Johnson & Johnson says two doses of its single-dose Covid vaccine boosts efficacy

Johnson & Johnson on Tuesday said a second dose of its vaccine substantially increased its protection against Covid-19. It was initially rolled out as a single-shot vaccine.

A new clinical trial compared participants who received one dose to those who got two doses eight weeks apart. Results showed that two shots delivered 94 percent efficacy against mild to severe Covid in the US, compared to 74 percent with a single dose.

The extra shot increased antibody levels four to six times higher than after the first, the company said.