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A second dose of Johnson & Johnson's COVID-19 vaccine given 2 months after the standard one-dose regimen was 94% effective against moderate to severe disease in the U.S., the manufacturer said in .

However, the confidence interval was wide, with one case in the vaccine arm and 14 in the placebo arm (95% CI 58%-100%).

Globally, the two-dose regimen resulted in 75% efficacy against moderate to severe disease (95% CI 55%-87%), with 14 cases in the vaccine arm and 52 in the placebo arm, according to data from the international ENSEMBLE 2 study.

The additional dose also had 100% protection against severe or critical cases of COVID-19 at least 14 days after the second shot, albeit with an even wider confidence interval (95% CI 33%-100%). There were no cases of severe/critical COVID in the vaccine arm and eight in the placebo arm.

"We now have generated evidence that a booster shot further increases protection against COVID-19 and is expected to extend the duration of protection significantly," said Johnson & Johnson's chief scientific officer, Paul Stoffels, MD, in a statement.

is a randomized phase III trial comprised of 31,836 adults ages 18 and up, though it is unclear whether the efficacy numbers reflect the full data set. Johnson & Johnson did not provide the total number of participants in the press release. The second dose was given 56 days after the first.

Median follow-up after the second dose was 36 days, and 29% of participants had at least 2 months of follow-up after their second shot, Johnson & Johnson said.

The company also provided immunogenicity data, which found that when a booster shot was given at 6 months, antibody levels rose to 12-fold higher a month later, irrespective of age.

Johnson & Johnson said it plans to submit these data to the FDA, as well as to peer-reviewed journals for publication. The one-dose viral vector vaccine is currently under emergency use authorization for adults 18 and up.

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