Here’s why some Covid-19 numbers keep improving. (Hint: It’s not widespread vaccinations)

Kudos to everyone who has been doubling down on mask wearing and social distancing. Health experts say your efforts are paying off.

But with only 4% of the US population fully vaccinated, experts say Americans must continue safety precautions at full force to prevent highly contagious variants from undoing all the progress.

Researchers reported Sunday that they identified seven troubling new coronavirus variants circulating in the US, according to Dr. Wilbur Chen, an infectious diseases specialist at the University of Maryland Medical School.

“We certainly want to preserve the full efficacy of these vaccines by preventing more variant viruses, but again, these vaccines are effective and they continue to be useful,” Chen, who is a member of the US Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices, said.

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The US shattered its records for daily new infections, hospitalizations and deaths in early January after widespread holiday gatherings and travel.

Since then, Covid-19 cases and hospitalizations have plummeted. Doctors say there are several reasons why:

“One, we came off of really high numbers from the holidays,” said Dr. Ashish Jha, dean of the Brown University School of Public Health.

“Second, there is pretty good evidence that people are doing a better job of social distancing and mask wearing,” he said.

Pharmacist Jef Bratberg draws the Moderna Vaccine into syringes at Central Falls High School in Central Falls, Rhode Island, on February 13, 2021. - The city of Central Falls is currently offering Covid-19 vaccinations to any adult 18 years and older in contrast to the rest of the state which is following state guidelines and offering only to certain groups, frontline workers and people 75 years old and older. According to the US Census bureau, Central Falls is made up of over 66% Hispanic and almost 13% Black residents. "Everybody that lives or works here is a priority, if we don't fix the problem, where the problem is. We are just going to continue spreading this disease," said Mayor Maria Rivera. (Photo by Joseph Prezioso / AFP) (Photo by JOSEPH PREZIOSO/AFP via Getty Images)

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“Third, I think in a lot of communities, we’ve had so much infection that you have some level of population immunity. Not herd immunity, but enough population immunity that it is causing the virus to slow down.”

The US is still months away from having most Americans vaccinated. About 14 million Americans have been fully vaccinated with both doses of their Covid-19 vaccines. That’s only about 4% of the US population. And it takes weeks for vaccines to fully kick in.

But more than 11% of Americans have received at least one dose of a vaccine. And that might contribute slightly to the decreasing Covid-19 hospitalizations, said Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia.

“The doses that are out there … are mostly first doses, and that will provide incomplete immunity, short-lived immunity,” Offit said. “You need that second dose.”

In the meantime, increased masking – and wider acceptance of wearing masks – has driven Covid-19 numbers down, said Dr. Jonathan Reiner, professor of medicine and surgery at George Washington University.

“This is a consequence, probably, of a much better message uniformly distributed about masking in the United States,” he told CNN on Monday. “More people really than ever before in this pandemic are wearing masks.”

Where the US stands now

The seven-day average of daily new cases is now 90,416 – down from a peak of about 250,000 in early January, according to Johns Hopkins University.

As of Sunday, 67,023 people were hospitalized with Covid-19 – about half the record-high of 132,447 set on January 6, according to the COVID Tracking Project.

But Covid-19 deaths are still painfully high. More than 42,500 Americans have died from Covid-19 in just the past two weeks. That’s an average of more than 3,000 lives lost every day.

And disturbing variants keep spreading, threatening another surge.

“We’ve had three surges. Whether or not we have a fourth surge is up to us,” said former CDC Director Dr. Tom Frieden.

“And the stakes couldn’t be higher – not only in the number of people who could die in the fourth surge, but also in the risk that even more dangerous variants will emerge if there’s more uncontrolled spread.”

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That’s why health experts say state leaders shouldn’t ease restrictions such as mask mandates now.

“It’s encouraging to see these trends coming down, but they’re coming down from an extraordinarily high place,” CDC Director Dr. Rochelle Walensky told NBC on Sunday.

“If we want to get our children back to school, and I believe we all do, it all depends on how much community spread is out there,” she said.

“We need to all take responsibility to decrease that community spread, including mask wearing, so that we can get our kids and our society back.”

Variants in the US include homegrown strains

More than 1,100 cases of the highly contagious B.1.1.7 variant, first spotted in the UK, have been reported in 39 US states, according to the CDC. About a third of the cases were reported in Florida.

The troubling B.1.351 strain, first detected in South Africa, has been found in 17 US cases. That strain is concerning because some vaccines may be somewhat less effective against it.

