A Closer Look at the Janssen Vaccine: How Effective is it?

Madison Norris — McMaster University Honours Life Sciences 2023

Over the last two years, the COVID-19 pandemic has completely changed the way in which we go about our everyday lives. The shift to online classes, working from home, social distancing restrictions and stay at home orders have left many yearning for a sense of normalcy. The only feasible solution to see us through the pandemic is the mass vaccination of the majority of the population.

Johnson and Johnson recently announced the completion of phase 3 clinical trials of the single dose Janssen vaccine. The Janssen vaccine contains viral DNA within a modified adenovirus that codes for the spike protein found on the surface of the SARS-CoV-2 virus. (1) Adenovirus-based vaccines use adenoviruses as vessels that will release genetic material upon entry into the cells of an inoculated host. The purpose of the vaccine is to protect against moderate to severe COVID-19 infections, including emerging variants of the virus, such as the B.1.351 variant proliferating in South Africa. (2)

In phase 3 clinical trials, 43,783 participants, 18 years of age or older, participated in a randomized double-blind study that compared the safety and efficacy of the Janssen vaccine to a placebo control. (2) The study population represented 8 different countries, including the United States, countries in Central and South America and South Africa. The study also included participants with comorbid conditions, such as obesity, type 2 diabetes, hypertension and immunocompromising conditions. (2) 28 days following vaccination, the Janssen vaccine demonstrated 66% efficacy. (2) However, it is important to note that differences in efficacy were found across geographical regions. The vaccine was observed to be most effective in the United States (72%) and less effective in Latin America (66%) and South Africa (57%). (2) In addition, the Jassen vaccine demonstrated similar rates of efficacy across age groups, including adults over the age of 60. The only side effect observed was a fever in approximately 9% of participants. As of March 5th, Health Canada has approved the Janssen vaccine.            

A notable difference between the Janssen vaccine and other competitor vaccines (Moderna and Pfizer) is that it can be stored for longer periods of time at refrigerator temperatures. To illustrate, the Janssen vaccine can be stored at 2°C to 8°C for 3 months, whereas the Moderna vaccine can only be stored at these temperatures for 30 days (3/4). Moreover, the Pfizer vaccine cannot be stored at refrigerator temperatures at all, and must remain in -60°C to -80°C conditions. (4) The ability to store the Janssen vaccine for longer periods of time at refrigerator temperatures presents the opportunity to transport vaccines to remote locations.

Another distinction of the Janssen vaccine is that while it has been observed to be less effective (66%) it only requires one dose. In contrast, both Moderna and Pfizer require two doses but are remarkably more effective at 94.1% and 95% efficacies, respectively. (5,6) This offers a potential solution to countries financially unable to fully vaccinate their citizens with a two dose vaccine. A more feasible alternative could be to vaccinate the elderly and at-risk populations with either the Pfizer or Moderna vaccines, as they are more susceptible and will experience more severe symptoms of the virus. The remaining population could then be vaccinated with the Janssen vaccine, as they have stronger immune systems that will allow for greater rates of recovery. (7)

In sum, the Janssen vaccine demonstrated an overall efficacy of 66%. The vaccine offers solutions to transportation, storage and financial barriers to vaccination. Following FDA approval, the administration of the Janssen vaccine brings the global population closer to herd immunity and one step closer to returning to normal life.


  1. Corum J. Zimmer C. How the Johnson & Jonson vaccine works. The New York Times. [Internet]. 2021 Feb 24 [cited 2021 Feb 24]; Health:[about 5 p.]. Available from:
  1. Janssen Vaccines & Prevention B.V. A randomized, double-blind, placebo-controlled phase 3 study to assess the efficacy and safety of Ad26.COV2.S for the prevention of SARS-CoV-2-mediated COVID-19 in adults 18 years and older. [Internet]. New Jersey: Janssen Vaccines & Prevention B.V; 2021 Jan 29 [cited 2021 Feb 24]. 184 p. Available from: -met-primary-endpoints-in-interim-analysis-of-its-phase-3-ensemble-trial.
  1. CDC. Moderna COVID-19 vaccine: storage and handling summary. [internet] CDC; 2020 Dec 20 [cited 2021 Feb 24]. 2p. Available from:
  1. Pfizer Inc. Pfizer and Biotech submit COVID-19 vaccine stability data at standard freezer temperature to the U.S. FDA. New York: Pfizer Inc; 2021 Feb 19 [cited Feb 24]. P4. Available from:
  1. Baden LR, El Sahly HM, Essink B, Kotloff K, Frey S, Novak R, et al. Efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine. N Engl J Med. 2021; 384: 403-416. Available from:
  1. Polack FP, Thomas SJ, Kitchin N, Absalon J, Gurtman A, Lockhart S, et al. Safety and efficacy of the BNT162b2 mRNA COVID-19 vaccine. N Engl J Med. 2020; 383: 2603-2615. Available from:
  1. CBC News. Johnson & Johnson single-shot COVID-19 vaccine appears 66% effective overall in global trial. The Associated Predd [internet]. 2021 Jan 29 [cited 2021 Feb 24]; Health: [about 1 p.]. Available from:

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