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AstraZeneca said Monday that the Covid-19 vaccine it developed with the University of Oxford reduced both mild and serious forms of the disease, paving the way for a likely U.S. authorization of the vaccine.

Doctors, regulators, and government officials the world over are likely to breathe a sigh of relief at the results, which are better than expected and appear materially higher than those in previous studies. 


The two-dose vaccine reduced symptomatic disease by 79%, the company said in a press release, and reduced severe Covid-19 and hospitalization by 100%. AstraZeneca said that the vaccine was equally effective in people over 65, where it had 80% efficacy.

Two volunteers in the study received vaccine for every one that received placebo. Across the study, 141 had symptomatic Covid-19 and five, all in the placebo group, had severe disease. Mene Pangalos, the executive vice president of biopharmaceutical R&D at AstraZeneca, said on a call with reporters those figures made the company very confident about the results, but that more data are being collected from the study.

The company said the study identified no new safety concerns. A specific review found no risk of blood clots, worries about which led many European nations to pause their vaccine rollouts last week. The European Medicines Agency said last week the vaccine’s benefits outweighed its risks. The study also did not see a specific type of clot in blood vessels near the brain that the EMA said might be associated with the vaccine. However, this type of clot, called a cerebral venous sinus thrombosis, is so rare it might not be expected to occur in even a large clinical trial.


“This analysis validates the AstraZeneca COVID-19 vaccine as a much-needed additional vaccination option, offering confidence that adults of all ages can benefit from protection against the virus,” said Ann Falsey, a professor of medicine at the University of Rochester School of Medicine, in the press release.

AstraZeneca plans to request an emergency use authorization from the Food and Drug Administration, and said it is preparing for “the rollout of millions of doses across America.”

The results are from an interim analysis of a 32,000-volunteer study conducted in the U.S., France, Chile, and Peru with funding from the U.S. Biomedical Advanced Research and Development Authority and the National Institutes of Health. Details in the AstraZeneca press release are sparse, even compared to press releases made by other vaccine makers. Full results will likely be published in a medical journal, and released in even more detail as part of the public vetting process of the FDA.

AstraZeneca’s results are “surprisingly positive,” Peter Welford, a pharmaceuticals analyst at the investment bank Jefferies, wrote in a note to clients. A previous analysis of four large studies of the vaccine in the United Kingdom, South Africa, and Brazil, showed the vaccine had efficacy of about 60%. Welford argued that the efficacy compared favorably to previous vaccines, especially given the prevalence of new variants of the SARS-CoV-2 virus, which causes Covid-19, that may decrease efficacy. Previous studies have hinted, he noted, that the vaccine could be more effective if doses are given further apart. The current study, like previous studies, gave two doses one month apart.

Among volunteers in the study, 79% were white, 22% were hispanic or LatinX, 8% were Black, 4% were Native American, and 4% Asian. About 20% of participants were 65 years and over, and approximately 60% had other health problems that can worsen Covid-19 like diabetes, severe obesity, or heart disease.

Even before AstraZeneca’s involvement, researchers at Oxford University had an ambitious goal: to create a vaccine that could be available to large swaths of the global population. The shot they designed is less expensive than other vaccines, and easier to manufacture and distribute. Unlike the Pfizer and Moderna shots, which must be kept very cold, it can be stored for six months at between 2 to 8 degrees Celsius and administered without on-site preparation.

Steps have already been taken to enlist other manufactures, including the Serum Institute of India, to make large amounts of the vaccine. The vaccine has been broadly deployed in the United Kingdom, which has vaccinated half its population.

But studies of the vaccine have been plagued by mishaps. Early on, the U.S. study was stopped for a month-and-a-half because of a case of a neurological condition in one patient that was later determined to be unrelated to the vaccine. Later, early trial results in the U.K., South Africa, and Brazil left many experts scratching their heads, as the second dose of the vaccine seemed to decrease its efficacy.

South Africa halted its own rollout of the vaccine in February after a small study showed the two-shot vaccine might not be as effective against a new variant of the coronavirus that causes Covid-19 that is circulating there. 

The two-dose AstraZeneca vaccine, like the one-dose vaccine developed by Johnson & Johnson, uses a modified common cold virus, known as an adenovirus, to ferry genetic code for a key component of the SARS-CoV-2 virus known as the spike protein into recipients’ cells. The cells make this protein, which is then recognized as foreign by the immune system. The result is that the recipient becomes protected against the virus.

Earlier vaccines from the team of Moderna and Pfizer/BioNTech use a different technology, called mRNA, in which particles of messenger RNA are snuck into cells, and also make versions of the spike protein.

