AstraZeneca announced the long-awaited results of the U.S. and South American study of its COVID-19 vaccine. It was the first bit of positive news about the shot in recent weeks after a parade of countries halted its use due to reports of blood clot complications.
In the Phase 3 study involving more than 32,000 people, AstraZeneca found that its vaccine was 79% efficacious in protecting against symptoms of COVID-19. In the trial, the two-dose shot—developed by the British-Swedish multinational pharmaceutical company in conjunction with Oxford University—was also 100% efficacious in protecting people from severe symptoms and hospitalization from the disease.
Those results differ slightly from the company’s earlier late-stage human trial, released in February, conducted in the U.K., U.S., South Africa, and Brazil. In that study, the overall efficacy of the vaccine was 66%. In part, that difference might be because more of the people in that earlier study were likely exposed and infected with new, mutant versions of SARS-CoV-2, particularly one first discovered in South Africa—against which the vaccine provides slightly less protection—compared to those in the U.S., Chile, and Peru, where the later trial took place.
Still, those earlier findings indicate an effective vaccine and led many countries, the European Union, and the World Health Organization to authorize its use. Then, in mid-March, reports of blood clots emerged, and many of these countries decided to suspend vaccination with the AstraZeneca regimen while they investigated the reports. On March 18, the European Medicines Agency determined after reviewing the cases that there was no increased risk of clotting or other related issues due to the vaccine but said it would continue to monitor vaccinated people for any side effects.
In the new, U.S.-based study, the data safety monitoring board also found no increased risk of blood clots among vaccinated people compared to those given a placebo. “This study puts to bed any doubts that this isn’t a highly effective vaccine against COVID-19 disease and COVID-19 symptoms,” says Mene Pangalos, executive vice president for biopharmaceuticals research and development at AstraZeneca. AstraZeneca’s vaccine is based on technology developed by scientists at Oxford University’s Jenner Institute. It involves using a chimpanzee adenovirus modified so it cannot cause the cold infection it usually does. The chimp virus acts as a vehicle to deliver genes into the body, where it encodes the SARS-CoV-2 spike protein; cells then process the protein so the immune system can recognize it as foreign and mount a strong response against it.