What We Know About AstraZeneca’s Head-Scratching Vaccine Results

Carl Zimmer and Rebecca Robbins  in The New York Times:

This month has seen a torrent of news about experimental vaccines to prevent Covid-19, with the latest development from AstraZeneca and the University of Oxford. On Monday they announced that a preliminary analysis showed their vaccine was effective — especially when the first dose was mistakenly cut in half. The announcement came on the heels of stunning reports from Moderna, as well as Pfizer and BioNTech. But AstraZeneca’s news was murkier, leaving many experts wanting to see more data before passing final judgment on how effective the vaccine may turn out to be. Researchers at the University of Oxford built the vaccine using a kind of virus, called an adenovirus, that typically causes colds in chimpanzees. They genetically altered the virus so that it carried a gene for a coronavirus protein, which would theoretically train a person’s immune system to recognize the real coronavirus.

…On Monday, AstraZeneca and Oxford released details about the first 131 volunteers to get Covid-19 in late-stage trials in the United Kingdom and Brazil. All of the volunteers got two doses about a month apart, but in some cases the first dose was only at half strength. Surprisingly, the vaccine combination in which the first dose was only at half strength was 90 percent effective at preventing Covid-19 in the trial. In contrast, the combination of two, full-dose shots led to just 62 percent efficacy. No one knows. The researchers speculated that the lower first dose did a better job of mimicking the experience of an infection, promoting a stronger immune response. But other factors, like the size and makeup of the groups that got different doses, may also be at play.

It was a lucky mistake.

More here.