A fourth dose of existing mRNA Covid-19 vaccines may have only “marginal benefits” for younger, healthy adults, according to a new Israeli study published Wednesday in the New England Journal of Medicine. The research assessed 270 health care workers at the Sheba Medical Center in Tel Aviv who received a second booster shot of either the Pfizer or Moderna vaccines, four months after initially getting three Pfizer shots. The study observed low vaccine efficacy against infections in health care workers, as well as relatively high viral loads suggesting that those who were infected were infectious. Thus, a fourth vaccination of healthy young health care workers may have only marginal benefits.