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Both the Covid-19 vaccine from Pfizer Inc. and Germany’s BioNTech SE and the one from Moderna Inc. require two doses, administered three or four weeks apart. In the U.S., President-elect Joe Biden plans to release all available doses right away, rather than holding back supplies for a second shot. The proposed departure from the Trump administration’s policy comes as U.K. authorities have decided to delay the second shot.

Here is what we know and don’t know about deviating from the recommended schedule.

Why do the vaccines require two doses a specified interval apart?

It takes two doses of the Pfizer-BioNTech and Moderna vaccines to fully prepare the immune system to guard against Covid-19, study results showed. The first shot helps the body recognize the virus and primes the immune system to defend against it, while the second shot strengthens that immune response, scientists say.

Researchers assessed the effectiveness of that two-dose regimen, spaced three or four weeks apart depending on the vaccine, in trials with more than 30,000 subjects. In Moderna’s late-stage trials, the first dose provided around 80% to 90% efficacy after 28 days. Two weeks after the booster shot, the efficacy had reached 94.1%, according to study results.

Pfizer said the late-stage trial found its shot was about 52% effective after the first dose, with study results showing protection starting as early as 12 days after receiving it. A week after the second dose, the shot’s effectiveness rose to 95%, Pfizer said its study found.

Why did the U.K. delay the second dose?

To increase the number of people being vaccinated. The U.K. has been hit hard by a more contagious new variant. To protect more people sooner against the virus, U.K. authorities decided to give as many people as possible a first dose, even if that meant delays in administering the second. U.K. officials estimate they can double the number of people with some level of immunity in the next three months by halving the number of shots each person receives initially.

Other countries in Europe and some provinces in Canada also are delaying second doses. The delays could cut hospitalizations and deaths by giving more people some protection against the virus, said Anna Durbin, professor of international health at the Johns Hopkins Bloomberg School of Public Health.

What do the WHO and FDA say about extending vaccination intervals?

The second dose of the Pfizer-BioNTech vaccine is supposed to come three weeks after the first, and Moderna’s is given four weeks later. The U.S. Food and Drug Administration rejected suggestions to adjust the schedules in the U.S., saying such changes weren’t supported by evidence and could put public health at risk.

Changing course, FDA officials said, could give people a false sense of confidence that they are protected from the virus and can proceed with riskier behavior when they could still get infected. World Health Organization officials this week recommended the Pfizer-BioNTech booster shot come no later than four weeks after the first injection, but the interval could go up to six weeks if there are extreme supply constraints.

Does changing the schedule affect vaccine effectiveness?

We don’t know. Stretching out the doses hasn’t been tested in people. Given the uncertainty, some researchers say it is best to stay with the studied and proven dosing schedule. Other scientists say a delay of a week or two might not make much of a difference, though the protection conferred over time could be less than what was found in the clinical trials.

It is unclear how long a first dose’s protection lasts without the booster shot. “If it’s three months later or four months later, that’s a big deal because you may have fading immunity and be at risk for disease," said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. For that reason, some scientists say waiting too long to get the second shot might require starting over.

Yet other scientists say a delay might confer longer-lasting protection. The U.K. has told doctors they don’t need to restart the vaccination regimen, even if the interval between doses is longer than the recommended four to 12 weeks.

“Longer intervals favor higher immune protection," said David Salisbury, an infectious-disease expert and former director of immunizations for the U.K. government.

Would delaying a second shot hurt efforts to curb the pandemic?

Giving fewer people the fullest possible protection to fight off the virus might be better for defending the broader population from infection, some researchers say. These researchers express concern that people with partial protection from just one dose might develop Covid-19 without symptoms—and wind up spreading the virus unwittingly.

Saad Omer, a virologist and director of the Yale Institute for Global Health, said the emergence of more transmissible variants argues for sticking with the recommended two-dose schedule. “If the outbreak becomes more intensive, in order to achieve your public-health goal you need your high levels of 95% efficacy," he said.

