Fast doesn't mean rushed. The trials were fast because it was easy to gather huge numbers of participants, and the phases were carried out in parallel rather than sequentially (normally, to save money/resources, phase II would only start after a successful phase I).

> and pressured approvals

Immense pressure to make sure it's safe, too. Like this precautionary pause for what seems like a lower blood-clot rate than would be expected just by chance in that size of population.