By Jon Cohen and John Travis

A third COVID-19 vaccine candidate has convincing evidence that it works, and it may be easier to distribute and cheaper than the two other vaccines already shown to protect people. Developed by the company AstraZeneca in partnership with the University of Oxford, the vaccine had an average efficacy of 70% in preventing the disease, the developers announce today in press releases. In one dosing scheme, its efficacy was 90%, according to results from the interim analysis of clinical trial data.

AstraZeneca says about 3 billion doses of the vaccine could be ready in 2021. Whereas the apparently powerful COVID-19 vaccines recently announced by Moderna and the Pfizer/BioNTech collaboration rely on a snippet of messenger RNA coding for the spike surface protein of SARS-CoV-2, the AstraZeneca/Oxford vaccine stimulates immunity by using a crippled chimpanzee adenovirus as a “vector” to deliver the gene for spike. (A Russian team has also presented evidence its vaccine works, but noted too few COVID-19 cases at the time to persuade many outside scientists.)

The AstraZeneca/Oxford collaboration is following more than 23,000 people vaccinated in the United Kingdom and Brazil. It reported a total of 131 COVID-19 cases in two groups: 8895 people given two full doses 1 month apart, and 2741 people who received a half dose first, followed by the full dose. The first scheme had only 62% efficacy, a clinical trial measurement that may not translate exactly to the real world. But in the second one, efficacy jumped to 90%. The collaboration did not report the breakdown of cases between people in the vaccine arm of the trial and the control group. Nor did it provide any data on COVID-19 protection among the elderly or various ethnicities.

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