Experts cautioned
that more research is needed, but the results do provide some evidence that the second dose could be delayed.
Feb. 19, 2021,
4:16 PM CST By Denise
Chow
The Covid-19 vaccine developed by Pfizer and BioNTech
produces a robust immune response after just one dose, according to a new
Israeli study of vaccinated health care workers at the country's largest
hospital.
The research, published Thursday in the medical
journal The Lancet, followed
7,214 staff members at Israel's Sheba Medical Center, a government-owned
facility, who received their first dose of the Pfizer-BioNTech vaccination
between Dec. 19 and Jan. 24. Scientists from the medical center found that the
vaccine was 85 percent effective at preventing symptomatic Covid-19 within 15
to 28 days after the shot was administered.
Experts cautioned that more research is needed before broad
conclusions can be drawn, but the results do provide some evidence that robust
immunity is generated after one dose and that the second dose could be delayed
beyond the three weeks prescribed by Pfizer in order to ease distribution and
supply constraints.
The timing of the second dose has been the subject of much
recent debate, with some countries such as the United Kingdom opting to delay
it as a way to speed up the country's rate of immunization. In the United
States, where the vaccine rollout has been bumpy and winter storms over the
past week have hampered some states' ability to
administer shots, similar questions have emerged.
19,
Dr. Jonathan Temte, a vaccine expert at the University of
Wisconsin School of Medicine and Public Health who was not involved with the
Israeli study, said any sweeping changes to immunization recommendations would
require more data, but that the results of the new study are encouraging.
"It does provide reassurance that delays for whatever
reason, be it the weather or problems in the supply chain — it provides some
comfort knowing that those individuals who have received the one dose do
achieve a good level of protection," he said.
Official guidance from the Food and Drug Administration and
Pfizer-BioNTech state that the two doses should be administered 21 days apart
based on the results of clinical trials. For the Moderna vaccine, the only
other vaccine currently authorized for use in the U.S., the prescribed interval
between the two shots is 28 days.
But vaccine supplies are limited and different strains of
the virus are circulating around the country, putting pressure on states to
quickly vaccinate as many people as possible. Some have questioned whether
there is flexibility in the timing of the second dose.
Last month, the Centers for Disease Control and
Prevention updated its guidance to
say that the second shot should be administered within the prescribed time
frames where possible, but that the second dose of the Moderna and the
Pfizer-BioNTech vaccines could be delayed up to six weeks,
if necessary.
Pfizer has said it has not yet studied changes to the timing
between doses, and the drugmaker has maintained that any deviations to the
dosing schedule is up to health authorities.
In the new Israeli study, researchers reported 170 Covid-19
infections among health care workers at the Sheba Medical Center between Dec.
19 and Jan. 24. Of those, 78 people tested positive after receiving the first
dose and three individuals tested positive after receiving the second dose.
Since the vaccines are not 100 percent effective, it is expected that a small
number of people could contract the virus even after being fully vaccinated.
Early reductions of Covid-19 rates provide support of
delaying the second dose in countries facing vaccine shortages and scarce
resources, so as to allow higher population coverage with a single dose,"
the researchers wrote in the study.
They acknowledged, though, that testing limitations may have
resulted in undercounted asymptomatic cases and that follow-up research is
needed to assess the long-term effectiveness of a single dose.
The findings provide some of the first real-world data on
the effectiveness of a single vaccine dose. Israel's vaccination program has
far outpaced any other country, and more than 30 percent of the country's 9
million residents have already received both doses.
Temte said that while early results from Israel's vaccine
program are positive, there are still too many unknowns about a single dose's
efficacy beyond three to six weeks.
"Until such time that there are good clinical trials
that show that a single dose provides an equivalent level of protection, I
don't know that we should abandon our approach or create new policies," he
said.
It's also not clear how one dose will perform
against different coronavirus strains that are becoming more
widespread, including separate strains that were first reported in the U.K.,
South Africa and Brazil.
"We're running into uncharted territory," said
Deepta Bhattacharya, an associate professor of immunobiology at the University
of Arizona, who was not involved with the study. "The uncertain issue is
how long that protection is going to last against the current variants, as well
as some of the ones that might get selected for if you wait too long."
Bhattacharya said delaying the second dose up to six weeks,
in line with the CDC's guidance, seems reasonable. But beyond that, it may be
too soon to tell, which means health officials may be faced with tricky
decisions.
"We're stretching the limits of what we're able to
see," he said. "We’re sort of at a point now where we have to make
decisions based on imperfect evidence."
Denise Chow is a reporter for NBC News Science focused on
general science and climate change.
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