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.”

Haydon, a communications manager at a university, initially found out about the study, which was being run in Seattle, from a colleague who sent him a link. He, along with thousands of other people, applied. They called him 11 days after he applied.

He went to the trial site for a physical, and signed a 20-page consent form. The vaccine, it told him, could conceivably cause severe anaphylactic shock, and there was no way to predict exactly how his immune system would respond to the new vaccine. He’d looked at research on other Moderna experimental vaccines, which work via an entirely new technology that uses messenger RNA, the body’s key envoy of genetic information inside cells, and thought they seemed relatively safe. During the physical, researchers took blood; the lab work came back a week later, and he received his first dose of the vaccine on April 8.

Haydon doesn’t like needles, and was as worried about the blood draw — which uses a bigger needle — than the actual shot. He remembers waiting, and being told that the reason he was waiting was because researchers were giving doses in ascending order, and he was to receive the high dose of the vaccine. But the injection was uneventful. If his eyes were closed, he said, he would not have felt it. He was given a paper log on which to write down any symptoms, a digital thermometer, and a small ruler to measure any reactions at the injection site.

He had arm pain the next day, “like being punched in the arm,” he said, and for a day he had trouble lifting his arm at the shoulder. But within days he was back to normal.

Haydon said he was slightly nervous before the second dose. He knew that second doses were given to increase the immune system’s response, and wondered if he might have more side effects. His arm became sore much more quickly this time. He got home from the clinic at about noon. At around 10 p.m., he started to get chills. He’s normally too hot at night, but he bundled up in sweats. His fingertips felt cold. He fell asleep, but woke up a few hours later with a raging fever.

At 1:30 a.m., his temperature was 103.2 degrees. At 3:45, it was 103 degrees. He was nauseous, and his muscles hurt.

The clinic where he was vaccinated had given him a 24-hour phone number to call, but he’d been reticent. His girlfriend, with whom he lives, called. They said to go to urgent care. It was a 10-minute drive. They arrived at 5 a.m.

The doctors met him in what looked like space suits. Even though he’d had a vaccine, he was also a potential Covid patient. They took him into an exam room, took a lot of blood, and gave him a nasal swab. He asked them to avoid his left arm, where he’d gotten the vaccine, but they ended up taking blood from both of his arms. His fever had already fallen to 99.8 degrees. They gave him Tylenol. The physician taking care of him offered to try to get him admitted to a neighboring hospital, but he decided to head home.

He and his girlfriend arrived home at 7 a.m., and he slept until noon. His temperature was 101.5. He got up to go to the bathroom, and became so nauseous he threw up. On his way back from the bathroom, he fainted. His girlfriend caught him and kept his head from hitting the floor.

She then called one of the doctors working in the study, and asked what they should do. The doctor told them he could go back to urgent care, or call 911, and reminded them that all his medical costs would be covered by the study.

But he got to a couch and was given sports drinks. He spent the afternoon there, with a wet towel on his head, fighting the fever. By 9:45 p.m., it was back down to 99.1. It tapered off. He says he felt better within a few days, and has had no side effects since.

Haydon said the experience left him as sick as he’d ever felt. But standard flu-like symptoms that resolve within a day are not necessarily considered a reason not to use a vaccine that prevents a more serious illness.

Given the stakes of a Covid-19 vaccine, the side effects described in the Moderna release would likely be seen as acceptable even if they turned out to be seen in future studies. The severe effects were only seen at high doses that are not being taken forward. The other vaccine for which early data are available caused fever in almost half of recipients.

But it’s also not clear what will happen as the vaccine moves into larger studies. “Humans have a very diverse immune system,” said Larry Schlesinger, the president and CEO of the Texas Biomedical Research Institute, a nonprofit. “And then you add on top of that diabetes or, you know, age 70 and you can imagine that the immune response will be very, very different.”

The difficulty, Schlesinger said, is that right now we are only getting “tidbits” of information about the new vaccine.

“Tidbits of science are always dangerous for the public to read because they give a false understanding, or a false sense of security, that we’re making progress or not,” Schlesinger said. “And then tomorrow we hear something completely opposite. And before you know it, the credibility of the scientific process is undermined and people stop listening.”

What does he think now? “It’s just not enough information at the current time.” That’s why we need clinical trials — and volunteers like Ian Haydon.