Johnson & Johnson’s clinical trial of its COVID-19 vaccine was paused after a volunteer came down with an unexplained illness. The company did not disclose what the illness was, citing the participant’s privacy. The illness is still under investigation.

The pause was triggered by the company based on the internal guidelines for the study. Pauses are common in large trials like this one and typically aren’t publicly disclosed. They’re different than clinical holds, which are issued by the Food and Drug Administration (FDA) and prompt a more extensive evaluation of the trial. Intense public scrutiny of the COVID-19 vaccine development process means any changes — even relatively routine pauses — tend to leak out, and companies are being more transparent than usual.

Johnson & Johnson’s study is the largest COVID-19 vaccine trial, and it aims to enroll 60,000 participants. With a group of that size, unexpected events that lead to a trial pause aren’t surprising. “If we do a study of 60,000 people, that is a small village,” a source familiar with the study told Stat News, which first reported the pause. “In a small village there are a lot of medical events that happen.”

The clinical trial for the AstraZeneca COVID-19 vaccine candidate is also on hold in the United States. A patient in the United Kingdom’s trial of that vaccine was hospitalized with severe neurological symptoms. It’s not clear if the vaccine caused the complications or if the illness was set off by something else. Health regulators in the UK gave AstraZeneca the go-ahead to resume the trial there, but the FDA is still investigating the incident.

There are four Phase 3 COVID-19 vaccine trials in progress in the US, and the other two — run by Pfizer and Moderna — are still underway. Both Pfizer and Moderna are testing gene-based vaccines, which inject tiny pieces of the coronavirus’ genetic material. AstraZeneca and Johnson & Johnson built their vaccines using weakened versions of mild viruses that usually cause symptoms like diarrhea or pink eye.

The trial pauses don’t necessarily mean that the vaccine doesn’t work or is unsafe. They’re normal parts of the process and mean that the companies (and regulators) are closely monitoring for any signs of a problem. “We’ve got to let the process play out and it’s going to take a while,” Ashish Jha, the dean of the Brown University School of Public Health, told CNN. “To me it’s reassuring that companies are acting responsibly and pausing when they need to.”