It is clear that Delta has a substantial transmission advantage, but scientists have not yet established why. Prof Catherine Noakes, a member of the UK’s Scientific Advisory Group for Emergencies (Sage) and an expert in airborne infections at the University of Leeds, suggested three possible reasons: that the people it infects have a higher viral load, meaning they would emit more particles; that people need to be exposed to less of the virus to become infected; or that a relatively short exposure time to an infected person is enough to spread the disease. It is possible that a person could be infected by being close to a carrier for a few seconds, if the carrier were to exhale a load of virus particles and the person just happened to breathe in at exactly the wrong moment, she said. “What it doesn’t necessarily mean is that it’s transmitting that way all the time for everybody. It may well just be one of these really unlucky events.” The World Health Organization is urging even fully vaccinated people to “play it safe” by continuing to wear masks, maintain social distance and practise other safety measures, to deal with the Delta variant.