Johnson & Johnson (J&J) on Tuesday announced data from a global study showing its Covid-19 vaccine is more effective when a second dose is administered at least two months after the initial shot, suggesting that booster shots may be beneficial for recipients of the vaccine.

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How effective is a second shot of the J&J vaccine?

In a press release, J&J announced the results of a late-stage study of around 32,000 adults in 10 countries, including the United States and the United Kingdom.

The study found that the vaccine was 100% effective at preventing severe Covid-19 at least two weeks after a second dose, J&J said. The study also found that a two-dose regimen of J&J's vaccine was 75% effective at preventing moderate-to-severe Covid-19. When looking at data for U.S. participants, the effectiveness against moderate-severe Covid-19 climbed to 94%. However, STAT News reports that the clinical trial data were collected before some of the newer variants of the virus were circulating in the United States.

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The study found that giving patients a second dose of the J&J vaccine two months after the first dose increased antibody levels four to six times. But patients who were given a second dose of the vaccine six months after their first dose saw even better protection, with antibody levels increasing up to 12-fold.

J&J also released results from a separate study of its one-dose regimen, which found a single dose was 53% effective at preventing moderate-to-severe Covid-19 and 76% effective at preventing hospitalizations.

Could J&J booster shots roll out widely?

J&J's results were announced just days after FDA's Vaccines and Related Biological Products Advisory Committee voted to recommend booster shots of the Pfizer-BioNTech Covid-19 vaccine to adults 65 and older and those at risk for severe Covid-19.

In its release, J&J argued that available data shows that booster doses would make its vaccine more effective. J&J has submitted its booster shot data to FDA and the World Health Organization.

Dan Barouch, an immunologist at Beth Israel Deaconess Medical Center who helped develop the J&J vaccine, said the data supports "giving a second shot of the J&J vaccine, anywhere from two months onwards." He added, "The longer you wait, the better the boost will likely be."

According to Barouch, waiting longer provides more protection because of the way the body's immune system responds to viruses.

Antibodies prompted by immunization can build up quickly but tend to wane over time. However, B cells and T cells also produced by the body help contribute to long-term protection, Barouch said. Stimulating B cells after they've become less active seems to help them produce new antibodies more effectively.

Barouch said it's possible some countries will elect to stay with the single-shot regimen while others may offer boosters. "Different people or different countries might actually make different choices based on what their desires or needs are," he said. (Herper, STAT News, 9/21; Schwartz, Wall Street Journal, 9/21; Fox, CNN, 9/21; Guarino, Washington Post, 9/21)