(NEW YORK) -- Centers for Disease Control and Prevention Director Rochelle Walensky told lawmakers Tuesday the delta variant now makes up 83% of cases, up from 50% at the beginning of this month.
"CDC has released estimates of variants across the country and predicted the delta variant now represents 83% of sequenced cases. This is a dramatic increase from -- up from 50% for the week of July 3rd," she testified in a hearing before the Senate Health Committee.
Walensky said the alarming increase was happening the most in unvaccinated areas and that they were "allowing for the emergence and rapid spread of the highly transmissible Delta variant."
"In some parts of the country, the percentage is even higher, particularly in areas of low vaccination rates," she said.
The best way to stop the spread is with vaccines, she said.
"To date, our data indicates that vaccines are available to neutralize the circulating variants in the United States and provide protection against severe disease, hospitalization, and death," she said. "The message from CDC remains clear: the best way to prevent the spread of COVID-19 variants is to prevent the spread of disease and vaccination is the most powerful tool we have.”
And on the subject of whether booster shots -- which could offer extra protection against the variant for immunocompromised people -- will be recommended, Walensky and Food and Drug Administration acting chief Janet Woodcock said they don't yet have a timeline on an answer.
They were pushed multiple times by Republicans who argued Israel has already made the call to use boosters.
Sen. Mitt Romney, R-Utah, pressed Woodcock on when boosters might be available to people who are immunocompromised.
A CDC advisory panel is scheduled to discuss on Thursday what the research shows on that point. While no vote is planned, the discussion could pave the way for the FDA to alter its authorization of the vaccines to allow for booster shots for the immunocompromised.
“Why should we not allow people who, who are elderly or have other compromised conditions to be able to get that booster?” Romney asked, noting that Israel was allowing it.
“Certainly, we are looking at all that,” Woodcock responded.
“Remember this vaccine right now -- the vaccines are under emergencies use authorization and require an additional authorization for a booster,” she said.
“Well, how long is that going to take? That's the question,” Romney asked. "We have people who want to get that booster and I'm hearing that from people who are at risk and concerned… Why can’t they?"
Woodcock didn’t answer directly, but noted that Pfizer was submitting data "to potentially make the case" for a booster and that “the FDA will be looking at that.”
Romney responded: "I don't like the timeframe, frankly, given the fact that this is being done elsewhere."
Although the nation's COVID officials would not give a timeline on when Americans will need boosters, they detailed what they're looking for to make that decision.
Immunity will slowly decrease, not unexpectedly plummet, the top COVID experts said, and when vaccines get around 70-80% efficacy, boosters will be necessary.
"Fortunately, we're anticipating that this will wane and not plummet so as we see that waning, that will be our time for action," Walensky said.
Walensky said the CDC is following a handful of groups, including thousands of people in nursing homes, health care workers, and essential workers, testing them weekly. The CDC is also doing lab studies.
Woodcock also said while immunity will wane, it will not "vanish."
"It's waning, it isn't vanished, and that is clear because everybody who's getting hospitalized is unvaccinated and they're being exposed to delta variant. So, the fact is the vaccination is holding right now in the U.S.," Woodcock said.
GOP Sen. Bill Cassidy also pressed Woodcock on a more exact timeline.
Woodcock said there are people who don’t respond to shots in the first place because they are immunocompromised, but for the general population, it needs to be done only when necessary.
"We can't just boost them all the time, right?" she said. "We need to boost when it's appropriate."
Fauci said that if immunity wanes from the current efficacy for mRNA vaccines -- the Pfizer and Moderna shots -- down to around 70-80% efficacy from 93-94% efficacy -- he would expect boosters would be needed.
"We know, according to the clinical trial, take for an example, the mRNA, they are 93 to 94% effective in preventing clinically recognizable disease. If you see a fall below that, into the 80s or even unfortunately -- hope it never happens -- into the 70s, then you know you've reached the point where the durability needs a boost," Fauci said.
"Those studies are ongoing right now," he said.