Yet scientists in the U.K., where one of the more contagious variants was identified, say giving more people a single dose would help combat the virus by increasing the number of people who have some protection against it.

It is unclear whether even full inoculation prevents the spread of the virus. The late-stage studies that led to vaccine authorizations found the shots prevented Covid-19 with symptoms—not transmission. Studies are under way looking at whether the vaccines also reduce the virus’s spread.

Would delaying second doses help the virus develop resistance to vaccines?

We don’t know, but some researchers worry it could. A single or partial dose could generate low levels of antibodies in people who might not fully defeat the virus, instead giving it an opportunity to mutate, some researchers say. Michel Nussenzweig, who heads the Laboratory of Molecular Immunology at the Rockefeller University in New York, said, “It could be a real public-health disaster to create a cohort of individuals that have suboptimal immune responses to the virus, creating an environment that would favor the emergence of variants that are resistant to the vaccine."

Would stretching out the shots accelerate vaccine distribution?

Some scientists and doctors say delays of a few weeks aren’t likely to significantly impact supplies, since companies are ramping up manufacturing and more vaccines appear on the way. They also say bottlenecks in the U.S. are more about administering shots on the ground. Making sure people return for a second shot could become challenging, too, because the longer boosters are stretched out, the less likely people are to return, according to experts and research.

Finally, vaccine experts are concerned that changing schedules might jeopardize already shaky levels of vaccine acceptance, given the number of people who worry the rapid progress might have affected the shots’ safety. “It could undermine public confidence," said Peter Jay Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. “People are just going to walk away."

What other changes to vaccine rollouts are being considered?

Mr. Biden, the incoming U.S. president set to take office Jan. 20, plans to release nearly all available doses to help speed up vaccinations, shifting away from the Trump administration’s policy of holding back stock for booster shots.

Federal officials have previously considered giving half doses of Moderna’s vaccine, according to Moncef Slaoui, chief adviser to the Trump administration’s Operation Warp Speed vaccine program. Some Moderna testing, he said, found people between ages 18 and 55 who got two half doses had the same immune response as those who got two full doses.

The FDA has warned of public-health risks in delaying second doses. The National Institutes of Health is studying antibody data from previously conducted research on Moderna’s vaccine that analyzed whether a half dose was effective at protecting against the disease.

The president-elect plans to use the Defense Production Act to boost supplies, his transition team said Friday, adding that Mr. Biden believes manufacturers can produce enough shots to deliver second doses quickly. The incoming administration doesn’t plan to change the timing interval between doses, a person familiar with the plans said.

Will Covid-19 vaccines work against the variants found in the U.K. and South Africa?

Some researchers say the mutations seen in the South African variant might reduce the shots’ effectiveness, meaning a larger proportion of vaccinated people would still be susceptible to Covid-19 or could still develop mild Covid-19 symptoms. There is less concern about this happening with the U.K. variant—which is the one that has also been detected in the U.S.

The Covid-19 vaccines generate in the body a range of antibodies and other immune cells targeting the entire spike protein, while the variant tweaks just a small part of the spike protein. So despite the mutations, vaccinated people are expected to have antibodies and immune cells that can effectively attack the coronavirus even if some of the spike protein has changed, said Bettie Steinberg, provost for research for the Feinstein Institutes, the research arm of Northwell Health.

What happens if the virus evolves and reduces the vaccines’ effectiveness?

New vaccines might be needed depending on how significantly the virus changes. Every year researchers develop new vaccines to tackle the seasonal flu strains expected to appear. One advantage of the gene-based technology, known as mRNA, used to make both the Pfizer-BioNTech and Moderna vaccines is that it can be adjusted more easily to make vaccines than older technologies, said Angela Rasmussen, a virologist and affiliate at the Center for Global Health Science and Security at Georgetown University.

Gabriele Steinhauser contributed to this article.

This story has been published from a wire agency feed without modifications to the text.

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