The ultimate hope is to have a vaccine that will protect everyone, and many companies and governments have already rushed the design of candidate vaccines. But as Dr. Fauci has explained multiple times, testing those candidate vaccines for safety and effectiveness takes time.

The process will take at least a year, even if nothing goes wrong. The roadblock, vaccine experts explained, is not bureaucratic. It is that the human immune system takes weeks to produce antibodies, and some dangerous side effects can take weeks to appear.

After extensive animal testing, vaccines are normally given to about 50 healthy human volunteers to see if they cause any unexpected side effects and to measure what dose produces enough antibodies to be considered protective.

If that goes well, the trial enrolls hundreds or thousands of volunteers in an area where the virus is circulating. Half get the vaccine, the rest do not — and the investigators wait. If the vaccinated half do not get the disease, the green light for production is finally given.

In the past, some experimental vaccines have produced serious side effects, like Guillain-Barre syndrome, which can paralyze and kill. A greater danger, experts said, is that some experimental vaccines, paradoxically, cause “immune enhancement,” meaning they make it more likely, not less, that recipients will get a disease. That would be a disaster.

One candidate coronavirus vaccine Dr. Hotez invented 10 years ago in the wake of SARS, he said, had to be abandoned when it appeared to make mice more likely to die from pneumonia when they were experimentally infected with the virus.