What You Need To Know About The Johnson & Johnson (J&J)/Janssen Vaccine

A History of Using Ad26-based Vaccines

  • The J&J/Janssen vaccine is based on the Ad26 vector platform.   
  • As of December 31, 2020, Ad26-based vaccines have been used to vaccinate over 193,000 participants in clinical studies and vaccination programs for a variety of diseases.   
  • An Ad26-based Ebola vaccine is licensed and being used in African countries. 
  • Ad26-based vaccines for RSV and HIV are being researched and no safety concerns have been identified to date. 
  • Ad26 vaccines have been shown to have an acceptable safety profile in other diseases including HIV, Malaria and RSV, where ongoing clinical trials have shown no safety concerns to date.  

Who was in the J&J/Janssen Ensemble COVID-19 Vaccine Study? 

  • Approximately 40,000 participants aged 18 years and older 
  • 45% females 
  • Geographic Distribution: 
    • 47% United States 
    • 40% Central/South America: Argentina, Brazil, Chile, Colombia, Mexico, Peru 
    • 13% South Africa 
  • Race (rounded to the nearest whole number): 
    • 59% White 
    • 19% Black or African American  
    • 10% Indigenous (1% American Indian/Alaskan Native in the US) 
    • 3% Asian  
    • less than 1% Native Hawaiian/Pacific Islander  
    • 6% Multiracial 
    • 1% Unknown 
  • 45% Latin origin (14% Hispanic/Latino/a in the US) 
  • 20% aged 65 years old or older 
  • Enrollment of participants aged 18-40 was limited to approximately 20% 
  • About 41% reported underlying health conditions (known as comorbidities).  The top were: 
    • Obesity (29%) 
    • High Blood Pressure (Hypertension) (10%) 
    • Diabetes (8%) 
    • HIV (3%)  

This vaccine requires 1 dose to prevent COVID-19 disease, hospitalization, and death 

How Effective is the Vaccine? 

Vaccine Efficacy in the J&J/Janssen Ensemble study is reported as how well the vaccine protects against moderate and severe COVID-19 disease. This is what that means: 

Moderate COVID-19 = having a positive SARS-CoV-2 test and symptoms of COVID-19 (fever, increased heart rate, chills, cough, sore throat, etc.). 

Severe COVID-19 = having a positive SARS-CoV-2 test; needing a ventilator or oxygen; having severe high blood pressure; having significant acute kidney, liver or nervous system dysfunction; admission to the intensive care unit; or death. 

Overall, the Jansen COVID-19 vaccine: 

  • Prevented 66% of moderate to severe COVID-19 disease 
  • Prevented 85% of severe COVID-19 disease 
  • Prevented 95% of hospitalizations due to COVID-19 disease 
  • Prevented 100% of deaths due to COVID-19 disease 
  • Has similar efficacy across age, racial and ethnic groups  

Viral Variants: Analyses in US, Brazil and South Africa


  • 96% efficacy in preventing moderate to severe COVID-19 against the Washington strain (known as A.1) virus.


  • Approximately 65% of COVID-19 cases that occurred in Brazil were found to be caused by the viral variant first identified in Brazil (known as P.1).  Vaccine efficacy in Brazil was similar to what was seen in the US. 

South Africa: 

  • Approximately 95% of the COVID-19 cases that occurred in South Africa were due to the variant first identified in South Africa (known as B.1.351). 
  • Efficacy of 82% in preventing severe COVID-19 disease in South Africa. 
  • Efficacy of 64% in preventing moderate to severe COVID-19 disease in South Africa. 
  • All 7 deaths in the study occurred in the placebo group and were in South Africa.  All of these participants had one or more pre-existing conditions (Obesity, Hypertension, Diabetes, Heart Failure, Asthma), placing them at higher risk for severe COVID-19. 

Is the J&J/Janssen Vaccine Safe? 

YES! Independent experts who reviewed the safety data have concluded that the vaccine is safe. 

  • Researchers are still studying the long-term safety of this vaccine in ongoing studies. 
  • Most side effects happened within 7 days of vaccination. 

The most common side effects were reported to be mild or moderate and lasted 1 to 2 days after vaccination: 

  • Injection site pain 
  • Headache 
  • Fatigue/Feeling Tired 
  • Muscle Pain 

Allergic Reactions 

  • Hives were reported more often in the vaccine group, so there is a possible relationship to the vaccine. 
  • There have been cases of severe allergic reactions, including anaphylaxis, after receiving the J&J/Janssen COVID-19 vaccine.  If you have a history of allergic reactions, consult a healthcare provider before getting vaccinated. 

Rare Adverse Reactions 

  • In extremely rare cases, vaccine-induced thrombotic thrombocytopenia (VITT) – a condition characterized by blood clots involving blood vessels in the brain, abdomen, and legs along with low levels of platelets (blood cells that help your body stop bleeding) – has occurred after receiving the J&J vaccine. 
  • In the US, VITT has occurred in 16 out of the 7.5 million people (≤ 0.0002%) vaccinated with the J&J vaccine, with 13 cases in females ages 18-49 years (3 have been fatal), 2 in females 50+ years, and 1 case in males.  
    • Doctors are still investigating the specific risk factors for VITT after receiving the J&J vaccine and the level of potential excess risk due to vaccination. 
  • Symptoms of VITT reportedly appear between 5-28 days after receiving the J&J vaccine. 
  • You should seek medical attention right away if you have any of the following symptoms after receiving the J&J vaccine: 
    • Shortness of breath 
    • Chest pain 
    • Leg swelling 
    • Persistent abdominal pain 
    • Severe and persistent headaches or blurred vision 
    • Easy bruising or tiny blood spots under the skin

What We Don’t Know 

  • We do not know how long the vaccine protection lasts. 
  • We do not know if the vaccine can prevent coronavirus infection. 
  • We do not know whether a vaccinated person can transmit coronavirus to others. 
  • We do not know whether the vaccine will have an impact on long-term effects of COVID-19 disease in people who are infected after vaccination. 
  • We do not know if this vaccine is safe for the groups of people that were not enrolled in the Ensemble Study.   
    • Children younger than 16 years of age 
    • Pregnant and lactating individuals 
    • Text BoxText BoxPeople with weaker immune systems 

Content last reviewed on July 15, 2021