AstraZeneca’s COVID-19 vaccine may provide less protection against severe disease than previously thought, underlining the importance of booster shots, a major new study has found.
The study, which looked at more than 42 million people in Brazil and 1.9 million people in Scotland who received two doses of the AstraZeneca vaccine, found that immunity against severe disease began to fade three months after people had received their second dose. The study was published late Monday in the peer-reviewed medical journal The Lancet.
In Brazil, protection against hospitalization and death from COVID-19 fell as low as 42.2% between 18 and 19 weeks after a second dose. In Scotland, where the Delta variant of the coronavirus was dominant during the study period, immunity held up slightly better, falling to 63.7% in the same time interval. Initial immunity, two to three weeks after the second dose, had been as high as 86% in Brazil and 83% in Scotland.
While previous studies had shown that immunity to symptomatic COVID-19 conferred by the AstraZeneca vaccine faded within six months, these same studies had seemed to indicate that protection against hospitalization and death lasted considerably longer. But the new research casts those earlier findings into doubt.
The new study, which was jointly conducted by scientists from universities in Scotland, Brazil, and New Zealand, is another blow to the AstraZeneca vaccine, which has suffered from a string of setbacks, including bad publicity during its initial testing, slightly poorer protection against symptomatic COVID-19 than competing vaccines from Pfizer-BioNTech and Moderna, poorer effectiveness as a booster, and concerns about a rare blood-clotting side effect.
The vaccine, which AstraZeneca brought to market following initial work on it by scientists at the University of Oxford, was used extensively for initial vaccination drives in the U.K., Brazil, India, South Africa, and a number of other countries. The vaccine had the advantage of being significantly cheaper than competitors’—AstraZeneca had pledged to produce it at cost until at least July 2021—and could be stored at normal refrigerator temperatures, making it easier to distribute, especially in countries with spotty refrigeration and cold transport.
Although AstraZeneca conducted a large clinical trial of the vaccine in the U.S., it has still to be approved by the U.S. Food and Drug Administration.
Global concerns
Given how extensively AstraZeneca was used, the new study will be a serious concern in the countries where it was employed, especially as the highly transmissible Omicron variant sweeps through them. The Omicron variant is now the dominant strain in England, having displaced the Delta variant earlier this week, and has become the dominant variant in South Africa. The variant has only just started spreading in Brazil.
AstraZeneca did not respond to a request for comment on the new Lancet study.
Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London, who was not affiliated with the researchers conducting the study, said that the Lancet paper underscored why it was critical that people receive booster doses. “The data reiterate, as we already know, that a booster vaccination shot is required to confer higher levels of sustained protection,” she said. “Everyone should ensure they get out and get boosted as fast as possible to retain protection against COVID illness of any severity.”
In the U.K., the Pfizer-BioNTech and Moderna vaccines are being used as boosters after other research indicated that doing so provided better immunity for those initially vaccinated with the AstraZeneca vaccine, versus having a third AstraZeneca dose. The country is racing to administer booster doses—injecting as many as 900,000 people per day this week—as it battles a tsunami of Omicron infections that scientists and officials fear could swamp its health system.
While earlier research had shown that antibodies to the spike protein of SARS-CoV-2, the virus that causes COVID-19, peaked two to three weeks after a second shot of the AstraZeneca vaccine and then faded, the company had repeatedly said that it believed its vaccine produced a robust B-cell and T-cell response, other aspects of immunity, that would confer “long-lasting” protection against hospitalization or death. This new study casts doubt on that conclusion.
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