Skip to main content

The Facts on the Recommended J&J Vaccine ‘Pause’

By April 16, 2021COVID-19


Update, April 13: In a statement on Twitter, the FDA and CDC announced they had recommended “a pause in the use” of the Johnson & Johnson vaccine “out of an abundance of caution.” The agencies cited “6 reported U.S. cases of a rare & severe type of blood clot” in the 6.8 million individuals who have received the vaccine. 

The pause is “important to ensure that the health care provider community is aware of the potential for these adverse events and can plan due to the unique treatment required with this type of blood clot,” the statement said.

“CDC will convene a meeting of the Advisory Committee on Immunization Practices (ACIP) on Wednesday to further review these cases and assess their potential significance,” the statement added. “FDA will review that analysis as it also investigates these cases.” For more, see “The Facts on the Recommended J&J Vaccine ‘Pause’.”

Quick Summary

Vaccine name: Ad26.COV2.S

Design type: Adenovirus viral vector

Dose number: 1 dose

Efficacy: Overall, 66.1% efficacy in preventing moderate to severe COVID-19 in adults 28 days or more after vaccination, but efficacy was higher in the U.S. population (72% efficacy in preventing moderate to severe COVID-19 and 85.9% efficacy in preventing severe or critical COVID-19) than in South Africa (64% and 81.7% efficacy in preventing moderate to severe or severe/critical disease, respectively), where a new strain of coronavirus emerged.

Safety: No serious safety concerns reported during clinical trials. The shot, however, did elicit temporary side effects in some recipients, including pain at the injection site, headache, fatigue, and muscle pain or ache.

Expected dose availability: Nearly 4 million available immediately in the U.S. upon FDA authorization; 20 million by the end of March and 100 million by the end of June.

Expected timeline: On Feb. 27, the FDA authorized the vaccine for emergency use in adults age 18 and older.

Operation Warp Speed involvement: The U.S. government provided about $1 billion for clinical trials and vaccine development. It spent another $1 billion for vaccine manufacturing, contracting for 100 million doses with an option to purchase up to another 200 million.

Storage considerations: normal refrigerator temperature (36°F to 46°F) for at least three months; -13°F to 5°F for long-term storage.

More than two months after its last COVID-19 vaccine authorization, the FDA authorized for emergency use a one-shot vaccine from Janssen Biotech Inc., a Johnson & Johnson pharmaceutical company, on Feb. 27.

Unlike the Pfizer/BioNTech and Moderna vaccines, which use an mRNA design, the Johnson & Johnson shot uses a harmless adenovirus — a type of virus that typically causes the common cold — modified with the genetic material for SARS-CoV-2 to trigger an immune response. The Johnson & Johnson vaccine has the advantages of being one shot, not two, and being stored at regular refrigeration temperatures for up to three months. The Pfizer and Moderna two-shot vaccines can only be kept at refrigerator temperatures for five or 30 days, respectively.

Johnson & Johnson submitted its request for emergency use authorization on Feb. 4. An external panel advising the FDA met on Feb. 26, and voted unanimously (22 to 0) that “based on the totality of scientific evidence available … the benefits of the Janssen COVID-19 Vaccine outweigh its risks for use in individuals 18 years of age and older.” The FDA then authorized the vaccine for emergency use the following day.

Johnson & Johnson Q&A

What is in the vaccine and how does it work?

The design is different from the mRNA vaccines authorized in December, but all of the vaccines fundamentally work in the same way: They trigger an immune response against the SARS-CoV-2 virus’s spike protein, which sits on the surface of the virus and is what the virus uses to enter cells.

Johnson & Johnson COVID-19 vaccine vials. Photo by Phill Magakoe/AFP via Getty Images

The Johnson & Johnson vaccine uses another, harmless virus — adenovirus 26 — modified with DNA for the SARS-CoV-2 spike protein. Adenoviruses typically cause common-cold symptoms, and for the vaccine, the virus is also modified so it can’t replicate in the body.

The modified adenovirus enters human cells, which then read the genetic material and begin making the coronavirus spike protein. This triggers the immune system, generating protective antibodies and activating other immune cells known as T cells.

In other words, the vaccine doesn’t include the COVID-19 virus, but rather, genetic material that prompts the body to produce an immune response to SARS-CoV-2 so that the body will recognize and react to the actual coronavirus if needed.

Dr. William Schaffner, an infectious disease expert with the Vanderbilt University School of Medicine, summarized how the vaccine works to CNN. “So essentially it’s a sheep in wolf’s clothing, and when your immune system sees it, it responds to it and creates protection against it and in the future, against the real virus that causes Covid-19,” he said.

How was the vaccine developed?

Johnson & Johnson has experience working with adenovirus-based vaccines, and gained approval in Europe in July for an Ebola vaccine that uses this technology.

The company is also working on adenovirus-based vaccines for HIV and Zika.

In January 2020, when the sequence for SARS-CoV-2 became available, Johnson & Johnson collaborated with Harvard’s Beth Israel Deaconess Medical Center to test multiple COVID-19 vaccine candidates. At the end of March, researchers announced they had identified a lead candidate.

In July, J&J launched a phase 1/2a randomized controlled trial in the U.S. and Belgium with 1,051 participants ages 18 to 55 and over 65 to assess dosage, safety and immune response. The company also is conducting a phase 1 trial in Japan and a phase 2a in Germany, Spain and the Netherlands.

