I've seen a lot of chatter to the effect that the FDA/CDC advise to pause administration of the Johnson and Johnson vaccine is ignorant, counterproductive, foolish and contrary to the public interest. The same reaction followed when many countries in the EU halted administration of the AstraZeneca vaccine, accusing the authorities of succumbing to the post hoc, ergo propter hoc fallacy. Well no. There is a legitimate debate about whether this was an overreaction, I suppose, but it isn't simple.
Here's a good summary of the issues. It isn't as simple as saying there have been six cases of the adverse effect -- cerebral venous sinus thrombosis, a rare event in the first place, but associated with low platelet count in these cases which is unusual -- out of more than six million doses. So that's one in a million, far less dangerous than actually getting Covid-19, so why are we even paying attention to it?
Well, in the first place, all of the cases are in young adult women, who are at low risk from Covid-19 complications, so the cost-benefit calculus isn't quite as extreme as it might appear at first glance. And it's not just "blood clots," as most news stories have it, it's something more specific and much more rare. Also, most doctors have never seen this and won't know to recognize it or how to treat it. Also, if this can happen maybe there is some biological mechanism going on that could have other ill effects which haven't yet been recognized. The same complication seems to be associated with the AstraZeneca vaccine and they both use adenovirus vectors, so something seems to be going on there that the regulatory agencies want to understand.
They may move very quickly to make recommendations, which might be, for example, that this is not the best choice of vaccine for women younger than 50; that information be disseminated to the medical community about how to recognize this and treat it; and that research continue and very careful monitoring be instituted for this vaccine. There is little cost to the pause because there is a lot of Pfizer-Biontech and Moderna vaccine available and this one isn't really needed in the U.S., although it's a lot easier to distribute and administer under some relatively challenging circumstances. It only requires one dose and it doesn't need to be kept as cold, so it's handier.
The fear is that this may contribute to more general distrust of vaccines. It shouldn't, logically. On the contrary, the risk is very low, no matter how you slice it, and the regulators have demonstrated extremely high vigilance. Unfortunately facts and logic aren't that influential any more. But if you're willing to take it from me, not to worry. Get vaccinated.