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 their 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the first dose, and appeared 89% effective from 36 days after the second dose, according to the research, which has not yet been peer-reviewed.
More than 59 million vaccine doses have so far been administered in the US, according to data from the US Centers for Disease Control and Prevention.
But Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington, said Friday that the US is unlikely to achieve herd immunity for the virus before the winter.
"We know Covid is really seasonal, so when the next winter rolls around, we need to have a much higher level of protection to stop Covid in its tracks than we are likely to achieve," he said.
Herd immunity doesn't take effect until 80% or more of the population has immunity, either through infection or vaccination. And the new variants may complicate the picture, Murray said. If people can be reinfected with the new variants, the pandemic may take off again.
Though officials hope to have vaccines widely distributed by the end of the summer, President Biden said Friday that issues like weather, mutating strains and manufacturing delays make it hard to nail down a timeline.
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.
"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' spread, CDC director says
In the hopes of returning to some sense of normalcy, a priority to many families and officials has been allowing students to resume in-person learning.
And 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
Meanwhile, 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.
"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.
CNN's Christopher Rios, Amanda Sealy, Lauren Mascarenhas, Kevin Liptak, Maggie Fox, Nicholas Neville and Nicquel Terry Ellis contributed to this report.
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