Johnson & Johnson said the pause should only be temporary, however, as it reviews data linked to the trial and determines whether the unnamed participant received a dose of the vaccine candidate or a mere placebo.
The Johnson & Johnson trial, which is part of the U.S. government's 'Operation Warp Speed' effort to develop an effective vaccine for COVID-19 and includes some 60,000 volunteers, is the second study in as many months to pause its dosing, following a similar decision by AstraZeneca in September.
"Adverse events – illnesses, accidents, etc. - even those that are serious, are an expected part of any clinical study, especially large studies. Based on our strong commitment to safety, all clinical studies conducted by the Janssen Pharmaceutical Companies of Johnson & Johnson have prespecified guidelines," the company said in a statement.
"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 statement added.
Johnson & Johnson shares were marked 1.8% lower in early trading Tuesday to change hands at $149.07 each, a move that would trim the stock's six-month gain to around 6.7%.
Johnson & Johnson also published its third quarter earnings Tuesday, with a stronger-than-expected adjusted profit of $2.20 per share on worldwide revenues of $21.1 billion.
COVID-19 vaccine trials are being closely watched as the best hope for ending the global pandemic that has killed more than a million people, sickened nearly 38 million and caused economic turmoil worldwide. Nearly 215,000 of the deaths have been recorded in the U.S. amid a disorganized and highly politicized response to the disease.
President Donald Trump has staked much of his re-election hopes on the accelerated approval of a vaccine, prompting fears that safety guidelines would be ignored in the rush to announce a success ahead of the Nov. 3 election.
Dr. Anthony Fauci, the U.S.'s foremost expert on infectious diseases, said AstraZeneca's decision to pause was "unfortunate", but a likely necessary "safety valve" in the complicated clinical trials process.
Last month, CEOs of nine drug manufacturers released a signed pledge to adhere to proper safety guidelines as they pursue development of a vaccine.
His comments echoed those published in a letter signed by the CEO's of nine pharmaceutical companies on September 8, pledging to 'stand with science' and not rush any potential vaccine to market before proper safety procedures were followed in “large, high quality clinical trials”