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 professionals at the country’s largest hospital.
The research, published on Thursday in the medical journal The Lancet, followed 7,214 staff members from the Sheba Medical Center, a government facility in Israel, who received their first dose of the Pfizer-BioNTech vaccination between December 19 and January 24. Scientists at the medical center found that the vaccine was 85 percent effective in preventing symptomatic Covid-19 within 15 to 28 days after the injection was administered.
Experts have warned that more research is needed before broad conclusions can be drawn, but the results provide some evidence that a robust immunity is generated after one dose and that the second dose may be delayed beyond the three weeks prescribed by Pfizer to facilitate delivery and supply restrictions.
The timing of the second dose has been the subject of very recent debate, with some countries, such as the United Kingdom, choosing to postpone it as a way to speed up the country’s immunization rate. In the United States, where the vaccine’s launch was turbulent and last week’s winter storms hampered the ability of some states to administer the vaccines, similar issues have arisen.
Dr. Jonathan Temte, a vaccine specialist at the University of Wisconsin School of Medicine and Public Health who was not involved in the Israeli study, said that any radical changes in immunization recommendations would require more data, but that the results of the new study are encouraging .
“This provides assurance that delays for any reason, be it the weather or problems in the supply chain – it provides some comfort knowing that individuals who received the single dose achieve a good level of protection,” he said.
Official guidance from the Food and Drug Administration and Pfizer-BioNTech states 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 USA, the prescribed interval between the two vaccines is 28 days.
But vaccine supplies are limited and different strains of the virus are circulating across the country, putting pressure on states to rapidly vaccinate as many people as possible. Some questioned whether there is flexibility at the time of the second dose.
Last month, the Centers for Disease Control and Prevention updated their guidelines to say that the second injection should be administered within the prescribed time limits, whenever possible, but that the second dose of Moderna and Pfizer-BioNTech vaccines can be postponed until six weeks if necessary.
Pfizer said it had not yet studied changes in the interval between doses, and the drugmaker said that any deviations in the dosing schedule depend on health authorities.
In the new Israeli study, researchers reported 170 Covid-19 infections among healthcare professionals at Sheba Medical Center between December 19 and January 24. Of these, 78 people tested positive after receiving the first dose and three subjects tested positive after receiving the second dose. Since vaccines are not 100 percent effective, it is expected that a small number of people will be able to contract the virus, even after being fully vaccinated.
The study also found a 75 percent reduction in symptomatic and asymptomatic infections shortly after the first injection. The scientists said these reductions suggest that one dose provides an immune response robust enough to justify postponing the second injection.
“Early reductions in Covid-19 rates provide support for the postponement of the second dose in countries facing shortages of vaccines and scarce resources, to allow greater coverage of the population with a single dose,” wrote the researchers in the study.
They recognized, however, that test limitations may have resulted in underestimated 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 dose of vaccine. Israel’s vaccination program has far surpassed any other country, and more than 30 percent of the country’s 9 million residents have already received both doses.
Temte said that while the first results of Israel’s vaccine program are positive, there are still many unknowns about the effectiveness of a single dose beyond three to six weeks.
“Until such time as there are good clinical trials showing that a single dose provides an equivalent level of protection, I don’t know whether to abandon our approach or create new policies,” he said.
It is also unclear how one dose will work against different strains of coronavirus that are becoming more widespread, including separate strains that were first reported in the UK, South Africa and Brazil.
“We are entering unknown territory,” said Deepta Bhattacharya, an associate professor of immunobiology at the University of Arizona, who was not involved in the study. “The uncertain question is how long this protection will last against current variants, as well as some that can be selected if you wait too long.”
Bhattacharya said that delaying the second dose by up to six weeks, according to CDC guidance, seems reasonable. But in addition, it may be too early to say, which means that health officials may face complicated decisions.
“We are pushing the limits of what we can see,” he said. “We are at a point now where we have to make decisions based on imperfect evidence.”
