WSJ US BusinessAstraZeneca Shot Is Safe, 79% Effective in Latest U.S. Trials

March 22, 20210
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AstraZeneca AZN -0.24% PLC’s Covid-19 vaccine was shown to be safe and 79% effective in preventing symptomatic disease in U.S. clinical trials involving more than 32,000 people, the U.K. drugmaker said Monday.

The company said it would continue to analyze the data and prepare to request emergency authorization in the U.S. in coming weeks, a move that—if approved—will add another vaccine available for Americans.

The shot is already widely used outside the U.S. The American trials, however, reflect the largest-scale tests yet of the vaccine and could bolster confidence in its use following questions about its efficacy and serious blood-clotting issues in a very small number of people in Europe who received the shot. The U.S. trials identified no increased risk of serious blood clotting.

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AstraZeneca said participants aged 65 years and over were shown to have vaccine efficacy of 80%.

Blood clotting concerns led more than a dozen European countries, including Germany and Italy, to temporarily halt use of the vaccine this month. Most resumed administering the vaccine after European and U.K. medicine regulators again endorsed the shot’s use despite the rare clotting issues, which weren’t proven to be linked to the vaccine. The regulators said they couldn’t rule out a connection, but said the vaccine is an important weapon in preventing death and serious illness and that its benefits outweigh potential risks.

Much is riding on global acceptance of the AstraZeneca shot, which was co-developed with the University of Oxford and is the most widely used in many parts of the globe. The U.S. Food and Drug Administration is expected to review the trial data and could decide whether to authorize the vaccine for use as early as next month.

Previous trial results of the shot from the U.K. and elsewhere were criticized for lacking elderly volunteers, who were recruited later in the trial process for safety reasons. The U.K. and other countries used the vaccine in over-55s, but some countries initially held back and only deployed the shot in younger age groups.

The U.S. trial involved 32,449 participants aged 18 and over, and about 20% were 65 or older. The vaccine’s effectiveness was 100% in preventing severe disease and hospitalizations, with similar results across all age and ethnicity groups.

The U.K. trial results of the vaccine, released late last year, presented a range of effectiveness, between 62% and 90%. That range was difficult to compare with U.S. trial results of rival vaccines, one from Pfizer Inc. and Germany’s BioNTech SE and another from Moderna Inc. Both were found to be more than 90% effective in U.S. trials.

​Europe’s top drug regulator endorsed AstraZeneca’s vaccine after it was suspended in several countries over blood-clot concerns. WSJ explains what’s at stake for a shot that’s been widely used around the world and may soon be considered for emergency use in the U.S. Photo: Mykola Tys/SOPA Images

The U.S. trial results haven’t yet been reviewed by independent researchers. AstraZeneca said the analysis will be submitted for publication in a peer-reviewed journal.

AstraZeneca said independent safety monitors ran a specific review of severe blood clotting, or serious thrombotic events, based on the trial data, including a severe condition known as cerebral venous sinus thrombosis that has been reported to be linked to a very small number of deaths in Europe among people who received the vaccine. The review was assisted by an independent neurologist. It found no increased risk of thrombosis among 21,583 participants who received at least one dose of the vaccine, AstraZeneca said.

The U.S. AstraZeneca vaccine trial, which started recruiting volunteers in the U.S., Chile and Peru in August, was standardized to meet FDA guidelines, measuring the results of two doses four weeks apart. AstraZeneca said stretching out doses—which the U.K. and other countries have done—could boost effectiveness of the vaccine further.

The U.S. late-stage trial results provide the closest comparison yet between the AstraZeneca vaccine and rival shots that were the first to be adopted in the U.S. Clinical trials showed the Pfizer and Moderna shots to be more than 94% effective against symptomatic Covid-19.

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Oxford-led trials of the AstraZeneca-Oxford vaccine last year in the U.K. and Brazil, meanwhile, produced a confusing array of results, from 62% to 90%, depending on dose size and timing.

Real-world vaccination data, heavily reliant on the U.K’s. mass rollout, has since shown a single shot to be around 70% effective, and highly effective in preventing death and hospitalizations. Those results suggested that separating the two doses by about 12 weeks helped boost immunity, resulting in around 80% effectiveness among people 70 years or older, after the second shot. British researchers said that result was similar to the over-70 results of the Pfizer-BioNTech vaccine.

The developing world is counting heavily on the AstraZeneca-Oxford shot, with 3 billion doses promised at no profit this year. But public-health experts have worried the complex trial results last year and AstraZeneca’s prolonged tension with European officials over extreme shortfalls in vaccine-delivery targets could do lasting damage to the vaccine’s reputation.

Those factors and missteps by AstraZeneca in its public and regulatory communications last year could impact perceptions of the vaccine in the U.S., some experts and AstraZeneca analysts have said.

In late February, the U.S. authorized the Covid-19 vaccine from Johnson & Johnson for the U.S. market, bringing a third shot into the country’s vast supply chain. That one-dose shot was found to be generally safe and 66.1% effective at protecting people from developing moderate and severe cases of Covid-19 at least 28 days after vaccination, based on a study of about 44,000 volunteers 18 years old and older.

British Prime Minister Boris Johnson receives the first dose of the AstraZeneca vaccine in London on Friday.

Photo: Frank Augstein/Zuma Press

The U.S. vaccination rollout has faced challenges in supply and distribution. But both the production and administration of shots have picked up in recent weeks. Now, some 2.5 million people in the U.S. are vaccinated daily on average, up from about 500,000 in early January.

But many people who want a vaccine can’t get one. Stockpiles of AstraZeneca vaccine can’t be distributed in the U.S. until the shot gets FDA authorization.

Last week, the Biden administration announced plans to send 4 million doses of the stockpiled AstraZeneca vaccine in the U.S. to Canada in Mexico, as a loan ahead of potential authorization. Officials said no deal was finalized.

The U.S. said it has 7 million “releasable doses” of the AstraZeneca vaccine. In total, the U.S. has ordered 300 million doses of the AstraZeneca-Oxford vaccine, including a contract for 200 million doses for the Department of Defense.

The U.S. has been fast-tracking production of vaccines to meet domestic targets, including by fostering manufacturing partnerships among vaccine makers and ramping up funding to expedite production of ingredients and supplies.

AstraZeneca has built a global network of manufacturing partners, including the Serum Institute in India, the world’s largest vaccine maker, to meet its supply targets. Staggered approvals, production shortfalls and export battles have complicated the rollout. AstraZeneca executives have said they are working out problems in the supply chain and plan to eventually meet announced obligations.

After the blood clotting reports in Europe, regulators there said last week they will add a warning to usage guidelines and patient-information sheets for the vaccine, for healthcare providers and people receiving the shot, to be on the lookout for signs of rare so-called thromboembolic problems—including a serious brain condition that has caused a very small number deaths in people who received the vaccine—and continued to back the shot. But regulatory officials stressed there was no link proven between the vaccine and clotting issues, which also occur naturally in the general population and in many cases from Covid-19.

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Write to Jenny Strasburg at jenny.strasburg@wsj.com

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