What Do We Know About the EU.1.1 COVID Variant?

The CDC is tracking a handful of new COVID variants, including the EU.1.1 variant, which now makes up an estimated 1.7% of U.S. COVID cases.

EU.1.1 is an XBB sublineage related to the XBB.1.5 variant that currently accounts for an estimated 27% of COVID cases in the country.

So how worrisome is this variant? Experts who spoke to MedPage Today said it's not time to raise the EU.1.1 alarm, but some caution is warranted.

Utah leads the U.S. with at least 97 EU.1.1 cases sampled, although Kelly Oakeson, PhD, chief scientist of the Utah Department of Health & Human Services, told MedPage Today that "Utah is doing more sequencing than, I think, everybody else in our region."

Oakeson pointed out that EU.1.1 does not appear have a big advantage in terms of transmissibility or severity. He said that "we're not seeing any kind of increases in hospitalizations or anything. It's some XBB [sublineage] that's picked up some more mutations in the spike protein, but nothing that's telling us that it's more severe, or more anything, than the other XBB sublineages that are circulating."

Rajendram Rajnarayanan, PhD, of the New York Institute of Technology and Arkansas State University in Jonesboro, told MedPage Today that EU.1.1 is "definitely more transmissible than the parent lineage, XBB.1.5, but it doesn't have any advantage over other circulating lineages right now." Rajnarayanan maintains a COVID-19 variant database. He said he saw the variant emerge in early June with an uptick in Utah cases.

Marc Johnson, PhD, of the University of Missouri School of Medicine in Columbia, said EU.1.1 "isn't setting off any red flags for me. It only has 2 amino acid changes from XBB.1.5 and neither are in particular hotspots," he wrote in an email to MedPage Today. In a lineage mutation report from outbreak.info, four locations on the sequenced genome for EU.1.1 are flagged as "Variant/Mutation of Interest."

"So far, it's nothing that we've seen to [be the] cause for concern or alarm," said Oakeson. "I think we're going see a lot of this happening, where you're going to see some lineages increase in proportion and then fade away and new ones come in and fade away."

Rajnarayanan said he thought there were already signs that EU.1.1 is slowing down. But he stressed that people who are immunocompromised, or vulnerable to COVID-19 in other ways, "should take the booster, as soon as it's available," referring to the XBB.1.5 boosters, which should protect against EU.1.1 as an XBB descendant.

"It's good for the season ... any booster is good for 4 to 5 months," he said. The FDA stated in June that it expects these booster shots to be available in the fall.

EU.1.1 has its origins in Germany and the Netherlands, and was named on the lineage designation software Pango. New, non-recombinant descendants of recombinant lineages (when two lineages combine) like XBB are assigned new alphabetical prefixes -- such as EU, as an "alias" for the name of the parental lineage -- if there are more than three numerical suffixes for their predecessor already. Aliases are assigned in order of available prefix letters. EU.1.1, then, is an alias of XBB.1.5.26.1.1 because the predecessor name was too long, and EU was the next alphabetical prefix available (after ES and ET, which are both descendants of the B lineage).

New non-recombinant lineage names are given only when they represent "one or more events of epidemiological significance," according to Pango, which could include mutations of biological interest or concern, or more transmissibility, immune escape, or severity.

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