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Johnson & Johnson emphasized that so-called adverse events — illnesses, accidents, and other bad medical outcomes — are an expected part of a clinical study, and also emphasized the difference between a study pause and a clinical hold, which is a formal regulatory action that can last much longer. The vaccine study is not currently under a clinical hold. J&J said that while it normally communicates clinical holds to the public, it does not usually inform the public of study pauses.

The data and safety monitoring board, or DSMB, convened late Monday to review the case. J&J said that in cases like this ‘it is not always immediately apparent’ whether the participant who experienced an adverse event received a study treatment or a placebo.

Though clinical trial pauses are not uncommon — and in some cases last only a few days — they are generating outsized attention in the race to test vaccines against SARS-CoV-2, the virus that causes Covid-19.