It was always a possibility that some small fraction of the people who volunteered to be part of a coronavirus vaccine trial would have an adverse reaction to it. It just happens that one of the people who did have a negative reaction, a man named Ian Haydon, had become the most high-profile test subject thanks to his frequent interaction with the media. Back in April when Haydon received his first shot as part of the trial, he gave an interview to NBC News about the experience:

When I received my first shot on April 8, I had to wear a mask the whole time and everybody there was wearing masks, but otherwise it was pretty normal. The shot was basically just like a flu shot — it went into my left shoulder. It didn’t hurt at all when I got it. I had to do a blood draw immediately before receiving the injection and that’s what they’re going to compare my blood to going forward, looking for changes.

The appointment took three hours in total and that included staying at the clinic for an hour after receiving the injection to make sure I didn’t have any immediate negative reactions.

Haydon got his second shot as part of the trial a month later, or just about three weeks ago. But the second shot did not go as well for him. He had a strong reaction including a high fever and eventually wound up passing out at home.

“Then, 12 hours after getting the second injection, I suddenly had severe chills. I decided to go to sleep but woke up in the middle of the night with a fever that was over 103 degrees. I also found I was nauseous, fatigued and had quite a headache,” Haydon told TODAY.

His partner called the 24-hour hotline for the vaccine study and Haydon was advised to go to urgent care…Haydon received intravenous fluids and Tylenol…

After leaving urgent care, Haydon went home to rest. His fever rose back to 101 degrees and he had to throw up.

“On my way back to bed I fainted. My girlfriend caught me as I went down and kept me from hitting my head. She woke me up. I remember being confused at the sight of my living room ceiling,” Haydon recalled.

Even after what sounds like a pretty difficult experience, Haydon was hesitant to share his story. He was concerned it would become grist for the anti-vaccine mill. He told STAT, “I understand that sharing the story, it’s going to be frightening to some people. I hope that it doesn’t fuel any sort of general antagonism towards vaccines in general or towards even this vaccine.”

Here’s an interview Haydon gave to CNBC 9 days ago. This is before he talked about his reaction. He did admit in this interview that he felt “crummy” after the 2nd injection but didn’t really spell out how bad it was.

One of the goals of the phase 1 trial was to determine the appropriate dosage of the vaccine. And it turns out that Haydon and two other people who also had strong reactions had been given the highest dosage used in the trial.

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.

As far as Haydon is concerned, the trial did exactly what it was supposed to do. It indicated that too high a dosage of the vaccine could have side effects, but he says that even those weren’t life threatening. He recovered about a day after passing out and is now back to his active routine. He says he remains “cautiously optimistic” about the vaccine.

The symptoms I experienced weren't life threatening. They're over, and I'm back to marathon training.

Vaccines are the single most important medicines we have. It's important to test them carefully — which is what's happening here.

I remain cautiously optimistic.

— Ian Haydon (@ichaydon) May 26, 2020