Drug company workers, government officials and others have been injected outside the usual testing process. More will be soon, bewildering experts who worry about potential ill effects.

Raina MacIntyre, who heads the biosecurity program at the Kirby Institute of the University of New South Wales in Sydney, Australia, said she would not recommend the emergency use of vaccines before the conclusion of Phase 3 trials. AstraZeneca, the British-Swedish company, halted late-stage testing in the United States on a vaccine candidate this month after one volunteer fell seriously ill for unknown reasons.

Flawed vaccines can cause significant health problems. In 2017, children who were injected with Sanofi’s dengue vaccine became sicker. Children vaccinated against respiratory syncytial virus, or R.S.V., in the 1960s also suffered side effects, resulting in trials being scrapped.

Broad inoculation campaigns also increase the risk of getting multiple vaccines, which could have adverse effects on a person’s immune response.

“It may be three to six months before we get Phase 3 trial results — it’s not that long to wait,” Dr. MacIntyre said. “You are potentially muddying the waters for the time when we do have Phase 3 trial data for the best possible vaccine.”

“The goal is to first establish an immune barrier among special populations, so that the operations of the entire cities will have a stable guarantee,” Mr. Zheng said.

The vaccine makers and local governments stress that participation is voluntary, and many people who take the vaccines pay a considerable amount to do so. According to government notices, the vaccines would cost about $148, putting them out of reach for many in a country where 600 million people make that much in a month.
It is not clear whether recipients have been fully warned about the risks of taking an unapproved vaccine.

Dr. Mulholland, of the Murdoch institute, said he would give an experimental coronavirus vaccine only to health care workers, especially in places like Brazil, which has one of the world’s highest case counts, and continue monitoring them. “They can be educated on the potential risks,” he said.

On a more positive note, all three Moderna trial participants and one in the Pfizer trial reported that their side effects, while intense, melted away after at most one day. All five, meanwhile, expressed the belief that the discomfort they experienced will be worth it for the value of the research being undertaken. Both of the trials in question are phase 3 studies that have tens of thousands of participants. And because they are double-blind, neither the patients nor the medical workers inoculating them know whether any given individual is getting the vaccine or a placebo.

AstraZeneca has not confirmed that the patient was afflicted with transverse myelitis, but Nath and another neurologist said they understood this to be the case. Transverse myelitis produces a set of symptoms involving inflammation along the spinal cord that can cause pain, muscle weakness and paralysis. Britain’s regulatory body, the Medicines and Healthcare Products Regulatory Agency, reviewed the case and has allowed the trial to resume in the United Kingdom.

The precise cause of the disease is key to the decision by authorities whether to resume the trial. Sometimes an underlying medical condition is “unmasked” by a person’s immune response to the vaccine, leading to illness, as happened with the MS patient. In that case, the trial might be continued without fear, because the illness was not specific to the vaccine.More worrisome is a phenomenon called “molecular mimicry.” In such cases, some small piece of the vaccine may be similar to tissue in the brain or spinal cord, resulting in an immune attack on that tissue in response to a vaccine component. Should that be the case, another occurrence of transverse myelitis would be likely if the trial resumed, said Dr. William Schaffner, an infectious disease specialist at the Vanderbilt University School of Medicine. A second case would shut down the trial, he said.