https://www.cnbc.com/2021/02/24/covid-vaccine-these-are-the-most-common-side-effects-from-jjs-.html
While the rate of unexpected side effects was similar between the vaccine and placebo groups, there was a slight imbalance of some reactions in the vaccine group to those who received a placebo. The vaccine group reported 15 cases of “embolic and thrombotic events,” or blood clots, among 14 recipients, compared with 10 in the placebo group.
https://www.politico.com/news/2021/02/24/fda-johnson-johnson-single-dose-shot-covid-471327
Volunteers reported mild reactions such as injection site pain, headaches and fatigue, but there also were a number of blood clots in the vaccine group including one in a 25-year-old man with no prior medical history who needed surgery.
FDA said data is insufficient at this time to determine whether the vaccine caused or contributed to those events — especially because many of the people who experienced clotting had other risk factors. The agency said it “will recommend surveillance for further evaluation of thromboembolic events with deployment of the vaccine into larger populations.”
Johnson & Johnson said it did not consider the clotting events to be linked to the vaccine, since the rate of cases in the trial was similar to the frequency at which the cases occur in the general population.
