Budrul Chukrut | LightRocket | Getty ImagesThe study of Johnson & Johnson’s Covid-19 vaccine has been paused due to an unexplained illness in a study participant.A document sent to outside researchers running the 60,000-patient clinical trial states that a “pausing rule” has been met, that the online system used to enroll patients in the study has been closed, and that the data and safety monitoring board — an independent committee that watches over the safety of patients in the clinical trial — would be convened. The document was obtained by STAT.Contacted by STAT, J&J confirmed the study pause, saying it was due to “an unexplained illness in a study participant.” The company declined to provide further details.”We must respect this participant’s privacy. We’re also learning more about this participant’s illness, and it’s important to have all the facts before we share additional information,” the company said in a statement.J&J 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.Given the size of Johnson & Johnson’s trial, it’s not surprising that study pauses could occur, and another could happen if this one resolves, a source familiar with the study said.”If we do a study of 60,000 people, that is a small village,” the source said. “In a small village there are a lot of medical events that happen.”On Sept. 8, a large study of another Covid-19 vaccine being developed by AstraZeneca and Oxford University was put on hold because of a suspected adverse reaction in a patient in the United Kingdom. It’s believed that the patient had transverse myelitis, a spinal cord problem. Studies of the vaccine resumed roughly a week after it was paused in the United Kingdom, and have since been restarted in other countries as well. It remains on hold, however, in the United States. Johnson and Johnson began enrolling volunteers in its Phase 3 study on Sept. 23. Researchers planned to enroll 60,000 participants in the United States and other countries.