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

Updated: Feb 28, 2021
  • Author: David J Cennimo, MD, FAAP, FACP, AAHIVS; more...
  • Print



  • Viral vector vaccine
  • Shipping and long-term storage: Frozen (-20°C) for up to 2 years
  • After thawing: Standard refrigerator temperatures (2-8°C) for up to 3 months
  • Dose: 1 injection

The phase 3 trial (ENSEMBLE) for adenovirus serotype 26 (Ad26) recombinant vector-based vaccine (JNJ-78436735, VAC31518; Johnson & Johnson [Janssen]) was launched in September 2020 in the United States, South Africa, and South America. In December 2020, the trial 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. [17]  

The EMSEMBLE phase 3 trial results were released in late January 2021. Safety and efficacy data are based on 43,783 participants randomized 1:1 accruing 468 symptomatic cases of COVID-19. FDA analysis of the data at Day 28 determined the vaccine was 72% effective in the US, 61% in Latin America, and 64% in South Africa (where the B.1.351 variant was circulating) at preventing moderate-to-severe COVID-19 infection. Importantly, the vaccine was 85% effective in preventing severe disease in all geographic regions. Efficacy against severe disease increased over time with no cases in vaccinated participants reported after day 49. Additionally, the vaccine provided 100% protection against COVID-19 related hospitalization and death at Day 28. [12]   

Additionally, 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. [12]  

The EUA Fact Sheet for Health Care Professionals states efficacy of the vaccine in older study participants showed no overall differences in safety or efficacy 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 aged 18-59 years. In the United States, estimated efficacy was 85.9% at least 28 days after vaccination. 

A second phase 3 trial was announced November 16, 2020 to observe effects of 2 doses of the vaccine, instead of just 1 dose.

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