It might be tempting, but it simply isn’t possible to directly compare the effectiveness of vaccines on the basis of those results alone, cautions David Kennedy, who studies the ecology and evolution of infectious diseases at Pennsylvania State University in University Park. Each measure of efficacy comes with a degree of uncertainty, and trials might have differing definitions of important criteria, such as what constitutes a ‘severe’ bout of COVID-19 compared to a ‘moderate’ one.

Added to this are the demographics of each trial: in the case of the Oxford–AstraZeneca vaccine, for example, the developers collected few data about the vaccine’s efficacy in people over 65. This led Germany to authorize the vaccine only for those under 65, even though the European Medicines Agency recommends it for all adults.

And the vaccines were studied at different times in various countries. Each trial can only offer a snapshot of protection against the viral variants that were dominant in that time or place, says Kennedy. “That number relates to a particular point in time,” he says. “How that translates into protection over one to two years is not the same.”