The AstraZeneca vaccine is shown to cut transmission of the virus.

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The vaccine developed by the University of Oxford and AstraZeneca not only protects people from serious illness and death but also substantially slows the transmission of the virus, according to a new study — a finding that underscores the importance of mass vaccination as a path out of the pandemic.

Researchers at the University of Oxford measured the impact on transmission by swabbing participants every week seeking to detect signs of the virus. If there is no virus present, even if someone is infected, it cannot be spread. And they found a 67 percent reduction in positive swabs among those vaccinated.

The results, detailed by Oxford and AstraZeneca researchers in a manuscript that has not been peer-reviewed, found that the vaccine could cut transmission by nearly two-thirds.

Matt Hancock, the British health secretary, hailed the results on Wednesday as “absolutely superb.”

“We now know that the Oxford vaccine also reduces transmission and that will help us all get out of this pandemic,” Mr. Hancock said in an interview Wednesday morning with the BBC.

The results, he said, “should give everyone confidence that this jab works not only to keep you safe but to keep you from passing on the virus to others.”

Some scientists looking at the limited information released cautioned that more analysis of the data was needed before such broad conclusions could be firmly stated.

“While this would be extremely welcome news, we do need more data before this can be confirmed and so it’s important that we all still continue to follow social distancing guidance after we have been vaccinated,” said Dr. Doug Brown, chief executive of the British Society for Immunology.

The Oxford and AstraZeneca researchers also found that a single dose of the vaccine was 76 percent effective at preventing Covid-19. The data measured the three months after the first shot was given, not including an initial three-week period needed for protection to take effect.

The encouraging results, lend support to the strategy deployed by Britain and other countries to prioritize providing as many first doses of vaccines as possible, setting aside concerns that people will get their second doses later than initially planned.

The latest data do not have bearing on the debate over whether to further space out the doses of the two vaccines authorized in the United States, those from Pfizer-BioNTech and Moderna, since the data on AstraZeneca’s candidate cannot be generalized to other vaccines.

Some scientists have called on the United States to follow the lead of Britain and other countries that have opted to delay the second doses of vaccines by up to 12 weeks. But U.S. federal officials have resisted, saying such a move would not be supported by the data from clinical trials of the two vaccines currently available across the nation. Tuesday’s results could amplify pressure on U.S. health officials to delay second doses of AstraZeneca’s vaccine, though it has not yet been authorized by the country.

The vaccine appeared more effective when the interval between the two shots was longer than the originally intended four-week gap, the Oxford and AstraZeneca researchers found. Among clinical trial participants who got two standard-strength doses at least three months apart, the vaccine was 82 percent effective, compared to 55 percent effective when the doses were given less than six weeks apart.

Covid-19 Vaccines ›

Answers to Your Vaccine Questions

Currently more than 150 million people — almost half the population — are eligible to be vaccinated. But each state makes the final decision about who goes first. The nation’s 21 million health care workers and three million residents of long-term care facilities were the first to qualify. In mid-January, federal officials urged all states to open up eligibility to everyone 65 and older and to adults of any age with medical conditions that put them at high risk of becoming seriously ill or dying from Covid-19. Adults in the general population are at the back of the line. If federal and state health officials can clear up bottlenecks in vaccine distribution, everyone 16 and older will become eligible as early as this spring or early summer. The vaccine hasn’t been approved in children, although studies are underway. It may be months before a vaccine is available for anyone under the age of 16. Go to your state health website for up-to-date information on vaccination policies in your area

You should not have to pay anything out of pocket to get the vaccine, although you will be asked for insurance information. If you don’t have insurance, you should still be given the vaccine at no charge. Congress passed legislation this spring that bars insurers from applying any cost sharing, such as a co-payment or deductible. It layered on additional protections barring pharmacies, doctors and hospitals from billing patients, including those who are uninsured. Even so, health experts do worry that patients might stumble into loopholes that leave them vulnerable to surprise bills. This could happen to those who are charged a doctor visit fee along with their vaccine, or Americans who have certain types of health coverage that do not fall under the new rules. If you get your vaccine from a doctor’s office or urgent care clinic, talk to them about potential hidden charges. To be sure you won’t get a surprise bill, the best bet is to get your vaccine at a health department vaccination site or a local pharmacy once the shots become more widely available.

That is to be determined. It’s possible that Covid-19 vaccinations will become an annual event, just like the flu shot. Or it may be that the benefits of the vaccine last longer than a year. We have to wait to see how durable the protection from the vaccines is. To determine this, researchers are going to be tracking vaccinated people to look for “breakthrough cases” — those people who get sick with Covid-19 despite vaccination. That is a sign of weakening protection and will give researchers clues about how long the vaccine lasts. They will also be monitoring levels of antibodies and T cells in the blood of vaccinated people to determine whether and when a booster shot might be needed. It’s conceivable that people may need boosters every few months, once a year or only every few years. It’s just a matter of waiting for the data.

A vaccination strategy that spaces out doses by three months “may be the optimal for rollout of a pandemic vaccine when supplies are limited in the short term,” the researchers wrote.

The newly released study builds on data issued late last year, which found that the vaccine was 62 percent effective when given as two standard-strength doses. In those initial findings, the vaccine’s efficacy was much higher, at 90 percent, when the first dose of the vaccine was given at half-strength.

Oxford and AstraZeneca researchers initially attributed the different levels of effectiveness to the lower strength of the initial dose. But they gradually reached a different conclusion: the amount of time between doses was the more likely explanation.

In the United States, the Food and Drug Administration is waiting on data from a clinical trial that enrolled about 30,000 participants, mostly Americans. Results from that study are expected later this month.

The study is expected to arm AstraZeneca with enough safety data to allow it by around early March to seek authorization to provide the vaccine for emergency use.

The United States has agreed to buy 300 million doses of AstraZeneca’s vaccine, but neither the company nor the federal government has said when and in what quantities those doses will be available after the vaccine is approved.





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