http://www.sciencemag.org/news/2021/08/booster-shot-jj-vaccine-raises-antibody-levels-company-says

A person in Washington, D.C., is injected with the single-dose Johnson & Johnson vaccine in April. The company says new data support giving a booster dose.

Johnson & Johnson (J&J) announced today that a second dose of its COVID-19 vaccine, given months after a first dose, robustly boosted antibodies that target the SARS-CoV-2 coronavirus in a small group of volunteers.