The multinational maker of packaged consumer goods, medical devices and pharmaceuticals declined to provide details about the reaction or illness, leaving the public and thousands of other trial participants in the dark.

In a press release, the company said its decision to withhold details was made out of respect for the trial participant's privacy. But the company also said the illness "might be related to a vaccine."

According to Johnson & Johnson's clinical trial guidelines, trials are paused only if participants experience a vaccine-related adverse event. The Phase 3 study began on Sept. 23 with researchers planning to enroll 60,000 participants in the U.S. and other countries.

Moreover, the Johnson & Johnson trial uses a sodium chloride solution (saline) as a placebo. Saline injections, regarded by the scientific community as harmless, are widely accepted as a valid placebo control in vaccine trials.

In September, the director of the National Institutes for Health, Francis Collins, told reporters that vaccine trials taking place as part of "Operation Warp Speed" are larger and more rigorous than ever before.