It was not immediately clear what the "unexplained illness" was, and the company said it is reviewing all relevant medical information before deciding whether to restart the study.
Johnson & Johnson ordered a temporary halt in its late-stage trial this week after a patient became ill, while AstraZeneca has paused testing in the U.S. since early September after a patient came down with unexplained neurological symptoms. The data and safety monitoring board - an independent committee that watches over the safety of patients in the clinical trial - would be convened.
A team at Cincinnati Children's Hospital will begin vaccinating teenagers aged 16 and 17 this week, and will move to enroll 12-to 15-year-olds later, said Dr. Robert Frenck, director of the Vaccine Research Center at the hospital.
Illnesses, even those that are serious, "are an expected part of any clinical study, especially large studies", Johnson & Johnson said, and a study's pause "is a standard component of a clinical trial protocol".
Given the size of the trial, it's not surprising that study pauses could occur, and another could happen if this one resolves, a source said. When the Food and Drug Administration recently said that vaccine trials must allow two months after the last experimental dose was administered, these instructions "change" the schedule again.
This is the most concrete time frame a leading vaccine developer in the West has given for an emergency-use-authorization application.
The recruitment for phase three trial has been suspended as a precaution.
The US is attempting to fast-track vaccine development too, after President Donald Trump authorised Operation Warp Speed in April to support companies with federal funding.
Coronavirus vaccines should be rolled out in four phases, with initial supply going to front-line health workers and first responders, an independent expert panel tapped by top USA health officials recommended earlier this month.