The same was true at two other health care centers in Delhi, a hint that delta may be more likely to infect some vaccinated people, called breakthrough infection, than variants like alpha (SN: 5/4/21). A single dose of Pfizer or AstraZeneca’s vaccines is around 33 percent effective at preventing symptomatic disease for delta infections three weeks after the shot, researchers report May 24 at medRxiv.org. A single dose of Pfizer’s COVID-19 vaccine was 94 percent effective at keeping people out of the hospital after being infected with delta and one dose of AstraZeneca’s was 71 percent effective.