Johnson & Johnson is pausing its coronavirus vaccine clinical trials as one of the patients is exhibiting "unexplained" symptoms, the company said in a statement.

The company announced phase 3 trials on 23 September.

"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. Following our guidelines, the participant’s illness is being reviewed and evaluated by the ENSEMBLE independent Data Safety Monitoring Board (DSMB) as well as our internal clinical and safety physicians", the Monday statement says.

According to the release, clinical trials are placebo-controlled, so it is not always immediately clear whether a participant received the actual treatment or a placebo.

"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", Johnson & Johnson said in its statement.

The company's phase 3 coronavirus vaccine trials involve 60,000 volunteers in various countries, including the US, Brazil and Mexico. 

The Johnson & Johnson single-dose shot is the fourth vaccine candidate to enter late-stage testing in the United States, after Moderna, Pfizer/BioNTech and AstraZeneca. began similar trials.