(CNN) A grim new forecast confirms what experts have been warning amid declining Covid-19 cases and hospitalizations: when it comes to the pandemic, the United States is not yet out of the woods.
Another 130,000 Americans are projected to die of the virus over the next three and a half months, according to the latest model from the University of Washington's Institute for Health Metrics and Evaluation.
And while Covid-19 numbers may be trending in the right direction now, there are four key factors that will determine how the next few months unfold, the institute said in a briefing accompanying its model.
The first two will likely help the pandemic numbers continue a downward trajectory: increasing vaccinations and declining seasonality -- the pattern of lower transmission that's likely in the US during the spring and summer months.
"Two factors, however, can slow or even reverse the declines that have begun," the IHME team said.
One of those factors is the spread of the B.1.1.7 variant, which was first identified in the United Kingdom and experts warned could become the dominant strain in the US by spring. Data from the US Centers for Disease Control and Prevention released Sunday shows more than 1,173 cases of the variant have so far been detected across 39 states.
Another key factor, according to the IHME team, is "increased behaviors that favor Covid-19 transmission."
"Transmission has been contained over the winter through mask wearing, decreased mobility, and avoidance of high-risk settings such as indoor dining," the team said. "As daily case counts decline and vaccination increases, behaviors are likely to change towards increased risk of transmission."
That's why experts say now is not the time for the US to let down its guard, even as a growing list of governors loosen Covid-19 restrictions.
CNN Medical Analyst Dr. Leana Wen said Sunday that declining numbers were good news. "But I'm also very worried about these variants, because we have seen in other countries what happens when there is explosive spread of these more contagious variants," Wen said on CNN's "Inside Politics."
"I think it's really critical for us to ramp up vaccinations as much as we can, and in the meantime, do our best to continue with masking, physical distancing -- these other measures that we know to be really important in controlling the spread of infection."
In addition to the B.1.1.7 variant, the CDC said Sunday 17 cases of the B.1.351 Covid strain initially seen in South Africa have been found across seven states and in Washington, DC. There are also two cases of the P.1 strain first linked to Brazil -- one in Minnesota and one in Oklahoma -- it said.
The agency said the data did not represent the total number of such cases circulating in the US but just those that have been found by analyzing positive samples.
Nearly a year on, the debate over how to safely return students to the classroom persists, even after the CDC released long-awaited guidance last Friday outlining five key strategies, including universal mask wearing.
On CNN's "State of the Union" on Sunday, CDC Director Dr. Rochelle Walensky emphasized the need for masks and other mitigation measures to safely reopen school. She said "mask breaching" was the main reason behind the spread of coronavirus in schools, though transmission in schools is mostly associated with spread in the surrounding community.
The CDC guidance includes a color-coded chart to describe the levels of transmission, from blue to red, with the latter representing high transmission. About 99% of the US population under the age of 18 -- 73 million minors -- live in a so-called "red" zone, according to a CNN analysis of federal data.
Walensky acknowledged "many" counties remain in a red zone. Schools in red zones should pursue a hybrid model if they're an elementary school, she said, while middle and high schools either continue virtual learning or a hybrid model if they're able to adhere to strict physical distancing.
Meantime, Dr. Wen said she found the CDC's guidance both "good and problematic."
She liked that the guidelines specifically defined levels of transmission for certain mitigation measures to go into place. But she took issue with other aspects, including the fact vaccinations are not listed as a "key" strategy for reopening schools. She didn't understand why the subject was up for debate.
"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, "especially as in so may 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."
Walensky noted Sunday the guidance from the CDC's vaccine advisory committee puts teachers in the Phase 1B category for vaccinations.
"I'm a strong advocate of teachers receiving their vaccinations," she said, "but we don't believe it's a prerequisite for schools to reopen."
Despite lingering concerns, officials are hopeful the continued ramping up of vaccinations is beginning to shift the pandemic's course in a positive direction.
So far, just over 38 million people have received their first dose of the two-dose vaccines available to the US market, according to CDC data released Sunday, and about 14 million people have been fully vaccinated. In all, nearly 53 million doses have been administered -- about 72% of all doses that have been distributed.
The IHME expects 145 million adults to be vaccinated by June 1, it said in a statement, which would prevent 114,000 deaths.
"Our vaccine supply is going up, the positivity rate is going down and we're getting one step closer to winning the war against COVID each day," New York Gov. Andrew Cuomo said in a statement Saturday, referring to New York's vaccinations.
By Sunday, the state had administered 89% of the first dose vaccines it had received from the federal government and 83% of first and second doses, the governor's office said.
In California, officials announced millions of people will be added to the vaccination priority list, including residents "at high risk with developmental and other disabilities" and residents with serious underlying health conditions. The plan, which will begin in mid-March, broadens the ages of eligible individuals from 65 and older to ages 16 through 64 who are in those categories.
Still, the state continues to face a "scarcity of vaccine," according to California Health and Human Services Agency Secretary Dr. Mark Ghaly. The supply shortage has forced several Los Angeles mass vaccination centers to temporarily close.
Some parts of the US this weekend are facing another challenge that's slowing down vaccinations: winter weather.
Federal officials expect Covid-19 vaccine shipments to Texas will be delayed this week because of a powerful winter storm, Texas Division of Emergency Management Chief W. Nim Kidd said.
"Our vaccines that are set to arrive on Sunday, Monday will probably not arrive until Wednesday, Thursday," Kidd said Saturday, "so we will see delays in vaccine coming into the state."
Some local outdoor vaccination facilities also shut down ahead of the storm, Kidd added, though indoor vaccination administration will continue "as long as it is still safe to drive there."
Alabama Gov. Kay Ivey declared a state of emergency Sunday ahead of anticipated winter weather. The state of emergency will include 28 counties, according to a press release from Ivey's office.
"If weather permits and roadways remain safe, Alabamians who have plans to receive their Covid-19 vaccine should still do so," the press release said.
In Portland, Oregon, where rain and snow have caused widespread power outages, the mass Covid-19 vaccination site at the Oregon Convention Center is closed Sunday due to weather, according to a tweet from Kaiser Permanente NW. Those who had appointments will be contacted to reschedule.
Washington state's Department of Health said it was also expecting a delayed delivery of vaccine doses due to the weather, in addition to announcing it would prioritize the administration of second doses this week, limiting the available number of appointments for first doses.
Vaccine providers in the state had requested about 170,000 doses this week, but the state was only allocated about 92,000, the department said in a news release. The difference was likely because last month some providers had used vaccines doses as initial doses when they were intended to be second doses.
That unfortunately means a portion of this week's available first doses will instead need to be used to complete the two-dose regimen for those individuals.