As the greenlight looms for another dose of Johnson & Johnson's Covid-19 vaccine, experts on Friday urged those who received it to get a booster shot as soon as it's available because it will provide them with the best protection against the coronavirus.

"J&J is a very good vaccine. I also believe it's probably a two-shot vaccine," said Dr. Ashish Jha, dean of the Brown University School of Public Health. "It's really urgent that people get that second shot pretty quickly.

"Probably one is not enough. And everybody who has had one needs a second one two months after the first. They are going to get full protection if they do that," he said.

The US Food and Drug Administration advisory committee recommended Friday that all adults who received the single-dose J&J shot should get a second vaccine at least two months after their first dose.

On Thursday, the same committee backed emergency use authorization for booster shots of Moderna's Covid-19 vaccine after six months, but not for everyone. Moderna recipients over the age of 65 and adults with conditions that put them at risk for severe disease or who work or live in a place that puts them at higher risk of complications or severe illness may be eligible for the 50-microgram booster, which is half the size of the primary two-dose series.

But as for the J&J vaccine, which was launched as a one-dose regimen, experts point to a combination of waning immunity and the spread of the Delta variant as the primary drivers for a second dose.