Daily Trends in COVID-19 Cases in the U.S. Reported to CDC (9-21-21): red line shows 7-day moving average. Click image to enlarge.

According to the CDC, the delta variant now makes up more than 99% of COVID-19 cases across the U.S. Recently, though, the media has been drawing attention to the mu variant. Should we worry?

How serious are variants?

Viruses constantly change. Mutations might make a virus easier to spread or more virulent, change its clinical presentation, or alter its response to tests, vaccines, or treatment. Or there may be no observable change in its characteristics at all. Some variants simply fade away; others stick around, but can’t compete with more dominant variants and never really get a “foothold” in a particular population. At this time—and this is a pretty big caveat—the COVID-19 mu variant seems stuck in this last category, at least in the U.S.

What about mu?

Although the mu variant has been identified in more than 39 countries, including the U.S., there have been only about 2,000 cases here, mostly in California, Florida, Texas, and New York. The World Health Organization (WHO) calls mu a “variant of interest” (more on this label later), but the CDC does not. While the WHO is tracking mu’s global spread, according to the CDC the variant peaked here in late June at less than 5% of circulating variants, and since then its numbers have trended downward.

Although there is some evidence that the mu variant is more transmissible than the original COVID-19 strain, thus far it has seemed no match for the delta variant. Like most variants, it has, theoretically at least, the potential to evade COVID-19 vaccines. But there is as yet no data to suggest that it is able to do that.

How are COVID-19 variants categorized?

Both the WHO and the CDC classify COVID-19 variants as either “variants of interest” or “variants of concern.” Both agencies have similar definitions for these terms:

  • A “variant of interest” is one in which genetic changes are known or predicted to affect the virus characteristics listed above. In addition, the variant is becoming more prevalent and has been identified as the cause of significant community transmission or case clusters. The WHO labels the mu variant as a variant of interest.
  • A “variant of concern” exhibits increasing transmissibility OR more virulence or a change in clinical presentation OR decrease in effectiveness of vaccines, testing, or treatments used in the disease. Both the WHO and CDC label the delta variant as a variant of concern.

The U.S. adds a third category of variants:

  • A “variant of high consequence” would be one for which there is clear evidence that the effectiveness of prevention or treatment measures are significantly reduced as compared with other variants, testing is producing regular false negatives for the variant, or more breakthrough infections are occurring. The U.S. hasn’t applied this label to any variants.

The classification of variants helps to direct our response. A variant of interest should trigger enhanced surveillance and epidemiological investigations; a variant of concern calls for stepped-up testing and mitigation efforts and ongoing research on the effectiveness of COVID-19 vaccines against these variants.

Testing for Variants

In theory, a new variant could elude testing. But as of today, all variants will reliably result in positive COVID-19 tests. (However, routine testing will not identify which variant has caused infection; genomic sequencing is needed for that.) As with all lab tests, we consider the test result along with the patient’s clinical presentation and history as well as the local COVID-19 epidemiology. When there is doubt about a negative test (for example, in a person who has symptoms that indicate COVID-19), a different manufacturer’s test, targeting different parts of the SARS-CoV-2 genome, may yield a positive result.

Viruses mutate.

The only way to slow this process is to limit transmission (and therefore, replication) of the virus. It has been demonstrated repeatedly in both clinical investigations and formal research studies that vaccination, masking, physical distancing, and measures to improve indoor ventilation are effective in limiting the spread of the virus.