In this case, the manufacturer itself is behind the decision, and at least part of the reason is to get information out to doctors and proper advice for giving the vaccine. If it should not be given to certain demographics, that needs to be worked out. And current informed consent information and documents needs to be updated. In addition they need to consider how to treat the complication. Standard rapid treatment for a brain clot is a blood thinner, which will make this type of clot worse.*
Here in the US, delays from switching to another vaccine are small. People deserve to be presented with accurate information for informed consent purposes, and the risk of waiting to get Pfizer or Moderna instead is not huge. Janssen has not been a huge part of our vaccination effort so far, and numbers of shots available just dropped anyway due to supply issues.
And so far, numbers look to be around 1 per million, but we know with scrutiny, similar numbers in Europe for AZ evolved into about 1 per 100,000. So pausing to get a handle on things does not seem like an overreaction to me. Again, I think the experts should be given the benefit of the doubt as having much much more information, knowledge, and expertise.
*I believe that, when a clot is suspected in the ER, heparin may even be given immediately, because rapid treatment for the by far more common type of clot is so tremendously beneficial, and this type of clot is so rare. If the incidence of this type of clot is changing due to it being a vaccine complication, treatment decisions might be impacted, and it could cause additional harm and deaths not to have a very good handle on the incidence of CVST.