A plain language summary of how well the single-dose Janssen vaccine works and how safe it is

Future Virol. 2021 Nov;16(11):725-739. doi: 10.2217/fvl-2021-0199. Epub 2021 Nov 1.


This is a summary of a publication about the ENSEMBLE trial of the Janssen Ad26.COV2.S vaccine against COVID-19, which was published in the New England Journal of Medicine in April 2021. The ENSEMBLE study started in September 2020 and is still ongoing. The study compared the effectiveness of the vaccine to a placebo in 43,783 adults from Latin America, South Africa, and the United States. Of those, 19,630 got a single dose of the vaccine. Compared to the placebo, the vaccine prevented: 66.9% of moderate to severe-critical COVID-19 cases after 14 days66.1% of moderate to severe-critical COVID-19 cases after 28 days85.4% of severe COVID-19 cases after 28 days100% of people with severe COVID-19 from needing to go to hospital for treatment None of the vaccinated participants died from COVID-19. There were 5 people who got the placebo who died from COVID-19. The vaccine was similarly effective in people from all age groups and different countries, including South Africa, where most cases were caused by the beta variant of the virus that originated there. The people in the study who got the vaccine who went on to get COVID-19 generally had milder and fewer symptoms than those who got the placebo. In most people, the vaccine started working after about 2 weeks. After receiving the vaccine, some people experienced pain at the injection site, headache, tiredness, muscle pain, and nausea. In most cases, these were mild and went away within a few days. Serious side effects were very rare. Blood clots, seizures, and tinnitus were very rare but were more common in the people who got the vaccine than in those who got the placebo. At the time of the study, it was not clear if these were caused by the vaccine or not. ClinicalTrials.gov NCT number: NCT04505722.

Keywords: COVID-19 vaccines; clinical trial; coronavirus; plain language summary; vaccine; vaccine development; vaccine efficacy.

Publication types

  • Review

Associated data

  • ClinicalTrials.gov/NCT04505722