The NYT makes the headline about an adverse reaction and the study is paused because of it. To my mind 'adverse reaction' means that they've established the cause and effect, and the study was halted because someone reacted badly to the vaccine.

My issue is that it's not clear whether the patient's illness is due to the vaccine or something else, so headlining with 'adverse reaction to the vaccine' is jumping to a conclusion. They don't say 'possible', which is the truth.