Just when we were getting a handle on delta, here comes mu.

The World Health Organization this week spotlighted a new and worrying COVID-19 variant it says might be resistant to vaccines. The mu strain accounts for a tiny fraction of all coronavirus cases in the U.S. — Illinois has detected only 18, according to the outbreak.info database — but like the delta variant that has become omnipresent, mu has properties that could make it more transmissible.

So how worried should we be?

WHO pointed to Colombia, where mu was first detected in January and has since grown to account for 39% of that nation’s COVID caseload. The variant has also been found in 38 other countries.

Ramon Lorenzo-Redondo, research assistant professor of medicine at Northwestern University Feinberg School of Medicine, said Colombia’s number could be misleading. The country has done relatively few genetic examinations of its virus cases, he said, so the high percentage of mu might not reflect the true situation.

“The noise can still be pretty high,” he said. “Or it can mean that there are outbreaks indicating that this variant is emerging in the country in some local regions.”

But Elena Navas-Nacher, an epidemiologist from Colombia who founded the Chicago-based Global Health Beat Foundation, said the variant has taken a harsh toll on the country. When she visited this summer, she said, intensive care units were 90% full.

“The most recent peak been very lethal, collapsing the medical systems in various (cities), including Bogotá and Medellín, which are the largest and most sophisticated,” she said. “In small towns, it has been a disaster.”

The caseload has since plunged, which Navas-Nacher attributes to a vigorous vaccination program in the South American country. But she noted that Ecuador, Colombia’s next-door neighbor with a mu prevalence of 13%, has seen cases rise.

Globally, WHO said, mu has declined to the point where it makes up less than 0.1% of COVID-19 cases. But Dr. Mark Dworkin, associate director for epidemiology at the University of Illinois at Chicago’s School of Public Health, said that doesn’t mean the variant is a goner.

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“We can’t determine what’s going to happen in terms of a strain changing,” he said. “We are learning which strains outcompete (the others) and at which times they do. So it could be that mu will pick up. It could be that mu will never gain a strong foothold.”

State officials confirmed that mu has been found in Illinois, but said the consequences of its appearance remain unclear. They noted that the Centers for Disease Control and Prevention has not listed it as a “variant of concern.”

Delta is one of those concerning variants, and is a demonstration of how quickly a mutated form of the virus can worsen the pandemic.

The strain wasn’t detected in Illinois until mid-June and now accounts for nearly all new cases. Dr. Jonathan Pinsky, medical director of infection control and prevention at Edward Hospital, said delta is responsible for the state’s spiking caseload that began in midsummer.

Though delta can infect vaccinated people, very few end up severely ill or hospitalized, he said. If mu were to become more prevalent, he said, he expects a similar outcome.

“I’m really not concerned that we would see a problem with our vaccines with this variant, based on what we know about variants with similar mutations,” he said.

Despite delta’s prominence, Gov. J.B. Pritzker expressed cautious optimism at a Thursday news conference that the pandemic might be easing in Illinois.

“A few days strung together in a row could give you hope but not necessarily tell you what the trend is, so we’re watching these numbers closely,” he said. “Certainly over the last few days, when put together and averaged, it seems like things are flattening, and that’s a really good piece of news for all of us.”

Dr. Chethra Muthiah, an infection control specialist with NorthShore University HealthSystem, said the tried and true methods of curbing the spread — “getting vaccinated, masking indoors and continuing to follow public health guidance” — apply to the mu variant as well. But Lorenzo-Redondo said the strain’s appearance here shows that defeating the pandemic requires worldwide solutions.

“If we only vaccinate the U.S. or Europe, we won’t solve the problem because other variants will emerge in other parts of the world,” he said. “We need a global effort to end infections everywhere.”

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