Unlike the authorizations for boosters for the Pfizer/BioNTech and Moderna vaccines, no restrictions were put on the J&J booster. The panel effectively said that the J&J vaccine, like the other vaccines, requires two doses to be effective.

But panelists were also critical of data suggesting that the vaccine be given as a booster dose at six months, saying the data came from only 17 patients. However, they said they would not want to deny boosters to anyone who had not yet received one.

https://www.statnews.com/2021/10/15/fda-advisory-panel-votes-19-0-to-endorse-booster-dose-of-jj-vaccine/