Dr. Anthony Fauci, the nation’s top infectious disease expert, issued a warning Tuesday regarding the fast-spreading delta variant of COVID-19.

“Similar to the situation in the U.K., the delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19,” Dr. Fauci said.

According to the Centers for Disease Control and Prevention, the variant now makes up over 20 percent of new cases in the United States. That is twice as much as it was in the previous CDC report.

The warning comes as a recent study revealed the Pfizer and AstraZeneca vaccines were effective against the variant.

“Conclusion: we have the tools, so let’s use them and crush the outbreak,” Fauci said.

The effectiveness of the vaccines has Fauci optimistic regarding a fourth large-scale surge in cases in the U.S.

“I don’t foresee what we refer to as a surge as we have known it, namely the three major surges that we’ve experienced over the past year and a half,” Fauci said. “I don’t think even under those circumstances that you’re going to see things like a thousand deaths a day.”

However, not everyone is safe from a case surge just yet.

“There is a danger, a real danger that if there is a persistence of a recalcitrance to getting vaccinated, that you could see localized surges,” Fauci said. “Which is the reason why I want to emphasize what all four of us have said, all of that is totally and completely avoidable by getting vaccinated.”

As Fauci continues his push to vaccinate, CDC advisers are set to meet to discuss reports of heart issues among younger people who have been vaccinated.

The CDC is tracking nearly 800 reports, although not all of them have been definitively connected to the vaccine. The symptoms in most of the cases have been mild.