delta variant did, but that does not mean it might not," Piedra says.
Monitoring of Mu over time remains warranted, he says.
The Mu variant was first reported in the U.S. in March and April, although cases only numbered in the single digits. The 7-day rolling average of cases from the Mu variant increased to 1% of all U.S. cases on May 8, to 2% a few weeks later on May 29, and continued climbing to a peak of 3% on June 22, according to figures from Outbreak.info.
Then, the Mu cases began to drop at about the same rate during July, decreasing to 2% on July 9 and 1% on July 22, and less than 0.5% in August. The number of cases from Mu was back to single digits by September. The most recent was reported Sept. 20.
Still, the Mu variant was detected in 49 of 50 states in July (all except Nebraska, which was not reporting COVID-19 data at the time). Later, Mu was reported in all 50 states, the District of Columbia and Puerto Rico.
COVID Variant Mu Made a Splash, Disappeared Without a Peep - WebMD
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