There weren’t any particular red flags during the trials (although I think one of the 14 was a young man who died and is part of the EMA investigation data)
The Janssen COVID-19 vaccine was publicly approved for international use on Wednesday by UN health agency expert advisory board, SAGE, which allayed concerns over clotting events being associated by some countries, without definitive evidence, with coronavirus jabs.
news.un.org
During trials for the Janssen vaccine involving nearly 44,000 people, 10 of the 22,000 people who received the blank dose developed a blood clot – or thrombo-embolic events - while 14 of the remaining 22,000 who were inoculated, developed a clot. “This is about the same”, for both groups said Dr Annelies Wilder-Smith, SAGE Technical Advisor. “There’s a slight imbalance, but it’s still not statistically significant,”
It has been halted in a couple of places but these seem more due to allergic reactions or a corrupted batch rather than the blood clots