Johnson & Johnson said Tuesday that it learned of the “unexplained illness,” which caused it to pause its Phase 3 coronavirus vaccine trial in the US, on Sunday and immediately informed the Data and Safety Monitoring Board, or DSMB, which monitors clinical trial developments.

The drugmaker said it did not know whether the volunteer who became ill had received the vaccine or placebo.

“We are now awaiting further medical information and evaluation, which we will then forward to the DSMB for further independent recommendations,” Mathai Mammen, Global Head of Research and Development at Janssen, said Tuesday on the company’s third-quarter earnings call.

“It’s not at all unusual for unexpected illnesses to occur in large studies over their duration,” Mammen said. In some cases, these are called serious adverse events, or SAEs, and may have something or nothing to do with the drug or vaccine being investigated,” Mammen said.

Mammen said the company has “very little information right now” regarding the illness which paused the trial but said that the DSMB has submitted a number of specific questions for them to answer.

As of Tuesday morning, the trial remains blinded, meaning that participants and those administering the vaccine candidates are unaware if they are receiving the vaccine or a placebo. Mammen added that the DSMB has the ability to unblind the study in order to investigate the unexplained illness if necessary.

Johnson & Johnson announced the clinical trial pause on Monday, making it the second Phase 3 coronavirus vaccine trial to be paused.

AstraZeneca’s vaccine trial was paused last month because of a neurological complication in a volunteer in the UK. While the trial resumed there and in other countries, it remains paused in the United States while the US Food and Drug Administration investigated.