A plain language summary of the Janssen COVID-19 vaccine effectiveness and safety as a single dose and with a booster

Summary This is a summary of the results of 2 global clinical studies of the Janssen Ad26.COV2.S vaccine against COVID-19. The ENSEMBLE study looked at the effectiveness of a single injection of the vaccine. The separate ENSEMBLE2 study looked at the effectiveness of a booster dose of the vaccine given 2 months after the first dose. In both studies, people received either the vaccine or a placebo. Vaccine effectiveness was evaluated 14 and 28 days after vaccination to allow sufficient time for generation of an immune response. In ENSEMBLE, compared to the placebo, a single dose of the vaccine prevented: 56% of moderate to severe-critical COVID-19 cases occurring at least 14 days after vaccination 53% of moderate to severe-critical COVID-19 cases occurring at least 28 days after vaccination 75% of severe-critical COVID-19 cases occurring at least 28 days after vaccination 76% of people with COVID-19 from needing to be hospitalized for treatment 83% of COVID-19–related deaths The vaccine continued to work well for at least 6 months after a single vaccine injection. In ENSEMBLE2, compared to the placebo, a single dose of the vaccine followed by a booster dose 2 months later prevented: 75% of moderate to severe-critical COVID-19 cases occurring at least 14 days after booster vaccination 100% of severe-critical COVID-19 cases occurring at least 14 days after booster vaccination In ENSEMBLE2, there were too few cases of COVID-19 to estimate vaccine effectiveness for preventing COVID-19–related deaths or hospitalization. ENSEMBLE2 was done during early 2021, when several COVID-19 vaccines became available by emergency use authorization. For ethical reasons, people could check whether they had received vaccine or placebo and decide whether they could be vaccinated outside of the study. This meant that the researchers could not look at the long-term effectiveness of the vaccine. In both studies, 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. In ENSEMBLE, blood clots, seizures, hives, and ringing in the ears were more common in the people who got the vaccine than in those who got the placebo. These side effects were very rare. In ENSEMBLE2, bleeding, hives, and ringing in the ears were slightly more common in people who got the vaccine than those who got the placebo. In ENSEMBLE2, blood clots were more common in people who got the placebo. At the time of the study, it was not clear if these side effects were caused by the vaccine. The vaccine was effective at protecting against moderate to severe-critical COVID-19 at 14 days after a single injection. Effectiveness was increased by a booster injection given 2 months after the first injection. You can find more detailed information and references in the original articles. Links to these articles can be found at the end of this summary. Clinical Trial Registration: NCT04505722  and  NCT04614948 ( ClinicalTrials.gov )