(CNN) -- Health experts have said that data so far has shown that Covid-19 vaccines prevent symptoms of the virus -- but a new study suggests that the Moderna and Pfizer/BioNTech vaccines may also prevent infections.
A team at the Mayo Clinic health system looked at more than 31,000 people across four states who had received at least one dose of either vaccine -- and found the vaccines were upwards of 80% effective in preventing infection 36 days after the first dose.
Vaccine efficacy was 75% 15 days after the first dose, and it appeared to be 83% after a total of 36 days passed. But that number rose to nearly 89% when researchers examined only people who received two doses, according to the study, which has not yet been peer-reviewed.
While the trials that Moderna and Pfizer used to win Food and Drug Administration authorization for their vaccines looked for symptomatic infection, the Mayo researchers said they looked for both symptomatic and asymptomatic infection.
After harsh winter weather recently cut vaccine shipments and caused vaccine appointment postponements in parts of the country, US officials say states should be prepared to start making up for it this week.
The US had a backlog of about 6 million doses that couldn't be shipped because of bad weather this week.
"We're going to be shipping this week's doses and next week's doses" starting Saturday, Andy Slavitt, a senior adviser to the White House Covid-19 response team, told CNN on Friday.
"(Governors and states) are going to have to be ready. They're going to have to (facilitate) more appointments. They're going to have to extend their hours," Slavitt said.
Public health experts have been pressing for faster vaccinations, before more transmissible variants have a chance to spread, fearing they could reverse recent declines in daily new infections, hospitalizations and deaths.
The CDC has said an apparently more-transmissible variant first identified in the UK could be the dominant strain in the US by next month.
"This is why we're telling people to not stop masking, not stop avoiding indoor social gatherings quite yet, because we don't really know what's going to happen with this variant -- and we saw what happened last winter when we didn't take Covid seriously enough," Dr. Megan Ranney, and emergency medicine physician with Rhode Island's Brown University, told CNN Saturday.
The pace of daily Covid-19 vaccinations slowed somewhat in this past snowy week. The country averaged about 1.59 million shots a day over the last week as of Friday, against 1.65 million a day the week prior, according to CDC data.
As of Friday morning, more than 41.97 million people in the US had received at least one dose of a Covid-19 vaccine, and 17.03 million have received two doses, according to CDC data.
Too risky to give single doses, Fauci says
One way to protect more people quickly, some experts suggest, is to prioritize administering first doses of the vaccine.
Dr. Ashish Jha, dean of the Brown University School of Public Health, advocated Friday for US officials to consider delaying the second dose.
"Would that really be a problem, because if we could do that, we could vaccinate a lot more high-risk people, quickly...Everybody needs a second dose, but I think we can do it in a way that's still safe and get a lot more people protected," Jha told CNN's Poppy Harlow.
But Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said Friday that a single-dose plan would be too risky.
Fauci said he worried that if large numbers of people got a single shot and had less than optimal immune responses, they could be exposed to the virus and start incubating viral mutations. In theory, new variants could arise, he said.
"We will stick with the scientifically documented efficacy and optimal response of a prime followed by a boost with the mRNA vaccines," Fauci told a White House briefing.
Jha, for his part, said he agreed that everybody needed a second dose, "I think the question is, right now we wait four weeks between first and second dose. What if we went six weeks or eight weeks or 10 weeks -- not much longer than that."
School can reopen no matter the virus's spread, CDC director says
CDC Director Dr. Rochelle Walensky said Friday that given the right precautions, schools may open no matter how much virus is spreading in the community.
As of Tuesday, CNN analysis indicated about 90% of children live in so-called red zones under the CDC's guidance -- meaning there is a high level of community spread of the virus. But even in those conditions, schools can safely reopen if they take precautions, Walensky told a White House briefing.
The CDC has said schools can reopen if they make sure they are mitigating the risk of spread with universal mask use, measures to keep children and staff six feet apart, frequent cleaning and disinfection and testing and contact tracing.
The CDC director's assurances came as Fauci announced that the US should have vaccine safety data for high school-age kids by the beginning of the Fall.
Companies are just starting tests of younger age groups but have been testing their vaccines on 12- to 17-year-olds, Fauci told a White House briefing. Safety data for younger children will likely not be available until early next year, he said.
Vaccine hesitancy not an excuse for inequities, Fauci says
Vaccine trials and distribution have shed light on inequities in the medical field.
People of color have been vastly underrepresented in US-based vaccine trials for the last decade, according to a new study released Friday by researchers at Fred Hutchinson Cancer Research Center, Harvard, Emory and other institutions.
The study found that White people made up the majority, or 78%, of participants in trials conducted between June 2011 and June 2020.
The study, published in the JAMA Network Open, comes as the nation grapples with a pandemic that has disproportionately impacted people of color. Health care leaders are working to combat vaccine distrust among Black and brown people, saying the shot is the key to preventing further devastating in their communities.
But that hesitancy should not be an excuse for officials to explain away racial disparities in vaccination, Fauci said Friday.
"It's that we've got to really extend ourselves into the community to get the access to minority populations that they don't have," Fauci said in an interview with MSNBC.