A volunteer in a key coronavirus vaccine trial has revealed that it left him the sickest he has ever been in his life — but still “cautiously optimistic” about the drug’s potential.

Ian Haydon, 29, identified himself in a series of interviews as one of four volunteers who had bad reactions during the trials that Moderna hailed for “positive” early findings of antibody responses.

The Seattle-based communications manager told STAT News that he started getting chills within hours of getting home from his second dose — waking in the early hours with a 103.2-degree fever.

Already nauseous and with aching muscles, his girlfriend called a 24-hour hotline from those leading the study, following their advice to go to urgent care, getting there at 5 a.m., he told STAT.

Ian Haydon

Haydon said he turned down the offer to be taken to a local hospital, and instead went home to rest with Tylenol.

After sleeping a few hours, he still had a temperature of 101.5 — feeling so nauseous he threw up in the bathroom, he said.

He then fainted on his way back to the bedroom — with his girlfriend luckily on hand to stop his head from slamming into the floor, he said.

Calling doctors in the study for a second time, he ultimately decided to stay home and rest, with the fever eventually dropping that night, he said.

Ian HaydonIan Haydon

Haydon initially hid the full drama of his reaction, having only told CNN’s Anderson Cooper that he had a “rough go of things for 24 hours.”

He was wary about revealing the horror he suffered because he was wary about “triggering anti-vax” people, he admitted in a series of tweets.

“I understand that sharing the story, it’s going to be frightening to some people,” he told STAT News.

“I hope that it doesn’t fuel any sort of general antagonism towards vaccines in general or towards even this vaccine.”

He stressed that he had received the highest dose from the trials — one 10 times stronger than others — one he has been told “will no longer be tested.”

“There’s no failure here — this is a big reason why we do clinical trials,” he insisted on Twitter. “Even safe meds can’t be taken at 10x doses.”

He also insisted as sick as he was it was never “life-threatening,” saying, “They’re over, and I’m back to marathon training.”

Even after his sickness, he said he remains “cautiously optimistic” about a vaccine breakthrough from the trial.

“Vaccines are the single most important medicines we have. It’s important to test them carefully — which is what’s happening here,” he wrote. “What I went through pales in comparison to being hospitalized with COVID-19. Or to the pain of losing a loved one to the pandemic,” he said.

The study, run with the National Institute of Allergy and Infectious Diseases, tested 45 patients between the ages of 18 and 55. Initial results showed the drug created antibody levels that were equal to or exceeded those found in patients who recovered from COVID-19. Moderna CEO Stephane Bancel told Bloomberg that the results “couldn’t have been better.”

Moderna did not immediately respond to requests for comment on Haydon’s interviews.