The Scottish vaccination program has significantly reduced hospital admissions in Covid-19, according to a study published Monday, providing 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, which included both AstraZeneca and Pfizer-BioNTech vaccines, looked at the number of people hospitalized after receiving a single dose of the vaccine. The UK has delayed the administration of the second dose for up to three months after the first and has chosen to offer more people the partial protection of a single shot.

However, the study cautioned about how long high levels of protection would last with a single dose. The risk of hospitalization decreased from a week after the first shot and bottomed out four to five weeks after vaccination. But then it seemed to rise again.

The scientists who conducted the study said it was too early to know if the protection from a single dose wore off after a month and warned that more evidence was needed.

The results in Scotland supported earlier results from Israel showing that the vaccines provided significant protection against the virus. The Israeli studies have focused on the Pfizer vaccine, but the Scottish study has been expanded to include the AstraZeneca shot, which has been administered in the UK since early January. The AstraZeneca shot is the backbone of many nations’ vaccination plans: it is far cheaper to manufacture and can be shipped and stored in regular refrigerators rather than the ultra-cold freezers used for other vaccines.

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

The Scottish researchers looked at around 8,000 hospital admissions related to coronavirus and looked at how the risk of hospitalization varied among people who had or had not received a shot. In total, more than 1.1 million people were vaccinated during the study period.

The number of people vaccinated and treated in hospitals was too few to compare AstraZeneca and Pfizer vaccines or give exact numbers for their effectiveness, the researchers said.

28 to 34 days after the first shot, the AstraZeneca vaccine reduced the risk of hospitalization after Covid-19 by around 94 percent. Over the same period, the Pfizer vaccine reduced the risk of hospitalization by around 85 percent. In both cases, these numbers fit into a wide range of possible effects.

Because the Pfizer vaccine was approved in the UK prior to the AstraZeneca shot, researchers had more data on the Pfizer vaccine and found that protection from hospitalization was somewhat limited after the first shot for extended periods of time.

“The peak protection is four weeks and then gradually falls off,” said Simon Clarke, a professor of cell microbiology at the University of Reading who was not involved in the study.

The AstraZeneca vaccine met with skepticism in parts of Europe after many countries decided not to give it to the elderly due to lack of clinical trial data in this group. The Scottish study failed to provide precise figures on the effectiveness of this vaccine in the elderly. However, the combined effects of the vaccines against AstraZeneca and Pfizer significantly reduced hospital admissions for people over 80. Many elderly people received the AstraZeneca vaccine.

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