Early data offers hope on AstraZeneca’s vaccine.

Scotland’s vaccination program substantially reduced Covid-19 hospital admissions, according to the results of study released on Monday, offering the strongest real-world signal of the effectiveness of the AstraZeneca-Oxford vaccine that much of the world is relying on to end the pandemic.

The study, encompassing both the AstraZeneca and Pfizer-BioNTech vaccines, examined the number of people who were hospitalized after receiving a single dose of the vaccine. Britain has delayed administering the second dose for up to three months after the first, opting to offer more people the partial protection of a single shot.

But the study sounded a cautionary note about how long high protection levels from a single dose would last. The risk of hospitalization dropped starting a week after people received their first shot, reaching a low point four to five weeks after they were vaccinated. But then it appeared to rise again.

The scientists who conducted the study said it was too early to know whether the protection offered by a single dose waned after a month, cautioning that more evidence was needed.

The findings in Scotland bolstered earlier results from Israel showing that the vaccines offered significant protection from the virus. The Israeli studies have focused on the Pfizer vaccine, but the Scottish study extended to the AstraZeneca shot, which has been administered in Britain since early January. The AstraZeneca shot is the backbone of many nations’ inoculation plans: It is far cheaper to produce, and can be shipped and stored in normal refrigerators rather than the ultracold freezers used for other vaccines.

“Both of these are working spectacularly well,” Aziz Sheikh, a professor at the University of Edinburgh who was involved in the study, said at a news conference on Monday.

The researchers in Scotland examined roughly 8,000 coronavirus-related hospital admissions, and studied how the risk of hospitalization differed among people who had and had not received a shot. Over all, more than 1.1 million people were vaccinated in the period the researchers were studying.

The numbers of vaccinated people who sought care in hospitals was too small to compare the AstraZeneca and Pfizer vaccines, or to give precise figures for their effectiveness, the researchers said.

But from 28 to 34 days after the first shot, the AstraZeneca vaccine reduced the risk of Covid-19 hospital admissions by roughly 94 percent. In that same time period, the Pfizer vaccine reduced the risk of hospitalizations by roughly 85 percent. In both cases, those figures fit within a broad range of possible effects.

Because the Pfizer vaccine was authorized in Britain before the AstraZeneca shot, the researchers had more data on the Pfizer vaccine, and found that the protection against hospital admissions was somewhat reduced at longer periods after the first shot.

“The peak protection is at four weeks, and then it starts to drop away,” said Simon Clarke, a professor in cellular microbiology at the University of Reading who was not involved in the study.

The AstraZeneca vaccine has faced skepticism in parts of Europe after many countries chose not to give it to older people, citing a lack of clinical trial data in that group. The Scottish study could not offer precise figures on that vaccine’s effectiveness in older people. But the combined effect of the AstraZeneca and Pfizer vaccines substantially reduced hospital admissions in people over 80. Many older people were given the AstraZeneca vaccine.

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