05:21 Advertisement "It's clear how important the second dose is to secure the strongest possible protection against COVID-19 and its variants," Matt Hancock, the UK's secretary of state for health and social care, said when an unreviewed preprint of the study was released in May. When patients were fully vaccinated, with two shots both given at least two weeks to take effect, Pfizer's COVID-19 vaccine became about 88% effective against symptomatic COVID-19 from the Delta variant, while AstraZeneca's vaccine was 67% effective against it.That is almost as good as those vaccines performed in clinical trials before the new variant was detected, but it does suggest that breakthrough infections in vaccinated people will become slightly more common now that Delta is here.