By Christina Maxouris and Holly Yan, CNN

(CNN) -- First, the good news: The rates of new Covid-19 cases are dropping in 48 states. And every state improved its rate of vaccinations.

Here's the bad news: The US is still averaging more than 170,000 new infections a day. And Covid-19 has killed an average of 3,088 people a day over the past week in the US -- more deaths each day than the 9/11 attacks.

Now it's a race between vaccines and variants, as health experts say the US must speed up shots before a highly contagious strain of coronavirus spreads more rapidly.

Moderna's vaccine is expected to work against variants

While all viruses mutate over time, some variants of Covid-19 are worrying scientists.

One strain, first identified in the UK, is more transmissible than other variants, scientists say.

The US Centers for Disease Control and Prevention has warned that the US could see "rapid growth" in its spread in early 2021. This B.1.1.7 strain has already been detected in more than 20 US states.

And there's "a realistic possibility" that B.1.1.7 could be deadlier than other variants, a UK report said.

Another strain, first detected in South Africa, is concerning because scientists have said current Covid-19 vaccines might not be as effective against it.

That strain has been found in more than 20 other countries, though it has not yet been detected in the US.

Two doses of Moderna's Covid-19 vaccine are "expected to be protective against emerging strains detected to date," the vaccine maker said Monday.

There was "no significant impact" on the vaccine's effectiveness against the strain first found in the UK. But there may be somewhat less effectiveness against the strain first detected in South Africa.

"The efficacy might be reduced somewhat, but it may still be very effective," said David Montefiori, a virologist at Duke University Medical Center. "Hopefully the vaccine will still be 70-80% effective."

Moderna said it's developing a new Covid-19 booster vaccine to protect against the variant first spotted in South Africa. The company plans to first test the vaccine in the lab and in a small Phase 1 clinical trial in the US.

100 million doses in 100 days is 'a floor. It's not a ceiling'

President Joe Biden has promised 100 million shots will be given in his first 100 days in office -- a goal that has been criticized by some as too modest.

"We need to do better than a million shots per day," said Dr. Jonathan Reiner, professor of medicine at George Washington University.

"We need to do better. We need to vaccinate about 2 million people a day. That should be the goal."

But Dr. Vivek Murthy, Biden's nominee for US surgeon general, said the goal of a million doses a day is "a floor. It's not a ceiling."

"We've got to vaccinate as many Americans as possible," Murthy told ABC's "This Week" on Sunday.

"And that's going to take a lot of work -- work dispelling misinformation, working on the supply, increasing distribution channels," Murthy said. "And that's some of what the vaccine plan that (Biden) announced over the last week is intended to achieve."

Both the Pfizer/BioNTech and the Moderna vaccines require a second dose a few weeks after the first. And most of the shots given now are first doses, Reiner said.

"As we go forward, more and more shots every day will be the second vaccination," he said.

"So the number of new vaccinations is going to start to drop until we get to a point in the not too distant future where every day the shots that are given are 50% follow-up and 50% new vaccinations."

How to stop the spread of variants

Less than 1% of all Americans have received both doses of a Covid-19 vaccine, according to CDC data.

"We're looking at probably middle of the summer, end of the summer before the average, healthy, young American has access to vaccination," said infectious disease expert Dr. Celine Gounder.

And with the B.1.1.7 variant spreading in the US, it's up the public to take personal responsibility to stop it's spread.

"The best way to prevent the emergence of new variants is to do all of the things we've been talking about for months," Gounder said.

That includes keeping your distance from others and wearing a mask anytime you might be near someone who doesn't live with you.

"The more you let the virus spread, the more it mutates, the more variants you'll have," Gounder said.

Send us your questions for President Biden's Covid-19 team

California lifts regional stay-at-home orders

In a sign of progress, one of hardest hit states is lifting regional stay-at-home orders Monday.

"We are turning a critical corner," said Dr. Tomás Aragón, director of the California Department of Public Health.

The orders applied to Southern California, the San Joaquin Valley and Bay Area regions, affecting more than 90% of the state's population.

The mandate was triggered when ICU capacity in a region fell below 15%. The four-week ICU capacity projections for these regions are now above 15%, the state reported.

"Californians heard the urgent message to stay home as much as possible and accepted that challenge to slow the surge and save lives," Aragón said.

"Together, we changed our activities knowing our short-term sacrifices would lead to longer-term gains," he said.

"Covid-19 is still here and still deadly, so our work is not over, but it's important to recognize our collective actions saved lives."