New Data Boost Calls for Single COVID-19 Vaccine Dose

Damian McNamara

February 18, 2021

Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Editor's note: This article was updated February 19 to include an additional interview, new research, and a statement from the White House COVID-19 Response Team.

A single dose of the Pfizer/BioNTech vaccine offers 92.6% efficacy in new calculations based on data submitted to the US Food and Drug Administration (FDA), researchers report.

Together with previous findings that a single Moderna vaccine dose provides 92.1% efficacy against SARS-CoV-2 infection, investigators propose it is time to defer the second dose to extend protection to more people through single-dose mRNA vaccinations.

Dr Danuta M. Skowronski

"Deferral of the second dose of the COVID-19 vaccine will enable more individuals within our high-risk and health care worker priority groups to receive a single dose," Danuta M. Skowronski, MD, told Medscape Medical News. "Since a single dose provides swift and substantial protection exceeding 90%, and a second dose provides little added benefit in the short term, second dose deferral will maximize the benefits of scarce vaccine supply while the pandemic risk remains elevated."

Skowronski, epidemiology lead, Influenza & Emerging Respiratory Pathogens, British Columbia Centre for Disease Control in Vancouver, Canada, and Gaston De Serres, MD, PhD, of the Quebec National Institute of Health, Quebec City, Canada, authored a letter published February 17 in The New England Journal of Medicine.

In addition, correspondence in the Lancet February 18 appeared to further fortify the single-dose Pfizer vaccine strategy. Researchers in Israel reported that one dose was associated with 85% protection against symptomatic COVID-19 in an adjusted analysis of more than 9000 healthcare workers eligible for immunization.

Other experts disagree, however, saying more data are needed before diverting from the two-dose regimen evaluated in clinical trials. They also point out that the FDA granted emergency use authorization based on the two-dose studies.

Sticking With the Studies

Anthony Fauci, MD, said regulators would look carefully and seriously at the Israeli study results, but that for now the United States would stick with a two-dose strategy for the Pfizer vaccine.

"And the reason is even though you can get a fair degree of protection after a single dose, it clearly is not durable," Fauci said at a February 19 White House press briefing. "We know that the durability is not as much as the durability that you would get with the boost," he said.

He pointed out that durability of a single dose was not measured in the Lancet study because most of the participants eventually received a second dose of the vaccine. Fauci stressed that after the second dose the concentration of neutralizing antibodies made in response to the vaccine rises 10-fold.

Dr Dial Hewlett Jr

"One of the things we should consider here is that when these vaccines are tested in clinical trials, they are tested under specific circumstances. In this case, two doses were used to achieve the efficacy. The studies did not actually look at just giving one dose and trying to just go with that," Dial Hewlett Jr, MD, medical director of the Division of Disease Control at the  Westchester County Department of Health in White Plains, New York, said Thursday during a media briefing, sponsored by the Infectious Diseases Society of America.

Dr Matthew Zahn

Matthew Zahn, MD, who also spoke during the IDSA briefing, agreed. "The CDC has worked really hard at putting a premium on adhering to what the known science is, and the trials looked at two vaccine doses separated by less than 6 weeks.

"That's really why we've maintained that recommendation. From my side, I think that makes a great deal of sense," said Zahn, medical director in the Division of Epidemiology and Assessment, Orange County Health Care Agency, Santa Ana, California.

In their correspondence, the authors report they examined documents submitted to the FDA from 2 weeks after the first dose up to the second immunization. They note that "even before the second dose…[the vaccine] was highly efficacious."

Do Two Doses Extend Protection?

Even if efficacy from one dose exceeds 90% at first, how long that protection persists without a second dose remains an open question, said Hewlett, who is also a member of the National Medical Association COVID-19 Task Force on Vaccines and Therapeutics.

"We don't have any trial data saying the vaccines will be just as effective if you lengthen the time between the two doses," said Zahn, who also serves as a liaison representative to the CDC's Advisory Committee on Immunization Practices.

Other researchers reported an efficacy of 94.8% against SARS-CoV-2 after two doses of the Pfizer/BioNTech vaccine in a previous study. The same report estimated a single-dose efficacy of 52.4% between the first and second doses, "but in their calculation, they included data that were collected during the first 2 weeks after the first dose, when immunity would have still been mounting," note Skowronski and De Serres.

"It is a fundamental principle of vaccinology that longer intervals between first and second dose of vaccine generally results in overall higher and more durable response to the booster dose, so that delaying the second dose beyond the interval used in the RCT protocol would not be a concern," Skowronski said.

She added that "it remains important to administer a second dose," but the optimal time to administer the second immunization could be determined  "through ongoing field evaluation of vaccine effectiveness over the coming weeks and months."

"There may be uncertainty about the duration of protection with a single dose, but the administration of a second dose within 1 month after the first, as recommended, provides little added benefit in the short term, while high-risk persons who could have received a first dose with that vaccine supply are left completely unprotected," they note.

"It may be true that in the short run, one dose might be effective," Hewlett conceded, "but we don't know how long that this protection will last, and is the second dose going to be adding to that?" He explained that many public health officials want to simplify vaccine administration, but "before we can support this, we are going to have to have data that looks at this."

Skowronski and De Serres conclude their letter by stating that "given the current vaccine shortage, postponement of the second dose is a matter of national security that, if ignored, will certainly result in thousands of COVID-19-related hospitalizations and deaths this winter in the United States — hospitalizations and deaths that would have been prevented with a first dose of vaccine."

Skowronski has disclosed no relevant financial relationships. De Serres reported grant support from Pfizer for an unrelated study of meningococcal antibody seroprevalence.

N Eng J Med. Published online February 17, 2021. Correspondence

Lancet. Published online February 18, 2021. Full text

Damian McNamara is a staff journalist based in Miami. He covers a wide range of medical specialties, including infectious diseases, gastroenterology, and critical care. Follow Damian on Twitter:  @MedReporter.

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