The vaccine started clinical trials in
June 2020, with PhaseIII trials involving around 43,000 people. On
29 January 2021, Janssen announced that 28 days after a completed vaccination, the vaccine was 66% effective in a one-dose regimen in preventing symptomatic COVID-19, with an 85% efficacy in preventing severe COVID-19, and 100% efficacy in preventing hospitalization or death caused by COVID-19.
The most common side
effects were pain at the injection site, headache, fatigue, muscle aches and nausea. Most of these side effects occurred within one to two
days following vaccination and were mild to moderate in severity and lasted one to two days.