Image of gloved hand holding a vial of Johnson & Johnson's investigational COVID-19 vaccine

A second shot of Johnson & Johnson’s coronavirus vaccine provides 100% protection against severe or critical COVID-19 when given two months after a first dose, the drugmaker announced Tuesday.

The finding is included in new data submitted to the Food and Drug Administration to support authorization of a second, or booster shot of the drug for adults.  

Vaccine efficacy can wane over time and booster shots can increase the protection an original vaccination provided. Long-term care residents who were vaccinated against COVID-19 with Pfizer-BioNTech’s vaccine at least six months ago are eligible for a booster shot. Meanwhile, those who were vaccinated with J&J’s or Moderna’s remain in limbo as federal regulators review data for those drugs.

The recent J&J results, from the company’s Phase 3 ENSEMBLE 2 study, also found that a booster given about two months after the primary dose offered patients 94% protection against moderate to severe COVID-19, with the full effect seen at 14 days after the booster was administered.

An earlier trial showed that a J&J booster vaccine dose given six months after the first shot increased SARS-CoV-2 antibody levels nine-fold one week after receiving the shot, and 12-fold in four weeks. 

Regarding safety, the new trial data showed that the booster dose “was generally well-tolerated.” That’s in line with the safety results for initial doses of the vaccine, J&J said.

“Our clinical program has found that a booster of our COVID-19 vaccine increases levels of protection for those who have received our single-shot vaccine to 94%,” said Mathai Mammen, M.D., Ph.D., Global Head, Janssen Research & Development, Johnson & Johnson. 

“At the same time, we continue to recognize that a single-shot COVID-19 vaccine that provides strong and long-lasting protection remains a crucial component to vaccinating the global population.”