[26][27][28] It is a viral vector vaccine based on a human adenovirus that has been modified to contain the gene for making the spike protein of the SARS-CoV-2 virus that causes COVID‑19.

[42] The Janssen COVID‑19 vaccine is used to provide protection against infection by the SARS-CoV-2 virus in order to prevent COVID‑19 in people aged eighteen years and older.

[46] In October 2021, Janssen reported at a meeting of the US Food and Drug Administration Vaccines and Related Biological Products Advisory Committee (VRBPAC) that a single dose produced durable protection against severe disease and hospitalization for at least 6 months in the United States, even when Delta emerged, but also a global decrease in protection against moderate disease attributed to emerging variants outside the US.

[51][52] The vaccine consists of a replication-incompetent recombinant adenovirus type 26 (Ad26) viral vector expressing the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein in a stabilized conformation.

[36][53] Unpunctured vials may be stored between 9 and 25 °C (48 and 77 °F) for up to twelve hours,[30][53] and the vaccine can remain viable for months in a standard refrigerator.

Coughing, joint pain, fever, chills, redness, and swelling at the injection site occurred in less than 1 in 10 people.

[1] An increased risk of the rare and potentially fatal thrombosis with thrombocytopenia syndrome (TTS) has been associated with mainly younger female recipients of the vaccine.

[86] Allergic reactions, including anaphylaxis, can occur in rare cases within a few minutes to one hour after receiving a dose.

[30][88][89] The European Medicines Agency (EMA) listed Guillain-Barré syndrome (GBS) as a very rare side effect of COVID‑19 Vaccine Janssen and added a warning in the product information.

[90] In August 2021, the Pharmacovigilance Risk Assessment Committee (PRAC) recommended updating the product information to the European Medicines Agency (EMA) that "cases of dizziness and tinnitus (ringing or other noises in one or both ears) are linked to the administration of COVID‑19 vaccine Janssen.

[106] In June 2020, Johnson & Johnson and the National Institute of Allergy and Infectious Diseases (NIAID) confirmed that they planned to start clinical trials of the Ad26.COV2.S vaccine in September 2020, with the possibility of phase I–IIa human clinical trials starting at an accelerated pace in the second half of July.

[107][108][109] A phase I–IIa clinical trial started with the recruitment of the first subject in July 2020 and enrolled study participants in Belgium and the US.

With one dose, after 29 days, the vaccine ensured ninety percent of participants had enough antibodies required to neutralize the virus.

[111][112] A substudy with 20 participants found that humoral and cell-mediated immune responses, including cytotoxic T cells, lasted for at least 8 months.

Study participants received a single intramuscular injection of Ad26.COV2.S at a dose level of 5×1010 virus particles on day one.

The interim analysis was based on 468 cases of symptomatic COVID‑19 among 43,783 adult volunteers in Argentina, Brazil, Chile, Colombia, Mexico, Peru, South Africa, and the United States.

[113] Beginning in December 2020, clinical trial of the vaccine candidate has been undergoing a "rolling review" process by the Committee for Medicinal Products for Human Use of the European Medicines Agency (EMA), a step to expedite EMA consideration of an expected conditional marketing authorization.

[32][37][131] In February, the Advisory Committee on Immunization Practices (ACIP) of the Centers for Disease Control and Prevention (CDC) recommended the use of the vaccine for those aged 18 and older.

[132][38] In April 2021, the CDC and the FDA issued a joint statement recommending that use of the Janssen vaccine be suspended, due to reports of six cases of cerebral venous sinus thrombosis—a "rare and severe" blood clot—in combination with low levels of blood platelets (thrombocytopenia), in six women between the ages of 18 and 48 who had received the vaccine.

[159] In June 2021, Malaysia's National Pharmaceutical Regulatory Agency (NPRA) issued conditional registration for emergency use of the vaccine.

[163] In September 2021, National Agency of Drug and Food Control (BPOM) issued emergency use authorization in Indonesia.

[164] In November 2021, the vaccine's authorization under interim order in Canada was transitioned to approval for use under the country's Food and Drug Regulations.

[166][167][168] In October 2021, the US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) authorized the use of either homologous or heterologous vaccine booster doses.

[169] Given the Janssen vaccine is a single dose and has a lower cost, it was expected to play an important role in low and middle-income countries.

[179] South African health minister Zweli Mkhize announced on 9 February 2021 that the country would sell or swap its one million doses of AstraZeneca vaccine.

The deal, worth more than $1 billion, is funded by the Biomedical Advanced Research and Development Authority (BARDA) and the U.S. Department of Defense.

[187] Shipments of the vaccine were scheduled to start in the second half of April 2021, with a commitment to deliver at least two hundred million doses to the EU in 2021.

[188] The European distribution of the vaccine was slightly delayed until the EMA decided that rare cases of vaccine-induced blood clots did not outweigh the benefits of helping to fight the COVID‑19 pandemic.

[189] The United States Conference of Catholic Bishops expressed concern about the vaccine because the cell line Per.C6, which is used in development and production, was originally derived from the retinal tissue of an 18-week-old fetus electively aborted in 1985.

[196] Others advance the view that the cells used for COVID‑19 vaccines are thousands of generations removed from their source material and do not contain any fetal tissue.