A third dose of Moderna's COVID-19 vaccine substantially improved protection for organ transplant recipients whose weak immune systems don't always rev up enough with the standard two shots, Canadian researchers reported Wednesday.

The study, published in the New England Journal of Medicine, was small but it's the most rigorous type of third-dose testing so far for this vulnerable group.

Moderna and similar vaccines provide robust protection for most people, even as the highly contagious delta variant is surging. But millions with suppressed immune systems because of transplants, cancer or other disorders don't always get that benefit. There's limited evidence that an extra dose helps some of them, something France and Israel already recommend and the U.S. is considering.

Researchers at Toronto's University Health Network enrolled 120 transplant recipients, and two months after their second Moderna shot, gave half a real third dose and the rest a dummy shot.

Soon after, 55% of the third-dose recipients had a high level of virus-fighting antibodies in their blood, compared to 18% who only got two doses plus a placebo. Antibodies are only one of the body's defenses; third-dose recipients also had more T cells that help prevent severe disease. Side effects were mild. 

The findings offer "yet more evidence" that many transplant recipients could benefit from an extra dose, said Dr. Dorry Segev, a Johns Hopkins University transplant surgeon who wasn't involved with the new research. But it's important to check patients' antibody levels before offering another shot, as some study participants had pretty good immune responses to regular vaccination, added Segev, who is leading a U.S. study of extra shots in unprotected transplant recipients.