The Mu strain was previously present in most states and accounted for 3% of COVID-19 cases nationwide at its peak on June 19.
The variant was initially found in Colombia in January and spread to at least 40 countries, but has since decreased to just 0.5 percent of global infections, Outbreak.info reports.
The website confirmed a total of 8,557 cases of Mu were detected since the reported initial diagnosis as of September 26.
Mu was initially speculated to be vaccine resistant, however, a study out of Japan showed the Pfizer-BioNTech COVID-19 vaccine provided substantial protection against the variant, the Daily News reports.
The World Health Organization and U.S. Centers for Disease Control and Prevention categorize COVID-19 variants in three watch lists -- variants of interest (VOI), variants of concern and variants of high consequence -- based on circulation rate and severity.
The Mu variant was previously considered a variant of interest as it had genetic differences to other known variants and caused infections in numerous countires, making it a threat to public health, as of August 30.
However, Mu -- unlike the Delta variant -- is not considered a variant of concern as its number of global cases remain considerably low.