First drug for any disease must 1) show efficacy, that is there is statistical evidence that is cures, reduces symptoms, or other benefits. 2) it must statistically demonstrate safety, that is it must do less harm that the disease would if left untreated.

For all drugs that follow for that disease they must demonstrate superiority. That requires one or more of the following 1) better efficacy - higher percentage of people receive some benefit from the new drug than the old drug. 2) the safety profile must be better than those drugs that are already approved. 3) in cases where existing drugs may lose effectiveness, such as infectious diseases, it must be at least equivalent for 1 and 2 and have some expectation that it might work when the others don't.

This situation is unusual by all measures. Pfizer and Moderna were approved first and are roughly equivalent. This was third, and meets the criteria, as long as it has good safety. These clots could indicate that it is less safe than the pre-existing vaccines, if so it would typically be pulled from market. However, because this is such an unusual situation what will happen after reviewing the data is anyone's guess. Especially when there is already a large amount of fear and distrust.