Patients in clinical trials are usually faceless. But as the experimental Covid-19 vaccine being developed by Moderna Therapeutics has begun advancing through studies, it has found a much more visible advocate: trial volunteer Ian Haydon, a 29-year-old in Seattle.

Haydon has spoken about the vaccine on CNN and CNBC. He even said he’d volunteer to be exposed to the novel coronavirus, SARS-CoV-2, if researchers want to test to see if the vaccine was actually effective. But up until now he has left out a key detail: He is, apparently, one of three people in the trial who had a systemic adverse reaction to the vaccine. 

Twelve hours after receiving his second dose, he developed a fever of more than 103 degrees, sought medical attention, and, after being released from an urgent care facility, fainted in his home. He recovered within a day.

He has not brought up the side effects previously, he said, out of “an abundance of caution.”

“I understand that sharing the story, it’s going to be frightening to some people,” he said. “I hope that it doesn’t fuel any sort of general antagonism towards vaccines in general or towards even this vaccine.”

But he decided to speak now because he hopes his story counterbalances the desperation that some people feel to push a vaccine to market regardless of the consequences. Haydon points out that the whole purpose of the study he was in, known as a Phase 1 clinical trial, is to find the right dose of the vaccine going forward. That means to find a dose that causes the body to produce antibodies, but does not result in too many side effects.

“As we rush to get a vaccine developed as quickly as possible, the reality of vaccine development is that it can only be rushed so much and the trial still needs to take place,” Haydon said. “They have to move at the speed they move at. And stories like what happened to me, they matter because they shape the approval process.”

In the 45-person Moderna study, four participants experienced what are known as “Grade 3” adverse events — side effects that are severe or medically significant but not immediately life-threatening. Neither the company nor the National Institute of Allergy and Infectious Diseases, which is running the trial, have previously detailed the nature of those incidents, but Moderna did disclose that three, likely including Haydon, received the highest dose of the vaccine that was tested, and had reactions that involved their whole bodies. A fourth received a lower dose and had a rash at the injection site. 

Such side effects are “noteworthy, but it doesn’t stop the train,” said William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center. The goal of studies is to establish a threshold at which something might go wrong.

With drugs, Schaffner said, patients tolerate the risk of side effects because they want to get better. “In contrast,” Schaffner said, “we give vaccines to healthy people in anticipation that they might contact the germ, the virus, down the road. But because we give them to healthy people, actually our standards for safety are higher than they are for drugs.” 

In Haydon’s words: “The point of the Phase 1 trial is to look out for health problems.” He said he received great medical care, and though he felt more sick than he ever has before, he was never afraid for his long-term health. “I don’t regret the decision I made to enroll in this study.”