In a clinical trial, researchers found that two doses of the vaccine delivered 94 percent efficacy against mild to severe Covid-19 in the United States, up from 74 percent conferred with a single shot, the company reported. And two shots showed 100 percent efficacy against severe disease, although that estimate had a wide range of uncertainty.
The data, presented in a news release, has been submitted to the Food and Drug Administration, Johnson & Johnson said. Since the company received emergency authorization in February, 14.6 million people in the United States have received its one-shot vaccine.
On Friday, an F.D.A. advisory committee recommended that the agency authorize Pfizer-BioNTech booster shots for recipients of the vaccine who are at least 65 or at high risk of Covid. That vaccine, like Moderna’s, offers high levels of initial protection after two doses, which then seem to diminish slightly over several months.
By contrast, Johnson & Johnson’s vaccine has shown little sign of waning. Researchers released a study last week comparing 390,517 vaccinated people to 1,524,153 unvaccinated ones. Up to five months after vaccination, the effectiveness of the Johnson & Johnson vaccine against hospitalization remained steady at around 81 percent.
As the pandemic has unfolded, people who received the Johnson & Johnson vaccine have waited for guidance about whether they’ll need a booster. The new clinical trial, which recruited 32,000 volunteers around the world, compared people who received one dose of Johnson & Johnson to those who received two doses eight weeks apart.
The researchers found that the second shot lifted the level of antibodies in the blood of volunteers four times as high as the level produced by the first shot. That improvement translated into stronger protection.
Many people got their Johnson & Johnson shot far more than eight weeks ago. Other research suggests that the extra time between doses could mean even better protection.
In a separate study announced last month, Johnson & Johnson gave boosters to clinical trial volunteers six months after their first dose, and then measured their antibody levels.
Initially, the researchers reported that the antibodies rose nine times as high as after the first dose. But in Tuesday’s news release, the company announced the level had continued to rise, reaching 12 times as high as the initial levels.
Some preliminary studies suggest that higher levels of antibodies against the coronavirus produce higher levels of protection against Covid. If that’s true, then a second Johnson & Johnson shot given after a wait of several months may prove even more effective than after just eight weeks.