CNN  — 

Millions of Americans have been vaccinated against Covid-19, and more are getting vaccinated every day. But that doesn’t mean the US can let its guard down, health officials say.

More than 100 million people in the United States have received at least one dose, according to a tweet from Dr. Cyrus Shahpar, the White House’s Covid-19 data director. Almost 4 million doses have been administered since Thursday, marking a record-high seven-day average of nearly 3 million doses per day, Shahpar said.

Almost 40% of American adults have received at least one dose, with more than 1 in 5 adults fully vaccinated, Dr. Rochelle Walensky, the director of the US Centers for Disease Control and Prevention, said in Friday’s White House Covid-19 briefing.

But she still has her concerns, she said.

“We are at 64,000 new Covid cases today and our numbers continue to increase,” Walensky said. “I still continue to worry that with 80% of the population unvaccinated, that we have a lot of work to do to control this pandemic.”

Walensky urged Americans to continue using the “mitigation strategies we know work,” like wearing masks and keeping a social distance.

“Getting more people vaccinated as quickly as possible and taking prevention measures to stop the spread of Covid-19 is the path out of this pandemic and back to our everyday activities,” Walensky said.

White House coronavirus coordinator Jeffrey Zients echoed that sentiment, telling reporters, “We are working to put this pandemic behind us as fast as we can, but we’re not there yet. So we need everyone to do their part.”

In a news conference Friday, Gov. Kate Brown of Oregon said cases were rising in the state and warned residents to stay vigilant.

“I know we’re all fed up with a pandemic, physical distancing and mask wearing, but we are in the last few miles of the marathon,” Brown said. “We have come so far, run so hard and we are close to the finish line. Don’t let the vaccines and the sunny spring weather give us a false sense that we’re in the clear, because we’re not.”

CDC: Travel is low risk if vaccinated

You have yet another reason to get vaccinated.

In new guidance Friday, the CDC said people who are fully vaccinated can travel at low risk to themselves, though nonessential travel is still not recommended.

So long as coronavirus precautions are taken – including wearing masks – fully vaccinated people can travel within the United States without first getting tested or quarantining afterward.

Fully vaccinated people traveling internationally do not need a Covid-19 test beforehand unless required by the destination, the CDC said. And they do not need to self-quarantine upon return to the US.

Travelers should have a negative Covid-19 test before boarding a flight back to the US, the CDC noted, and a follow-up test three to five days after their return.

According to the CDC, an individual is considered fully vaccinated two weeks after receiving the last required dose of a Covid-19 vaccine – whether that be the second dose of vaccines by Pfizer/BioNTech and Moderna, or the single dose required by Johnson & Johnson.

Anyone not fully vaccinated should continue to avoid travel, the CDC advised. If they must travel, they should get tested one to three days before and again three to five days after. They should also self-quarantine at home for seven days, or 10 if they did not get tested post-travel.

Additionally, all Americans should wear a mask and practice public health measures when traveling, regardless of their vaccination status.

The agency will continue to update its guidance as needed, Walensky said Friday, adding the “science on Covid-19 is constantly evolving.”

“With so many people still unvaccinated, it is important that everyone – regardless of vaccination status – continue to take prevention measures in public and adhere to our guidance on ways to reduce the spread of Covid-19,” Walensky said. “Wear a mask, socially distance, avoid crowds and poorly ventilated spaces, and wash your hands frequently.”

Vaccinations now can prevent future Covid-19 variants, surgeon general says

High on the list of factors worrying experts is the spread of coronavirus variants in the US.

Michigan on Thursday said it identified its first case of a variant that was originally found in Brazil, adding to reports of variants spreading across the US. That spread, along with relaxation of social distancing and mask mandates in many states, contributed to an influential model increasing its prediction of the number of people who will die of the virus by July 1.

According to US Surgeon General Dr. Vivek Murthy, while Covid-19 vaccines appear to provide protection against the identified variants of the coronavirus, vaccination is also important to prevent viral mutation in the future.

“The good news is that the vaccines that we have to date have proven to be both safe and effective, including against the variants,” Murthy told MSNBC on Friday. “Our concern is that in the future, some of those variants may be much more resistant to protection from a vaccine.”

He described the current situation in the US as a race between vaccines and variants. The potential development of coronavirus variants is a motivating factor to further increase vaccination rates, Murthy said.

“The more quickly we get people vaccinated, the more we’ll be able to lower the overall amount of infection in our community,” he said.

“When that infection is high, there are more chances for the virus to replicate, to mutate and lead to these variants.”

Pfizer vaccine protects for at least 6 months, but could last years

The protection offered by the Pfizer/BioNTech’s coronavirus vaccine remains high for at least six months, the companies said Thursday – but that’s “the floor, definitely not the ceiling,” according to Wen.

“Most likely the protection that the vaccine will provide will be years even,” she said. “But we just don’t know that.”

Wen added there was a possibility people may need a booster shot. “But I think that’s a small price to pay now that we have these safe and very effective vaccines that are out there.”

The protection from those vaccines will soon be available to many more people as states expand their eligibility. Zients, the White House coronavirus coordinator, said Friday that about 90% of adults across the US will be eligible for vaccination by April 19.

West Virginians 16 and older are now eligible to receive a vaccine, Gov. Jim Justice said Friday – just as active cases in the state have been increasing. Justice said the state had 6,642 active cases, with 420 new positive cases reported in the last 24 hours.

Alabama Gov. Kay Ivey on Friday announced all individuals 16 years and older will be allowed to get a Covid-19 vaccine beginning April 5. The state had initially said it was aiming for May 1 as the target date to have all eligible residents able to receive a shot.

“Truly, this vaccine is our ticket back to normal life,” Ivey said in a news release. “We are so close to getting COVID-19 in the rearview, and until then, we should all keep wearing our masks, get vaccinated and use the common sense the good Lord gave us.”

California to allow indoor gatherings and events

As the number of new infections in California fall, the state will allow indoor gatherings and private events, like weddings and conferences, with limitations, starting April 15, state health officials said on Friday.

Venues can have fully vaccinated sections without social distancing, but masks will still be required, California Department of Public Health Officer Dr. Tomás J. Aragón said. Other sections must continue to maintain the 6-feet social distancing requirement.

Attendees will have to go through a self-verification process, Dr. Aragón said, like bringing their Covid-19 vaccination card or showing negative test results within 72 hours of the event.

Venue capacity will be determined by the Covid-19 risk category an area is in. A capacity of up to 200 people will be allowed in the areas in the lowest-risk category, or tier, while the limit will be 100 for the next-to-highest tier.

“Today’s update to the Blueprint for a Safer Economy is a result of the progress we are making both in vaccinations and in controlling the spread of Covid-19,” said Dr. Mark Ghaly, secretary of California’s Health and Human Services Agency, in a statement.

CNN’s Melissa Alonso, Maggie Fox, Deidre McPhillips, Ryan Prior, Rebekah Riess, Melanie Schuman, Anna Sturla and Sarah Moon contributed to this report.