By Holly Yan and Christina Maxouris, CNN

(CNN) -- The US is at risk of losing all its recent gains in the battle against Covid-19 as highly contagious variants take advantage of Americans getting lax with safety measures.

"Please hear me clearly: At this level of cases with variants spreading, we stand to completely lose the hard earned ground we have gained," said Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention.

After weeks of tumbling case numbers, new infections are on the rise again -- about 2% more this past week compared to the previous week, Walensky said Monday.

"Similarly, the most recent seven-day average of deaths has also increased more than 2% ... to nearly 2,000 deaths per day."

Walensky also called out states that were easing up on Covid-19 safety mandates.

"I am really worried about reports that more states are rolling back the exact public health measures we have recommended to protect people from Covid-19," she said.

"Please stay strong in your conviction. Continue wearing your well-fitting mask and taking the other public health prevention actions that we know work," Walensky added.

"It is so critical that we remain vigilant and consistently take all of the mitigation steps we know work to stop the spread of Covid-19 while we work our way toward mass vaccination."

A 3rd vaccine will help with mass vaccination

The good news this week: Johnson & Johnson's single-dose vaccine will start being administered.

"There's a kind of new urgency to it," said Dr. Eric Rubin, a professor at the Harvard T.H. Chan School of Public Health.

"Because there are new viral variants coming out right now, with some chance that some of them could eventually become somewhat resistant to the protection provided by vaccines, it's really important to get it out there fast."

About 3.9 million doses of the J&J vaccine will be distributed to states, tribes, territories, pharmacies and community health centers, a senior Biden administration official said Sunday night.

"Those J&J doses will be delivered as early as this Tuesday morning."

As a one-dose vaccine, "people do not have to return for a second dose to be protected," Walensky said.

"In addition, this vaccine does not need to be kept in a freezer and can be stored at refrigerated temperatures. So it is easy to transport and store and allows for expanded availability in most community settings and mobile sites as supply scales up."

The other two vaccines being distributed -- one from Pfizer-BioNTech and one from Moderna -- both have efficacy rates of about 95%, with even greater protection against severe forms of disease.

The Johnson & Johnson vaccine has shown a 72% efficacy rate against mild to severe/critical disease among US trial participants. It's more effective against severe forms of disease, with about 85% protection.

Health experts say Americans should not be deterred by J&J's slightly lower numbers. Its vaccine was tested later than the other two vaccines, when infections were already surging and new variant strains were spreading more widely.

The J&J vaccine was also tested in South Africa when the troubling B.1.351 strain was dominant there, but it still gave strong protection against severe illness.

Don't be too picky about which vaccine you get

Health experts such as Dr. Anthony Fauci have said they would be happy to get the Johnson & Johnson vaccine if it were the only one available because all three vaccines getting distributed in the US are highly effective against severe Covid-19.

And if that's the only vaccine available in your area, you should go ahead and get it once you're able to, CNN medical analyst Dr. Leana Wen said Monday.

"Think about the other vaccines that we get. If we go in to get the flu vaccine, I don't think anyone is asking the brand of the flu vaccine (or) what company manufactures it."

Besides, "in the immediate future, people are not going to be getting a choice when supply is a limiting factor," she said.

"Right now, the key is to get immunity of some kind. Get whatever vaccine is first available to you. You can always get another vaccine or booster shot later on when supply is not the issue."

New variants keep spreading

March will be a very important chapter in this pandemic. The CDC has predicted the highly contagious B.1.1.7 variant first detected in the UK will become the dominant strain in the US this month.

More than 2,460 infections involving variant strains have been reported, according the CDC data. The vast majority of those cases -- at least 2,400 -- are of the highly contagious B.1.1.7 strain.

Those counts are probably much lower than the actual number of people infected by variants. The numbers represent only those variant cases found through genomic sequencing, the CDC has said.

The US has lagged behind dozens of other countries in the proportion of cases tested for variants, but the CDC said it's working to scale up those efforts.

Can vaccines fend off the new variants?

Johnson & Johnson, Moderna and Pfizer-BioNTech are all trying to make sure their vaccines get ahead of the variants.

Johnson & Johnson is working on a booster to help its Covid-19 vaccine deal with new strains of coronavirus variants, CEO Alex Gorsky said Monday.

"While we're encouraged and we're confident in the current vaccine that we have, you've always got to be preparing for the future and frankly for the unknown," Gorsky said.

Last week, Moderna said it has designed a booster shot to help stave off the B.1.351 strain, which has worried scientists because it has a mutation that might affect the efficacy of vaccines.

That shot would serve as a booster for people who have already been vaccinated and as a primary vaccine for people who haven't had coronavirus and have not yet been vaccinated.

Moderna is also testing a third, lower dose of its current vaccine in trial participants to see if that would protect against troubling variants.

Pfizer and BioNTech said last week that they've started testing how well a third dose of their authorized vaccine stacks up against new variants.

Many more need vaccine before herd immunity

While news of third Covid-19 vaccine is worth celebrating, the US is still a long way off from herd immunity. That's when enough people are protected against a virus that it cannot spread through the population.

About 15% of the US population has received at least one shot of their two-dose Moderna or Pfizer/BioNTech vaccine, according to CDC data Sunday.

About 7.5% of Americans have been fully vaccinated with both doses.

Track Covid-19 vaccinations in the US

Vaccines will be tested in children

Now that the Johnson & Johnson vaccine has proven to be safe and effective in adults, the company will begin studies with children, said Dr. Macaya Douoguih, head of clinical development and medical affairs with J&J's vaccine arm, Janssen.

"We will conduct several immunogenicity and safety studies in children from 17 years of age down to neonates," Douoguih told a CDC advisory committee Sunday.

"The study in adolescents, we hope, will open next week. We are also anticipating a study in pregnant women in the second and third trimesters toward the end of March, early April," Douoguih said.

Johnson & Johnson also plans to begin study in immunocompromised people in the third quarter of this year, Douoguih said.

The FDA's emergency use authorization for the Johnson & Johnson vaccine is currently for use in adults 18 and older.

The Pfizer/BioNTech vaccine has been authorized for people 16 and older, and the Moderna Covid-19 vaccine has been authorized for people 18 and older.

Both Moderna and Pfizer have started enrolling children in Covid-19 vaccine trials.