The Epsilon variant is a foe we've met before.
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"It is the one that originated here in Southern California that we discovered," said research scientist Jasmine Plummer with Cedars-Sinai Medical Center.
When she and her colleagues identified this dominant variant during the relentless winter surge it was known as B.1.429, or the California variant.
It was far more infectious than its predecessors.
Now in a new report, University of Washington researchers have unlocked its secrets.
"Mechanistically, it's really important to understand these mutations," Plummer said.
Scientists identified three changes in the spike proteins of the Epsilon variant. One affected the receptor binding domain. The other two mutations affected the N-Terminal domain. Researchers say these mutations can weaken our current vaccines by up to 70% and side-step the naturally produced antibodies a person forms after being infected with COVID-19.
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But scientists saw all this in a lab setting. In real life, the human immune system has far more defenses than just antibodies and Plummer said vaccines make that possible.
"Let's learn from the Epsilon variant," she said. "When we were able to get vaccinated, most of our cases were carrying this and we were able to kind of diminish it."
"In unvaccinated populations, either pockets of people or even states that just aren't as vaccinated as other states, we are seeing this Delta variant take hold," said Kaiser Permanente infectious disease specialist Dr. David Bronstein.
Bronstein said these findings show us which variants to target for a booster, but the vaccines we have now are holding up and the path to end this pandemic is clear.
"Rather than worrying about whether to boost or not, let's make sure everybody gets the first dose of the vaccine and then complete the series," he said. "That really needs to be our primary focus right now."