As of 31 August 2021, an estimated 33.5 million doses of Janssen (Ad26.COV2.S) vaccine had been administered. The most common side effects with COVID-19 vaccine Janssen are pain at the injection site, headache, tiredness, muscle pain and nausea. They are mild or moderate and resolve within 1 or 2 days after vaccination.

Based on post-marketing safety surveillance, the following rare safety concerns have been identified: thrombosis with thrombocytopenia syndrome (TTS) in approximately 2 cases per million doses administered, Guillain-Barre Syndrome (GBS) in 7–8 cases per million doses administered, and capillary leak syndrome (CLS) in 0.21 cases per million doses administered, some in persons with a prior history of CLS.

In countries with ongoing SARS-CoV-2 transmission, the benefit of vaccination in protecting against COVID-19 far outweighs the risks of any rare adverse reactions. The benefit–risk ratio is greatest in older age groups as the risk of severe COVID-19 disease outcomes, including COVID-19 related thromboembolic events, increases with age.

It is important to remember that serious adverse reactions are very rare and treatable when diagnosed early, and that the risks are higher with COVID-19 disease than following vaccination.