Another COVID-19 vaccine success? Candidate may prevent further coronavirus transmission too

Artist illustration of adenovirus used in a COVID-19 vaccine from AstraZeneca and University of Oxford.


Science’s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.

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 three 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: 8,895 people given two full doses a month apart, and 2,741 people who received a half-dose first followed by the full dose. The first scheme only had 62% efficacy, a clinical trial measurement that may not translate exactly to the real world. But in the second one, it jumped to 90%. The collaboration did not report the breakdown of cases between people in the COVID-19 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.

Peter Piot, head of the London School of Hygiene & Tropical Medicine, says he was “very pleased” to see these results, especially given the “encouraging news” that the half dose scheme worked best, meaning more people could receive the vaccine while it remains in short supply for the next several months. But he noted that none of the COVID-19 efficacy results reported to date have offered much data. “It is frustrating that all these announcements are by press release, and we can’t review actual data,” says Piot. Without knowing how the 131 cases divide up between the different vaccine regimens and people in the control group (who either received a meningococcal vaccine or a placebo), researchers cannot calculate what is known as the confidence interval around the reported efficacy. “We desperately need total transparency about trials and data,” Piot says.

Even if people who receive the COVID-19 vaccine become infected by SARS-CoV-2, the shots may protect them from serious symptoms. There were no hospitalizations or severe cases of COVID-19 in vaccinated participants, AstraZeneca and Oxford reported. They also said vaccinated people had fewer asymptomatic infections, which suggests they were less likely to transmit SARS-CoV-2 to others.

No serious safety issues have emerged so far in the efficacy trials, AstraZeneca says. The company now plans to file for emergency use of the vaccine with the United Kingdom and other regulatory bodies. Other efficacy trials of the vaccine that involve 60,000 participants are underway in the United States, Kenya, Japan and India.

The company says the vaccine, which only requires standard refrigeration, may cost about $3 to $4 a dose. Moderna, in contrast, may charge $25 or more a dose, according to statements from its CEO. Its vaccine and the one from Pfizer/BioNTech both require freezers for storage and transport to prevent the RNA and the lipid particle that holds it from degrading.

“These results suggest it is highly effective in protecting serious illness and it may reduce transmission,” Charlie Weller, head of vaccines at the Wellcome Trust research, said in a statement. “It is based on an established vaccine technology, which does not require the challenging cold-chains and should therefore ease deployment and global access. As with all interim results we have seen, it is critically important that the trial is completed and regulators can now independently and rigorously assess the data.”

Adrian Hill, who led the Oxford University team that developed the vaccine, says there are lots of theories, but little evidence, to explain why the half dose priming shot worked best. He says “perhaps the most likely–and measurable” explanation is that people develop an immune response against the chimpanzee virus vector that blunts the booster shot’s impact. A half dose in the beginning in theory would trigger a milder response against the vector.

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