Johnson & Johnson said Tuesday its Covid-19 booster shot is 94% effective when administered two months after the first dose in the United States. It also said the booster increases antibody levels by four to six times compared with one shot alone.

A J&J booster dose given six months out from the first shot appears to be potentially even more protective against Covid, the company said, generating antibodies twelvefold higher four weeks after the boost, regardless of age.

When given as a booster, the vaccine remained well tolerated, with side effects generally consistent with those seen after the initial dose, according to J&J.