Why was the J&J vaccine paused and then restarted?
The FDA and CDC wanted to evaluate a total of 15 rare events of cerebral venous thrombosis (a very rare type of clotting disorder) and low platelet count. One of these people died; and another was hospitalized. This syndrome (large vessel blood clots with low platelet count) was deemed to be biologically plausible to be associated with this type of vaccine.
The pause also afforded the opportunity to provide clarity to providers on a) who appears to be at increased risk and b) how to treat it appropriately if there is concern that it happened in someone who received the J&J vaccine.
The conclusion that they came to was that these rare events were in fact quite rare; and the benefits of the J&J vaccine greatly outweighed the risks of these events.