The new efficacy result compares favorably with the results seen in studies of other vaccines, although it is impossible to directly compare the vaccines because they used different criteria for deciding whether to give Covid-19 tests to patients and because they were conducted at different times when different variants of the SARS-CoV-2 virus may have been circulation.

Last November, Pfizer and BioNTech released data showing that its vaccine was 95% effective at preventing symptomatic disease. That clinical trial tested patients for SARS-CoV-2 whenever they had a symptom that might indicate they had the disease. There were only nine cases of severe disease in the study, all but one were in the placebo group.

Moderna and the NIH also released results in November showing the vaccine reduced symptomatic Covid-19 by 94%, but it required volunteers two symptoms to be tested. In its study, there were 30 cases of severe disease, all in the placebo group.

On Jan. 29, Johnson & Johnson and the NIH reported that J&J’s vaccine, the only one to require only one dose, prevented moderate Covid-19 by 66%, with results differing by geography due to different variants being in circulation. It reduced severe disease by 85% — there were 30 cases in the placebo group, but one in the vaccine group. The J&J vaccine also prevented death from Covid-19. There were seven deaths due to Covid in the placebo group, and none in the vaccine group. In other studies, not enough patients died of Covid to make that comparison.

All three of those vaccines were granted emergency authorization in the United States.

NovaVax has also released results showing 90% overall efficacy of its vaccine in a study in the U.K. In a separate trial in South Africa, where the B.1.351 strain is circulating, the efficacy was 49%. The vaccine still prevented severe disease in South Africa, with no cases in the vaccine group and five in the placebo group. All five of those patients were hospitalized, and two died, the company said on March 11.

One question for the AstraZeneca vaccine will be how new variants of the SARS-CoV-2 virus affect the vaccine’s efficacy. The data from South Africa, released in February and published in the New England Journal of Medicine last week, showed only limited vaccine efficacy. It’s not  clear how much the data from the current study can offer regarding those variants, which have been most common in South Africa and Brazil.

Still, the results are encouraging, meaning that one of the world’s most-used Covid-19 vaccines is also an effective choice.

Correction: A previous version of this story misstated the nature of the condition that led the AstraZeneca trial to be paused.

    • Though it appears the NIAID did not say why they were concerned enough to issue a kind of “emergency” press release, it seems to me we can guess that it was something important – One, because it is reported as unusual, Two, because NIAID has been so concerned with the public’s trust of vaccines, they would not have done anything to decrease that trust if it was avoidable.
      The linked article contains some comments by a scientist in England who seems to think the NIAID concerns are with efficacy against the variants, which is reported as “60% in South Africa” – where most of the infected people had B1351 – this “60% efficacy” among all participants indicates it is very close to failing the minimum 50% efficacy standard set by US health officials at the beginning of “Warp Speed”.
      I am not sure there is any advantage in the US to try to use this vaccine. It stores in any freezer apparently and is cheaper to make but still requires two doses. Since we are reportedly giving 2.5M doses/day of more effective vaccines already, it is not clear we need it, unless Pfizer, Moderna or JJ is LESS effective against them.

    • Absolutely. The immunity from vaccination is far greater than immunity from having had Covid, and a vaccination after a bout with Covid is found to even result in above-average immunity. It is also surfacing now, that “long haulers” have great benefits of improved recovery after Covid vaccination.

  • This seems like good news, but i am kind of wondering if it’s time for any reports of vaccine efficacies to include whatever info. is available on efficacy against the variants.
    For that matter, I am sure Stat would cover solid data from a preprint, but maybe it is time for Stat to talk to more virologists to examine the claims which go something like ” Well, in our test tube research the antibodies generated by Moderna and Pfizer are ten times less reactive to B1351, but we think they will still work”, which seems like wishful thinking to me.

    • I wanted to clarify- I very much appreciate Stat giving us the data from studies in South Africa and Brazil on efficacies in those places, but it is not broken down into efficacy against their most common variants. It sure looks to me like we (the US) will get a lot of shots in arms, as will many other countries, but this will not stop the epidemics of South African and Brazilian and even Californian variants which are coming maybe mid to late summer? It looks to me, if we are LUCKY, these variants will be only one epidemic, that is, natural immunity to one may confer immunity to the others – so it will be a simultaneous outbreak of all at once, rather than each variant causing it’s own outbreak, but still, it looms very large.
      I see Dr. Fauci saying vaccines CAN be updated rapidly to deal with the variants, and it may be very true, theoretically, but in reality, we can not get the US vaccinated in under 6 months with a vaccine we already have – and we do not read about a variant vaccine being in production yet. So, what, about a year from today we might have most of us vaccinated against the variants?

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