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One dose of Pfizer’s COVID-19 vaccine offers some protection. But experts say you still need the second shot

A vial of the Pfizer/BioNTech COVID-19 vaccine concentrate is diluted with 1.8ml sodium chloride ready for use at Guy's Hospital in London on Tuesday.VICTORIA JONES/POOL/AFP via Getty Images

Some bright news amid the gloom of the pandemic: A single dose of the coronavirus vaccine developed by Pfizer and its German partner, BioNTech, confers some protection against COVID-19, according to a new analysis of the clinical trial data by the Food and Drug Administration.

But infectious disease experts caution that doesn’t mean people should skip or delay their second shot.

“This vaccine, if it is approved, will be approved to give it as a dose and then to give the second dose three weeks later,” said Dr. Paul Offit, an attending physician at Children’s Hospital of Philadelphia and member of the FDA’s vaccines advisory committee. “It [will be] approved as a two-dose vaccine — not as a one-dose-and-then-hope-you-get-that-second-dose-in-time vaccine.”


Pfizer’s vaccine requires two injections, spaced three weeks apart. The vaccine has an efficacy rate of 52 percent between the first and second dose, according to the data assessed by the FDA on Tuesday. The efficacy of the vaccine climbs to 95 percent seven days after the second dose.

Vaccine “efficacy” is a measure of how well vaccines work in controlled clinical trials, versus “effectiveness,” which gauges real-world performance in less-than-ideal conditions. The efficacy of Pfizer’s two-dose regimen indicates that COVID-19 symptoms were prevented in 95 percent of trial participants who received the vaccine, compared with those who received the placebo.

Moderna’s vaccine, which is also awaiting FDA authorization, has an efficacy rate of 94.5 percent, according to its trial data, and requires two doses, spaced four weeks apart.

A panel of independent experts known as the Vaccines and Related Biological Products Advisory Committee will vote in a public meeting this Thursday on whether to recommend Pfizer’s vaccine for emergency-use authorization. The panel will meet again a week later to discuss Moderna’s vaccine candidate.

Before the end of the year, the United States expects to receive 35 million to 40 million doses of vaccines from Pfizer and Moderna, enough to inoculate up to 20 million people with two doses.


But some public health authorities have advised against this plan. Former FDA commissioner and Pfizer board member Scott Gottlieb told USA Today the federal government should focus on vaccinating as many people possible — 35 million to 40 million people — and presume that drug manufacturers will be able to make enough vaccines in time for the second injection.

“The idea that we need to cut [the doses] in half and give half of it now and hold onto it, so we have supply in January to get the second dose… I just fundamentally disagree with that,” Gottlieb said in an interview with USA Today Monday.

Offit dismissed the idea as unwise, cautioning about gaps in vaccine availability.

“If you say, ‘Well I hope I get 40 million doses three weeks from now,’ because Pfizer data were all based on giving dose one and then giving dose two three weeks later, and it’s not three weeks later, but four weeks later or five weeks later or two months later, then you’re basically doing something for which we have no data,” Offit said.

“You’d be making up your vaccine strategy for which you have no data,” he added, “and there’s always a risk in that.”

Dr. Helen Boucher, chief of infectious diseases at Tufts Medical Center, warned against drawing conclusions on the potential efficacy of a single dose of Pfizer’s coronavirus vaccine, based on the clinical trial data. It’s also unknown, she added, how long protection after one dose would last.


“We don’t know if that response, that observation that we see, would last for three weeks, three months, a year, because what we know is everyone got a second dose, a booster dose,” she said.

“It’s still very interesting and hopeful to see that potential efficacy of the first dose,” she continued, “and I’m sure it will be studied further, but it will need to be studied further before we can make any conclusions.”

Pfizer chief executive Albert Bourla also warned against skipping the second dose in a briefing Tuesday with the International Federation of Pharmaceutical Manufacturers and Associations.

“This is a two-dose vaccine, so people need to take two doses to be able to feel confident that they’re protected at 95 percent,” Bourla said, according to CNN.

“It’s a very big mistake if anyone tries to do it with only one dose,” he continued “when… with two, you almost double the protection.”

Deanna Pan can be reached at deanna.pan@globe.com. Follow her @DDpan.