Does the AstraZeneca Vaccine Also Stop Covid Transmission?

Three Mondays in a row have now yielded three apparently effective and safe vaccines against the pandemic disease Covid-19. Amid an unprecedented peak in cases in the United States and Europe, with US deaths pushing 250,000 and the country showing uncontrolled spread of the virus, that ain’t bad news. But slightly hidden in that non-bad news was news even less bad. This week’s entrant, a vaccine from the drug company AstraZeneca and researchers at Oxford University, came with tantalizing hints of a particular capability that would, if it bears out, make a huge difference in fighting the pandemic. The makers of the two other vaccines in play have reported only evidence that their drugs keep people from getting sick—which is to say, fewer vaccinated people have moderate to severe symptoms and test positive for infection. The vaccines do this very well. But researchers working on the AstraZeneca version said they also had signs of reduced transmission, of people spreading the disease from one person to another. The AstraZeneca results have some perplexing elements , for sure, but if the transmission thing holds up, it’s going to matter. A lot.
Here’s what’s known (or at least announced) so far: The first two vaccines to complete their large-scale trials, one from the drug companies Pfizer and BioNTech and the other from Moderna, are a new kind of medicine. They use bits of genetic material called messenger RNA, in this case a sequence that codes for a part of the virus called a spike protein. That protein helps the SARS-CoV-2 virus attack people’s cells; the mRNA, enfolded in proprietary bubbles of fat, teaches the human immune system to fight the virus instead. Pfizer’s version has an efficacy of above 90 percent, says a company press release; a Moderna press release says its efficacy is 94.5 percent. If those results hold when more data becomes public, these vaccines would be extraordinary.The one from AstraZeneca is a little more traditional, putting the gene for that spike protein into a sort of stealth carrier called a vector—in this case, an adenovirus that usually infects chimpanzees, modified so that it can’t replicate anymore. The company’s results—again, maddeningly, delivered via press release rather than peer-reviewed science—are a little more confusing. AstraZeneca is running different studies around the world, each with slightly different methodologies, which makes them hard to compare. But if you dump them all into the same pool, as AstraZeneca seems to have done, its two-dose regimen seems to have an efficacy of around 60 percent. That seems not great, though it’s higher than the 50 percent, plus or minus, that the US Food and Drug Administration was looking for. And in a group accidentally given a half-dose for the first shot and a full dose for the second, efficacy went up to 90 percent. Nobody knows why, and it is not good statistics to just average together a study done right with a study done wrong, re-analyzed after the fact.
But for the moment let’s not look this gift adenovirus in the mouth. The press release on the AstraZeneca vaccine from the Oxford side included this bulleted finding: “Early indication that vaccine could reduce virus transmission from an observed reduction in asymptomatic infections.” An Oxford immunologist told the news section of the journal Nature that some of the people in the UK part of the trial actually were testing themselves regularly for infection with the virus, and that different infection rates in the placebo and vaccine groups suggested that the drug was also blocking transmission of the disease. Researchers at Oxford also told reporters Monday that testing showed the vaccinated group in the UK had fewer asymptomatic infections, which means they'd be less likely to unwittingly spread the disease themselves.