These studies were conducted before the spread of the Delta variant, for which real-world data on asymptomatic infection are still lacking. But lab studies are promising, suggesting mRNA vaccines may inhibit asymptomatic Delta infection, too. Last week, separate groups at New York University and Yale University posted preprints analyzing blood serum from people vaccinated with the Pfizer and Moderna vaccines. Antibodies generated by those vaccines lost little of their potency against the Delta variant.

And Topol notes that a large real-world study from the United Kingdom, published last week in The New England Journal of Medicine, found that the Pfizer vaccine outperformed the Astra Zeneca vaccine at preventing symptomatic infection by the Delta variant that has taken over in that country by 21 points, 88% to 67%.

That substantial margin “likely translates to better suppression of transmission, unlike the similarities for the vaccines in reducing hospitalizations and deaths,” Topol says.