Public health experts were among those who urged cautious optimism on Monday as pharmaceutical giant Pfizer announced that the first formal review of its coronavirus vaccine showed the vaccine is more than 90% effective at preventing the virus in trial volunteers who had never before had Covid-19. 

Pfizer shared the news in a press release—not in a peer-reviewed medical journal, noted consumer advocacy group Public Citizen—amid its ongoing 44,000-person trial, which it is conducting with German drug manufacturer BioNTech. The company said no serious safety concerns have arisen from the trial so far, but it needs two months of safety data in order to apply for emergency authorization from the Food and Drug Administration (FDA). Currently Pfizer hopes to apply for the authorization later this month, which it says would allow it to manufacture enough of the vaccine to immunize 15 million to 20 million people by the end of the year. 

"For the foreseeable future, a mask remains a more potent weapon against the virus than the vaccine."
—President-elect Joe Biden

Considering Pfizer's method of disseminating its data to the public, Public Citizen said Monday that "enthusiasm" for the potential vaccine "must be tempered."

"The release of preliminary and incomplete clinical trial data by press release to the public is bad science," said Michael Carome, director of the organization's health research group.  

"Until the trial results are independently reviewed and scrutinized by staff at the U.S. Food and Drug Administration and the independent experts on the agency's Vaccines and Related Biological Products Advisory Committee, enthusiasm for the apparently promising interim results announced by Pfizer and BioNTech must be tempered," Carome continued. "Crucial information absent from the companies' announcement is  any evidence that the vaccine prevents serious Covid-19 cases or reduces hospitalizations and deaths due to the disease."

Public health experts expressed shock at the news of the vaccine's apparent efficacy, with Yale University immunologist Akiko Iwasaki telling the New York Times that she'd been expecting current vaccine trials to achieve "something like 55%" effectiveness."

But a number of public health experts cautioned Americans not to interpret Pfizer's news as evidence that the public can ease up on compliance to guidelines urging social distancing, mask-wearing, and other measures to prevent the spread of Covid-19.

"We need to see the actual data, and we're going to need longer-term results," Jesse Goodman, a professor of medicine and infectious diseases at Georgetown University, told the Times.

Dr. Tom Frieden, former head of the Centers for Disease Control and Prevention (CDC), called Pfizer's news "encouraging" and listed a number of caveats about the data released so far. 

Regardless of how successful the vaccine is found to be, other experts said, with Covid-19 cases surging across the country and public health officials warning that the coming winter will bring an estimated one million cases per week in the U.S. by the end of the year, Americans should double down on their commitments to wearing face coverings; avoiding crowded places, particularly indoors; and social distancing.

President-elect Joe Biden released a statement calling Pfizer's announcement "excellent news" but echoing the call by public health experts. 

Regardless of the outcome of Pfizer's trial, Biden said, "it will be many more months before there is widespread vaccination in this country."

"This is why the head of the CDC warned this fall that for the foreseeable future, a mask remains a more potent weapon against the virus than the vaccine," he added. 

Covid-19 has killed more than 238,000 people in the U.S. this year, and has infected 10 million people so far. Over the past week, an average of 111,175 new infections have been confirmed per day, a 59% increase in average cases from two weeks ago. 

On Sunday, officials in El Paso County, Texas said they expect to soon have 10 mobile morgue units, up from four that were set up last week, to cope with the area's spike in coronavirus deaths. 

The number of Americans hospitalized with Covid-19 stood at about 56,000 on Monday, up from 33,000 a month ago, according to Politico. 

"If you want to have a better 2021, then maybe the rest of 2020 needs to be an investment in driving the virus down," Dr. Cyrus Shahpar, a co-founder of the website Covid Exit Strategy, told the outlet. "Otherwise we're looking at thousands and thousands of deaths this winter."