The first dose alone of Pfizer-BioNTech's two-dose Covid-19 vaccine appears to generate a high level of immunity, according to a correspondence study published Thursday in The Lancet—a finding that may add to the debate about whether to delay the second dose so as to maximize limited vaccine supplies.

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Study finds vaccine produces strong immune response after single dose

In 2020, FDA issued an emergency use authorization for the two-dose vaccine developed by Pfizer and BioNTech based on clinical trial data showing the vaccine is 95% effective at preventing Covid-19 after patients receive two doses of the inoculation 21 days apart. The companies also submitted limited data suggesting the vaccine is 92.6% effective at preventing Covid-19 after the first dose is administered.

Now, new data from an Israeli study provides more evidence that the Pfizer-BioNTech vaccine is effective after a single dose. For the peer-reviewed study—published as a correspondence in The Lancet—researchers from the Sheba Medical Center in Israel examined real-world data on the effectiveness of the Pfizer-BioNTech vaccine collected outside of a clinical trial.

The researchers assessed the vaccine's effectiveness in preventing Covid-19, the disease caused by the novel coronavirus, using data on 9,109 health care workers who were eligible to receive the inoculation. The data included 7,214 health care workers who had received their first dose of the vaccine and 6,037 health care workers who had received their first and second doses.

Overall, the researchers found a 75% reduction in asymptomatic and symptomatic Covid-19 cases among health care workers within the 15 to 28 days after receiving the vaccine's first dose, when compared with unvaccinated health care workers. In terms of symptomatic cases alone, the researchers found an 85% reduction.

New data adds to the debate over whether second doses should be delayed

The latest evidence adds to the debate about whether it's safe and effective to delay a person's second dose of Covid-19 vaccines, particularly amid the United States' slow and rocky vaccine rollout and the emergence of new and more-contagious variants of the virus.

Specifically, officials are weighing whether the second dose of either of the two-dose vaccines currently authorized in the United States—one by Pfizer and BioNTech, and one by Moderna—should be delayed to allow more Americans to receive at least one dose of a vaccine, which might provide some level of protection against the virus.

However, it's not yet clear how robust that protection could be or how long it could last, experts say. And because of those uncertainties, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, last month said he "would not be in favor of that" for the United States. "We're going to keep doing what we're doing" in terms of how the United States' authorized coronavirus vaccines are administered, he said.

But the latest study adds new fuel to that debate, Eyal Leshem, director of Sheba's Center for Travel Medicine and Tropical Diseases and one of the study's authors, said. "This is the first study assessing effectiveness of a single vaccine dose in real-life conditions and shows early effectiveness, even before the second dose was administered," Leshen explained.

Separately, Arnon Afek, Sheba's deputy director general, added the findings "suppor[t] the British government's decision to begin inoculating its citizens with a single dose of the vaccine." According to the Wall Street Journal, Britain had decided to increase the timeframe between people's first and second doses of the Pfizer-BioNTech vaccine to three months, up from the originally recommended timeframe of three weeks.

However, Gili Regev-Yochay, an epidemiologist at Sheba Medical Center and one of the study's authors, said the findings should be viewed with caution because the study included "mostly young and healthy" health care workers, while data from Pfizer's clinical trial included a more diverse patient population. Moreover, she pointed out that the because most of the study participants eventually got the second shot, the study couldn't confirm how long the protection engendered by a single dose lasted.

For its part, Pfizer declined to comment on the new findings. However, Pfizer in a statement said it is conducting its own analysis of "the vaccine's real-world effectiveness in several locations worldwide, including Israel."

Currently, according to CDC data, the federal government as of Thursday morning had distributed about 73.4 million doses of the country's two authorized Covid-19 vaccines. Of those, about 57.7 million doses had been administered in the United States. That total includes about 41 million people who have received "one or more doses" of a vaccine and about 16.1 million who've received two doses, the data shows.

Where America's coronavirus epidemic stands

The latest findings on Pfizer's Covid-19 vaccine come as recent data indicates America's coronavirus epidemic has improved since last month's peak in reported cases, hospitalizations, and deaths. The reported rates of each of those metrics remain high, however, and data indicates that new and more transmissible variants of the novel coronavirus are spreading quickly throughout the country.

According to data compiled by the New York Times, U.S. officials on Thursday reported about 71,874 new coronavirus cases. As of Friday morning, officials had reported a total of about 27.9 million cases of the novel coronavirus since the United States' epidemic began.

According to the Times, the United States' average daily number of newly reported coronavirus cases over the past week was 72,848—down by 44% when compared with the average from two weeks ago, when the United States was in the midst of its worst peak in newly reported cases.

As of Friday morning, data from the Times showed that the rates of newly reported coronavirus cases were "staying high" in Colorado, New Hampshire, South Dakota, and Vermont, which have each reported a daily average of at least 15 newly reported cases per 100,000 people over the past week. In contrast, rates of newly reported coronavirus cases were "staying low" as of Friday morning in Guam, Puerto Rico, the U.S. Virgin Islands, and 14 states: Hawaii, Idaho, Illinois, Louisiana, Maine, Maryland, Michigan, Minnesota, Missouri, North Dakota, Oregon, Washington, Wisconsin, and Wyoming.

All other states and Washington, D.C., had been seeing comparatively higher rates of coronavirus transmission, but the daily average of newly reported coronavirus cases over the past seven days in those areas was "going down" as of Friday morning, according to the Times' data.

Experts say the recent drop in newly reported coronavirus cases is encouraging, but they also worry the United States could see another spike in cases, in part because new and more transmissible variants of the virus are spreading throughout the country. According to CDC data, the United States as of Thursday reported 1,549 total cases of novel coronavirus variants across 42 U.S. states, including 1,523 cases of the B.1.1.7 variant first identified in the United Kingdom, 21 cases of the B.1.351 variant first identified in South Africa, and five cases of the P.1 variant first identified in Brazil.

Meanwhile, U.S. hospitalizations for Covid-19 remained high as of Wednesday, but were down significantly from record highs reported last month. According to data from The Atlantic's COVID Tracking Project, there were 62,300 Americans with Covid-19 hospitalized for treatment on Thursday, including 13,045 who were receiving care in an ICU and 4,180 who were on a ventilator. Thursday marked the fourteenth consecutive day that fewer than 90,000 Americans with Covid-19 were hospitalized for treatment.

Similarly, the United States' rate of newly reported deaths linked to the novel coronavirus has declined over the past two weeks, though it also remains high. According to data from the Times, U.S. officials reported about 2,620 new deaths linked to the virus on Thursday. As of Friday morning, officials had reported a total of about 492,946 U.S. deaths linked to the virus since the country's epidemic began.

(Pancevski/Lieber, Wall Street Journal, 2/19; Williams, Reuters, 2/18; CDC variants data, updated 2/18; New York Times, 2/18; "COVID Tracking Project," The Atlantic, accessed 2/18).