This illustration, created at the Centers for Disease Control and Prevention (CDC), reveals ultrastructural morphology exhibited by coronaviruses. Note the spikes that adorn the outer surface of the virus, which impart the look of a corona surrounding the virion, when viewed electron microscopically.

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Now, researchers have identified a batch of worrisome mutations in US samples that also appear to make the virus more transmissible.

Those mutations all affect the same stretch of the spike protein – the knob-like extension of the virus that’s used to dock onto the cells it infects, researchers wrote in a pre-print report that has not yet been peer reviewed.

But so far, these mutations appear to be “relatively rare,” one of the researchers said.

There’s also an important lesson from the recent global drop in new Covid-19 cases.

In the past five weeks, “weekly reported cases (have) fallen by almost half,” said Tedros Adhanom Ghebreyesus, director-general of the World Health Organization.

“This shows that simple public health measures work – even in the presence of variants,” he said.

“What matters now is how we respond to this trend. The fire is not out, but we have reduced its size.”

States still struggle with vaccine supply

Public health experts say the US is now in a race to vaccinate as many Americans as possible before coronavirus strains keep spreading and mutating further.

But supply shortages continue.

Starting Thursday, CVS said it will be offering doses to people eligible in some of its stores in 11 states.

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In Washington state, officials said appointments for the first dose will be “extremely limited” this week as the state will focus on administering second doses.

“We are monitoring the distribution of doses closely and making adjustments as needed,” Secretary of Health Umair A. Shah said.

“While the limited availability of first doses will be challenging this coming week, focusing on second doses will help pave the way for an improved and more sustainable allocation of vaccines in future weeks.”

In San Francisco, officials said high-volume vaccination site will pause for a week and will reopen “once supply is sufficient to resume operations.”

A second high-volume site expects to resume vaccinations Friday – but only for second doses. A third high-volume vaccine site is set to launch this week, but “with available appointments far below full capacity,” officials said in a news release Sunday.

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“The City has the capacity to administer more than 10,000 vaccines per day but lack the vaccine supply,” they added.

Several of Los Angeles’ Covid-19 vaccination sites were forced to temporarily close because of a lack of vaccine doses.

Washington state and California are far from alone in their struggles. Officials have said supply will likely remain a challenge for a while, and experts say vaccines will likely not be widely available to the American public until late spring or summer.

“By the end of the summer, we will have enough vaccine in order to vaccinate the entire US population that is eligible,” Walensky, the CDC director, told Fox News on Sunday.

Debates over teacher vaccinations and school reopenings

About 89% of children in the US live in a county considered a red zone with high levels of Covid-19 transmission under new school opening guidelines shared by the CDC on Friday, according to a CNN analysis of federal data.

Red, or “high transmission,” communities are defined by the CDC as counties where there were at least 100 new Covid-19 cases per 100,000 people or a test positivity rate of at least 10% during the past seven days.

Emergency physician Dr. Leana Wen said teacher vaccinations are crucial for reopening schools – a stance that differs from the CDC’s school reopening guidance released last week.

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The agency’s guidelines did not list vaccination as a “key” strategy for opening schools, focusing instead on measures like masks and physical distancing, among others. Vaccinations for staff and teachers are “an additional layer of protection,” Walensky has said.

If schools in “high transmission” communities cannot “strictly implement all mitigation strategies,” the CDC says all extracurricular activities should be virtual. Plus middle and high schools should stick with virtual learning in these red zones, and elementary schools should maximize physical distance through hybrid learning or reduced attendance.

On Sunday, Walensky told CNN that while vaccination for teachers is not a prerequisite for reopening schools, current CDC guidance does say those who are at higher risk should have virtual options.

“I’m a strong advocate of teachers receiving their vaccinations,” Walensky said. “But we don’t believe it’s a prerequisite for schools to reopen.”

Wen, however, called teacher vaccinations “essential.”

“If we want students to be in school for in-person learning, the least that we can do is to protect the health and well-being of our teachers,” Wen said.

She said teacher vaccinations are especially important because “in so many parts of the country, teachers are already being made to go back to school in poorly-ventilated, cramped areas, with many students who may not always be masking and practicing physical distancing.”

CNN’s Jason Hanna, Amanda Watts, Keith Allen, Ben Tinker, Michael Nedelman, Maggie Fox and Naomi Thomas contributed to this report.