J&J Jab Efficacy; Delta Now 83% of New U.S. Cases: Virus Update Bloomberg 1 hr ago Bloomberg News (Bloomberg) -- One dose of Johnson & Johnson’s Covid-19 vaccine is much less effective against the delta and lambda variants than against the original strain of the virus, the New York Times reported, citing a study that hasn’t been peer reviewed yet. Delta now makes up 83% of all sequenced cases in the U.S., up from 50% in early July, health authorities said. A spokesman for U.K. Prime Minister Boris Johnson insisted that the country’s self-isolation rules aren’t optional following some mixed messaging from a government minister.