What is the Ad26.COV2.S coronavirus disease 2019 (COVID-19) vaccine?

Updated: Sep 24, 2021
  • Author: David J Cennimo, MD, FAAP, FACP, FIDSA, AAHIVS; Chief Editor: John L Brusch, MD, FACP  more...
  • Print



  • EUA in the United States for adults aged 18 years and older 
  • Phase 3 trial (ENSEMBLE) in adults complete  
  • Phase 2 trial in pregnant women launched February 2021
  • Phase 2a trial (ENSEMBLE 2) to assess efficacy of 1 or 2 doses began late 2020
  • ENSEMBLE 2 trial expanded to include adolescents April 2021 

Ad26.COV2.S is an adenovirus serotype 26 (Ad26) recombinant vector-based vaccine (JNJ-78436735, VAC31518; Johnson & Johnson [Janssen]) administered as a single injection. 

The phase 3 trial (ENSEMBLE) began in September 2020 in the United States, South Africa, and South America. In December 2020, it was fully enrolled. In phase the 1/2a trial (N = 805), neutralizing antibodies against SARS-CoV-2 were detected at day 29 in 90% or more of all participants, regardless of vaccine dose or age group, and reached 100% by day 57, as well as strong T-cell responses and a Th1 response. [33]  

The ENSEMBLE trial randomly assigned more than 40,000 participants worldwide to receive 1 injection of either Ad26.COV2.S (n = 19,630) or placebo (n = 19,691). Ad26.COV2.S protected against moderate to severe–critical Covid-19 with onset at least 14 days after administration (vaccine group, 116; placebo group, 348; efficacy, 66.9%) and at least 28 days after administration (66 vs 193 cases; efficacy, 66.1%). The vaccine showed higher efficacy against severe–critical Covid-19 (76.7% for onset at 14 days and 85.4% for onset at 28 days). The vaccine was 100% effective against COVID-19–related hospitalization and death at Day 28; 16 hospitalizations occurred in the placebo group. [34]  

Ad26.COV2.S showed consistent protection across race; age groups, including older adults (participants aged 60 years and older were 34.6% of the vaccine arm); and across all variants and regions studied, including South Africa, where nearly all cases of COVID-19 (95%) were due to infection with a SARS-CoV-2 variant from the B.1.351 lineage. [34]  

The EUA Fact Sheet for Health Care Professionals states the vaccine's safety and efficacy in older study participants showed no overall differences compared with younger participants. At least 28 days post vaccination, efficacy against moderate to severe/critical disease at all study sites (ie, US, Latin America, South Africa) was 66.2% for those aged 60 years and older compared with 66.1% for those 18-59 years. In the United States, estimated efficacy was 85.9% at least 28 days after vaccination.

Did this answer your question?
Additional feedback? (Optional)
Thank you for your feedback!