Coronavirus COVID-19 vaccine: Volunteer has severe reaction to second shot – TODAY

A Seattle man who is taking part in a COVID-19 vaccine trial is opening up about experiencing an adverse reaction that briefly left him unconscious.

Ian Haydon, 29, said volunteering was the right thing to do when he shared his story with TODAY last month after receiving the first dose of the vaccine candidate.

The adverse reaction came after the second shot on May 5. Haydon is only now revealing the details in part because it wasnt clear what happened at first and because he found it hard to talk about vaccine safety without triggering anti-vaccine activists, he tweeted Tuesday. "There's no failure here," he emphasized.

Haydon felt some arm pain at the injection site, just like after the first dose, but it came on much more quickly than before, he said.

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.

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His partner called the 24-hour hotline for the vaccine study and Haydon was advised to go to urgent care. There, he was met by the study's lead doctor and medical staff who were all wearing full personal protective equipment, including face shields, masks and gowns. One nurse even had a powered air-purifying respirator.

Haydon received intravenous fluids and Tylenol. He was given a COVID-19 nasal swab and had blood drawn from his arms to use for a series of viral panel tests. It turned out he did not have COVID-19.

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.

The symptoms he experienced weren't life threatening and they're over, Haydon said, adding hes now feeling fine and back to marathon training.

Haydon recently found out he received the highest dose of the vaccine tested in the trial 10 times the amount some other participants were injected with. In a news release last week, Moderna, Inc., the Massachusetts-based biotechnology company that helped develop the vaccine, said the most notable adverse events were seen at this highest dose level affecting three participants, but only after the second dose.

The symptoms included fever, muscle pains and headaches, and all went away after a day, Dr. Tal Zaks, chief medical officer at Moderna, told The New York Times. The high dose wont be used in future studies because the lower doses appear to work well, he added.

The company said its vaccine candidate can prompt an immune response in the human body and was found to be safe and well-tolerated in a group of 45 patients. The drug is now being tested in larger studies.

The potential of adverse reactions isn't a surprise: Participating in a clinical trial comes with the risk of serious side effects or feeling uncomfortable, the National Institutes of Health noted.

Haydon said he consented to be in the study and everyone admitted could have received any dose of the vaccine. He praised the excellent medical care he received during the incident.

This is exactly the kind of event that a phase 1 study is built for. The clinicians are looking to see how people react to different doses of the vaccine, Haydon said.

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.

Haydon is still in the study and will be monitored periodically for about another year, just like all the other participants, he said.

A. Pawlowski is a TODAY contributing editor focusing on health news and features. Previously, she was a writer, producer and editor at CNN.

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Coronavirus COVID-19 vaccine: Volunteer has severe reaction to second shot - TODAY