Covid-19 variants could fuel another surge. Fauci says we have two powerful tools against them

(CNN)Covid-19 variants are threatening to cause another surge of infections, health officials warned Wednesday, and it's now more important than ever for Americans to continue practicing safety measures that work against the spread of the virus.

"The continued spread of variants that are more transmissible could jeopardize the progress we have made in the last month if we let our guard down," Centers for Disease Control and Prevention Director Dr. Rochelle Walensky said during a White House briefing.

But by lowering cases, the US will give "fewer opportunities for the variants to spread and fewer opportunities for new variants to emerge," she added.

It's a warning that's been repeated by multiple experts recently as the country's tally of variant cases keeps climbing while more state leaders are opting to loosen Covid-19 restrictions.

Among the most concerning strains is the highly contagious B.1.1.7 variant, first identified in the UK, that the CDC has previously warned could become "the predominant variant in March." The agency has reported more than 1,270 cases of the variant across 41 states and Washington, DC.

But the US has two powerful tools that can help against its spread and move the country further out of the pandemic's tunnel, Dr. Anthony Fauci told CNN on Wednesday night.

"The two ways that we can counter that is one, continue to do the kinds of public health interventions that we talk about all the time," he said.

That includes the universal wearing of masks, social distancing, staying away from crowded areas and regular hand washing.

"But also, as the weeks and months go by, as you vaccinate more and more people, you have a vaccine that works against this variant," Fauci added. "So if we roll out the vaccine ... and get as many people vaccinated as quickly as we possibly can while we maintain the public health measures, we should be in good stead."

It won't be easy, he cautioned, because of the variant's transmissibility -- which is why "we have to act quickly on that," Fauci said.

And lab studies offered more encouraging news this week, suggesting the Pfizer and Moderna vaccines can protect against Covid-19 variants.

When the general public will get a shot

But while the country continues to grapple with vaccination challenges -- including the frigid weather that has slowed deliveries this week -- many Americans who aren't currently in priority groups are wondering when they'll get their shots.

On Tuesday, President Joe Biden said 600 million Covid-19 vaccine doses would be available to the country by the end of July, "enough to vaccinate every single American." His promise echoes earlier announcements that the US was on track to have enough vaccines for 300 million Americans by the end of July.

But actually vaccinating all those people will likely take longer.

"(The vaccines will) be available in July," Fauci told CNN. "It may take an additional couple of months actually, maybe toward the end of the summer, to get everybody vaccinated."

Just how long it will take will depend on "the efficiency with which you get doses into people's arms," he added.

US and state leaders have in the meantime worked to boost their vaccination numbers and increase efficiency -- by measures like adding more vaccinators to their rosters, launching vaccine events, opening mass vaccination sites and creating partnerships with companies and pharmacies.

On Wednesday, Kroger announced its health care division, Kroger Health, has administered more than 380,000 Covid-19 vaccines across 25 states to groups including essential health care workers, elderly populations and educators.

About 1,300 Kroger Health pharmacies are receiving and administering shots, the release said, and once all 2,200 pharmacies begin getting vaccine doses, they will be able to administer nearly 500,000 vaccinations every week.

Kroger Health also announced a new scheduling tool for vaccinations, which can manage "more than 250,000 requests daily" and will allow people to check vaccine eligibility.

Here's how many people have been vaccinated

So far, according to CDC data, more than 40 million Americans have received at least their first dose of a Covid-19 vaccine. Both of the vaccines that have so far received a green light for the US market -- Pfizer and Moderna -- require two doses.

More than 15.4 million people have been fully vaccinated, CDC data shows. That's roughly 4.6% of the US population.

Between 75% to 85% of the US population would need to be vaccinated for the country to reach herd immunity, Fauci said, adding the percentages are "purely an estimate."

"You don't know what herd immunity really is until you reach a certain level and then when you get below it, then you start getting infections for the community," Fauci said. "We know exactly what it is for measles, we don't know what it is yet for (Covid-19). But I imagine it's somewhere between 75 to 85%."

Fauci also said he hopes that by the time vaccine doses become widely available for the American public, enough people will get in line to get a shot in order for the country to be able to reach the herd immunity percentages.

"That's the reason why we're putting a big push on getting out and extending ourselves into the community, particularly the minority community, to address what some people (would) refer to as vaccine hesitancy," Fauci said.

CDC director warns against travel

Meanwhile, amid the repeated warnings for Americans to continue taking safety measures, the CDC director also urged the American public to avoid travel, even domestically.

"Don't travel," Walensky said during a JAMA livestream Wednesday, highlighting the advice is especially important because of the circulation of variants. "We really, really would advocate for not traveling right now."

Walensky was asked about whether there would be any guidance or recommendations from the CDC on Covid-19 testing prior to domestic flights.

"We don't have the tests now to make that really easy and possible ... for people who are trying to travel," she said. "You shouldn't be traveling anyway."

She said the agency is asking people to abide by traveling guidelines like quarantining and testing afterward as variants circulate and the country works to scale up its testing capacity.

But the US has instead recently seen some of its busiest air travel days since the holidays.

The TSA reported it screened more than 967,000 people at airports on Monday and a further 738,000 people on Tuesday.

"If you choose to travel, please wear a mask," TSA spokesperson Lisa Farbstein reminded travelers in a tweet.

CNN's Michael Nedelman, Pete Muntean, Andrea Diaz, Rebekah Riess, Andrea Diaz, Amanda Sealy and John Bonifield contributed to this report.