After the phase 1/2a trial showed a single dose of the vaccine produced neutralizing antibodies for COVID-19, J&J launched a phase 3 randomized controlled trial of a single dose. It began enrollment in the U.S. on Sept. 21, ultimately enrolling 44,325 people age 18 or older in the U.S., South Africa and six Latin American countries. Enrollment in that trial was complete on Dec. 17. The EUA application is based on the results of that trial, which is ongoing. Participants will be followed for two years.

J&J is also still enrolling participants for a second phase 3 randomized controlled trial in 10 countries, including the U.S., assessing a two-dose regimen. And it has planned future clinical studies of children, pregnant women and their infants, and immunocompromised individuals.

How effective is the vaccine?

Overall, J&J reported 66.1% efficacy globally in preventing moderate to severe COVID-19 in adults 28 days or more after vaccination, and an efficacy of 85.4% in preventing severe or critical COVID-19. The efficacy was similar across demographics including age, race and ethnicity.

The FDA noted in its briefing document that there were small numbers of participants 75 years and older in the phase 3 trial, creating “limited interpretability” on the efficacy rate for that subgroup, as well as “insufficient” data to assess efficacy in those who had prior SARS-CoV-2 infection.

Moderate to severe disease was defined as a positive COVID-19 test result and either one respiratory-related or deep vein thrombosis symptom, or two symptoms from a list including fever, chills, sore throat, cough, malaise, headache and gastrointestinal upset, the FDA said. A severe/critical case was defined as a positive test result and one symptom including clinical sign of severe systemic illness, respiratory failure, shock, admission to the ICU and death.

As we’ve explained before, a 66.1% efficacy roughly means a vaccinated person has a 66.1% reduced risk of developing moderate disease, compared with a similar person who wasn’t immunized.

Due to new variants of the virus emerging in recent months, J&J provided data on efficacy by location, showing that the vaccine doesn’t appear to be as effective against the B.1.351 variant, which was first observed in South Africa.

In the U.S. population, there was 72% efficacy in preventing moderate to severe COVID-19 and 85.9% efficacy in preventing severe or critical disease, while than in South Africa, those figures were 64% and 81.7%, respectively.

In Brazil, where another variant first emerged, efficacy was closer to that of the U.S. population: 68.1% in preventing moderate to severe disease.

The vaccine had 100% efficacy in preventing COVID-19 that would require medical intervention — meaning hospitalization, ICU admission, mechanical ventilation or a life support machine — 28 days or more after vaccination.

As of Feb. 5, there were no COVID-19-related deaths in the vaccinated group and seven COVID-19-related deaths in the placebo group, the FDA briefing document said.

There may be some efficacy against asymptomatic cases. J&J said preliminary data suggest an effect “based on a limited number of Day 71 results,” but more investigation was needed.

The participants in the vaccinated group who developed moderate symptoms had “fewer and less severe symptoms” than those in the placebo group, J&J said. After 28 days post-vaccination, 66 people developed such symptoms in the vaccinated group, compared with 193 in the placebo group. For severe/critical cases, there were five in the vaccinated group and 34 in the placebo group.

The table below was part of Johnson & Johnson’s briefing document for the Vaccines and Related Biological Products Advisory Committee.

How safe is the vaccine?

There were “no specific safety concerns” identified in the trial, the FDA briefing document said. Some of those receiving the vaccine did report side effects, including injection site pain, headache, fatigue and myalgia (muscle pain or ache), generally lasting a day or two post-vaccination. These side effects were more common in 18- to 59-year-old participants.

Among the 44,325 participants in the phase 3 trial, 21,895 received the COVID-19 vaccine and 21,888 were in the placebo group.

The FDA determined three non-fatal serious adverse events in the vaccinated group were likely related to the vaccine: a hypersensitivity reaction involving urticaria or hives, injection site pain unresponsive to over-the-counter pain medicine, and a case involving a few days of symptoms including “generalized malaise, weakness, myalgia, shortness of breath, headache, sensation of numbness and tingling in upper extremities, chest pain and fever.”

The FDA noted two reports of Bell’s palsy, a form of temporary facial paralysis, in the vaccine group and two in the placebo group, and one case of Guillain-Barre Syndrome in each group. The FDA said those reports “are unlikely related to study vaccine but a causal relationship cannot be definitively excluded.”

Six vaccine recipients reported deep vein thrombosis, while two placebo recipients did so. Four vaccine recipients reported pulmonary embolism, as did one person in the placebo group. The FDA said there were risk factors among those individuals, but the imbalance between the vaccine and placebo groups meant that “vaccine cannot be excluded as a contributing factor,” recommending surveillance as the vaccine was deployed in larger populations.

There were also more reports of tinnitus, or ringing in the ears, among vaccine recipients: Six reports, compared with none in the placebo group. The FDA again said there was insufficient data to determine a causal relationship to the vaccine, complicated by the fact the individuals had risk factors.

And there were a few cases urticaria, or hives — eight in the vaccine group, three in the placebo group — which the FDA said are “possibly related” to the vaccine.

There were no reports of anaphylaxis immediately after vaccination; however, on Feb. 26, Johnson & Johnson said it had received a report of one such reaction in South Africa.

Editor’s note: This story will be updated as necessary. does not accept advertising. We rely on grants and individual donations from people like you. Please consider a donation. Credit card donations may be made through our “Donate” page. If you prefer to give by check, send to:, Annenberg Public Policy Center, 202 S. 36th St., Philadelphia, PA 19104. 

Daniel TL

Daniel TL