"We have temporarily paused further dosing in all our COVID-19 vaccine candidate clinical trials, including the Phase 3 ENSEMBLE trial, due to an unexplained illness in a study participant," the company said in a statement.

"Adverse events – illnesses, accidents, etc. - even those that are serious, are an expected part of any clinical study, especially large studies," the company said in a statement.

Based on our strong commitment to safety, all clinical studies conducted by the Janssen Pharmaceutical Companies of Johnson & Johnson have prespecified guidelines.

"These ensure our studies may be paused if an unexpected serious adverse event (SAE) that might be related to a vaccine or study drug is reported, so there can be a careful review of all of the medical information before deciding whether to restart the study," the company said.

In the phase 3 trial, the company had started recruiting participants in late September.

In an earlier press release, the company had said: "Based on these results and following discussions with the U.S. Food and Drug Administration (FDA), ENSEMBLE will enroll up to 60,000 volunteers across three continents and will study the safety and efficacy of a single vaccine dose versus placebo in preventing COVID-19."

 The Company had earlier said it is committed to bringing an affordable vaccine to the public on a not-for-profit basis for emergency pandemic use and anticipates the first batches of a COVID-19 vaccine to be available for emergency use authorization in early 2021, if proven to be safe and effective.