Johnson & Johnson has temporarily halted its US trial of a vaccine for Covid-19 after a test volunteer developed a mysterious illness.

It’s the second time in two months that a Covid-19 vaccine trial has been halted following an unexplained illness in a test subject, raising concerns about the safety of the vaccines and dampening expectations of a quick fix for the pandemic.

Researchers conducting the Johnson & Johnson trial, which includes 60,000 volunteers around the world, were told Monday to pause testing of the single-dose vaccine. The New Jersey-based corporation is one of a handful of companies in the world that has reached the final phase of testing on people.

“We have temporarily paused further dosing in all our Covid-19 vaccine candidate clinical trials … due to an unexplained illness in a study participant,” according to a statement from the company. More details will be revealed after the trial participant’s condition is more thoroughly evaluated, the statement said.

“We are committed to providing transparent updates throughout the clinical development process of our vaccine candidate,” it added.

It also assured that “adverse events — illnesses, accidents, etc. — even those that are serious, are an expected part of any clinical study, especially large studies.”  

UK drugmaker AstraZeneca last month temporarily stopped tests of its vaccine candidate after a trial participant fell ill. That trial has resumed in several countries but not the US. It has been considered a leading contender in the race toward an effective and safe vaccine.

AstraZeneca, which is working with Oxford University, is awaiting a decision from US regulators on whether it can resume tests in America, according to Bloomberg. The clinical trial participant who became ill is from Britain. 

Both cases are a wake-up call about the hurdles to developing a reliable vaccine. President Donald Trump has repeatedly claimed that a Covid-19 vaccine would be available by the November 3 election, but most experts predict one or more might be available deep into 2021.

Dr. Ashish Jha, the dean of the Brown University School of Public Health, told CNN’s Chris Cuomo that such a testing pause is “completely expected” during the development of a vaccine. Jha also called it a “reminder of how ridiculous it is to try and meet a political timeline of having a vaccine before November 9.