Sep 9, 2021 - Science

How the Delta variant dominates Mu

Illustration of a delta symbol casting a shadow against a smaller mu symbol.

Illustration: Shoshana Gordon/Axios

The Mu variant of the coronavirus is something to monitor — as it appears to partially evade immunity from authorized COVID-19 vaccines — but Delta's continued dominance means "Mu is not any immediate threat," NIAID director Anthony Fauci tells Axios.

Why it matters: Sounding the alarm, Fauci says widespread vaccination is a priority to fight the coronavirus and cut down on the rate of new infections — which is currently 10 times higher than where it needs to be.

What's happening: The WHO recently labeled Mu (B.1.621 and first discovered in Colombia) a variant of interest as preliminary data indicated it may better elude immunity from prior infection or vaccination.

  • Mu may "indicate potential properties of immune escape, as it has some of these hallmarks of being able to get around that existing vaccine protection, but it doesn't mean that's what we're seeing play out in real life," says Anne Rimoin, an epidemiologist and director of the UCLA Center for Global and Immigrant Health.
  • Fauci says when comparing how specific variants may escape antibody protection, Beta appears more evasive than Mu, and Mu more than Delta.
  • Only about 0.5% of new cases are showing as Mu, Fauci says, with 99.3% testing as Delta, which has such an "extraordinary ability" to transmit that it won't likely lose its global dominance in the immediate future.

Between the lines: While some have raised concerns it may be time to look at updating the vaccines to better match these variants, Fauci says that's not needed as the current vaccines "do very well against the Delta variant."

  • "Even though the vaccine wasn't specifically [designed] against Delta, there's enough cross-protection against the different variants once you get the antibody level high enough," he says.
  • But a third dose may be needed to "extend that immunity," Rimoin says.

Yes, but: If a new variant came along that evaded antibodies and was transmissible enough to dominate Delta, scientists would take quick action, Fauci says.

  • The mRNA vaccine platform enables the development of a new version of the vaccine within roughly three months, and the FDA is expected to treat these similarly to the flu shot, as a "strain change" that can be approved relatively quickly, he adds.
  • Gigi Gronvall, an immunologist and senior scholar at the Johns Hopkins Center for Health Security, says the FDA is close to deciding what the correlates of immunity should be for COVID. These would give an indication of the minimum amount of antibodies, T-cells and other immune responders that a vaccine should generate.

"The biggest problem — the 10 elephants in the room — is the fact that we need to get people vaccinated, so we don't have people dying as much every day," Gronvall says.

  • "The biggest push has to be to get transmission down," she says, "because the unvaccinated are creating problems for the vaccinated, and they're dying, basically, of a vaccine-preventable disease."
  • Fauci says the U.S. remains in a pandemic because it doesn't have "even modestly good control," with 160,000 new cases a day when it needs to be below 10,000 a day.

The big picture: Global vaccination is key to halting this pandemic — to protect every individual from severe disease or death, to lessen the probability of long COVID, and to prevent a monster variant from developing.

  • After all, Rimoin says, globalization has ensured that "an infection anywhere is potentially an infection everywhere."

Read more from the interview with Fauci here.

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