“Please listen to me clearly: at this level of cases with spreading variants, we will completely lose the hard-earned ground we have gained,” said Dr. Rochelle Walensky, director of the US Centers for Disease Control and Prevention.

“Similarly, the most recent 7-day average death rate has increased more than 2% … to nearly 2,000 deaths per day.”

“I’m really concerned about reports that more and more states are rolling back the exact public health measures we have recommended to protect people from Covid-19,” she said.

“Please remain strong in your convictions. Keep wearing your well-fitting mask and taking the other public health prevention measures that we know work,” added Walensky.

“Ultimately, vaccination will get us out of this pandemic. To get there, we’ll have to vaccinate a lot more people.”

A third vaccine helps with mass vaccination

The good news this week: Johnson & Johnson’s single-dose vaccine is now available.

“There’s kind of a new urgency,” said Dr. Eric Rubin, professor at Harvard TH Chan School of Public Health.

“With new viral varieties emerging and some of them potentially becoming a little more resistant to vaccine protection, it is really important to get these to market quickly.”

Approximately 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 evening.

“These J&J cans will be delivered this Tuesday morning.”

As a one-dose vaccine, “people don’t have to come back to protect a second dose,” Walensky said.

“In addition, this vaccine does not need to be kept in a freezer and can be stored at refrigerated temperatures, making it easy to transport and store, and allowing for extended availability in most community and mobile locations as supply increases.”

Why Americans Shouldn't Turn Up Their Noses At Johnson & Johnson's Vaccine

The other two vaccines – one from Pfizer-BioNTech and one from Moderna – have efficacy rates of around 95% and offer even better protection against severe forms of the disease.

The Johnson & Johnson vaccine has shown a 72% effectiveness rate against mild to severe / critical illness in US study participants. It is more effective against severe forms of disease with about 85% protection.

Health experts say Americans shouldn’t be put off by J & J’s slightly lower numbers. The vaccine was tested later than the other two vaccines when infections were already rising sharply and new variant strains continued to spread.

The J&J vaccine was also tested in South Africa when the offending strain B.1.351 dominated, but it still offered strong protection against serious diseases.

Don’t be too picky about which vaccine you get

Health experts like Dr. Anthony Fauci said they would be happy to receive the Johnson & Johnson vaccine if it were the only one available as all three vaccines sold in the US are highly effective against severe Covid-19.Fauci: And if this is the only vaccine in your area, get it as soon as you are able, said CNN doctor Dr. Leana Wen on Monday.

“Think about the other vaccines we get. When we get the flu vaccine, no one asks the brand of the flu vaccine (or which company makes it.”

Also, “people will have no choice in the near future if supply is a limiting factor,” she said.

“Right now, the key is getting some kind of immunity. Get the vaccine that you have available first. You can always get another vaccine or booster later if it’s not about supplies.”

New variants continue to spread

March will be a very important chapter in this pandemic. The CDC has predicted that the highly contagious B.1.1.7 variant, first discovered in the UK, will be the dominant strain in the US this month. Researchers find worrying new coronavirus variant in New YorkAccording to the CDC data, more than 2,460 infections with variant strains have been reported. The vast majority of these cases – at least 2,400 – are from the highly contagious B.1.1.7 strain.

These numbers are likely much lower than the actual number of people infected with the variants. The numbers only represent the variant cases found by genomic sequencing, according to the CDC.

The US has lagged behind dozens of other countries in the percentage of cases tested for variants, but the CDC said it is working to step up those efforts.

Can vaccines ward off the new variants?

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

Moderna, Pfizer are testing vaccine strategies against new variants

First detected in South Africa, the B.1.351 strain has the most worrying effects on the ability of vaccines to elicit an immune response, said Dr. Heather Scobie at the CDC Advisory Committee on Immunization Practices (ACIP) meeting on Monday.

She also said two doses of the Covid-19 vaccine protect people better than just one dose.

“Five studies have shown that by postponing the second dose of mRNA, some people may be less protected against the SARS-Cov-2 variants,” Scobie told the ACIP meeting.

The Moderna and Pfizer / BioNTech vaccines are mRNA vaccines.

“All studies showed improved neutralization of B.1.1.7 and B.1.351 after the second dose of vaccine,” she said. “In some studies, people who had recovered from Covid-19 and received a dose of vaccine had moderate protection against B.1.351.”

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

“While we are encouraged and can rely on the current vaccine, we must always prepare for the future and, frankly, for the unknown,” said Gorsky.

Last week, Moderna said it developed a booster shot to fend off strain B.1.351, which has worried scientists because it has a mutation that could affect vaccine effectiveness.

This shot would serve as a booster for people who have already been vaccinated and as a primary vaccine for people who did not have the coronavirus and have not yet been vaccinated.

Without a global vaccination schedule, coronavirus variants could lead to countless deaths

Moderna is also testing a third, lower dose of its current vaccine in study participants to see if it would protect against troublesome variants.

Pfizer and BioNTech announced last week that they had begun testing how well a third dose of their approved vaccine would perform against new variants.

On Monday, Ugur Sahin, CEO of BioNTech, said it will take another six to eight weeks to have real-world data showing how effective the Pfizer BioNTech vaccine is against variant B.1.351.

Many more will need a vaccine prior to herd immunity

While the news of the third Covid-19 vaccine is worth celebrating, the U.S. is still a long way from herd immunity. Then enough people are protected from a virus that cannot spread to the population. About 15.3% of the US population have received at least one shot of their two-dose Moderna or Pfizer / BioNTech vaccine, according to CDC data available Monday.

Approximately 7.7% of Americans were completely vaccinated with both doses.

Track Covid-19 Vaccinations in the US

Vaccines are being tested in children

After the Johnson & Johnson vaccine is shown to be safe and effective in adults, the company will begin studies in children, said Dr. Macaya Douoguih, director of clinical development and medical affairs at J & J’s vaccine arm Janssen.The CDC does not know whether new variants cause rare complications in children

“We will conduct several immunogenicity and safety studies in children aged 17 and up through newborns,” Douoguih told a CDC advisory committee on Sunday.

“We hope the trial in adolescents will open next week. We also expect a trial in pregnant women in the second and third trimesters in late March to early April,” Douoguih said.

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

FDA emergency approval for the Johnson & Johnson vaccine is currently for adults 18 years of age and older.

The Pfizer / BioNTech vaccine was approved for people aged 16 and over, and the Moderna Covid-19 vaccine was approved for people aged 18 and over. Both Moderna and Pfizer have started enrolling children in Covid-19 vaccine studies.

CNN’s Jen Christensen, Fred Pleitgen, John Bonifield, Jason Hoffman, Michael Nedelman and Maggie Fox contributed to this report.