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If there is a silver lining to the flawed U.S. response to the coronavirus pandemic, it is this: The relatively high number of new cases being diagnosed daily — upward of 20,000 — will make it easier to test new vaccines.

To determine whether a vaccine prevents disease, the study’s subjects need to be exposed to the pathogen as it circulates in the population. Reopening the economy will likely result in the faster spread of the coronavirus and therefore more opportunities to test a vaccine’s efficacy in trial subjects.

Under a proposal under discussion by a committee set up by the National Institutes of Health, each of four or five experimental vaccines would be tested on about 20,000 trial participants with a placebo group of 10,000 for each vaccine. Some 50 U.S. medical centers — and perhaps an equal number overseas — would participate in these trials.

On Monday, Moderna, the biotech company, reported promising results in the first eight of 45 people enrolled in an initial test of the safety and immune responses to its vaccine. Analysts  in the Dow that day at least partly to this very preliminary data, so eager are investors for any signs of progress in efforts to control the pandemic.

Moderna is running animal and human studies simultaneously  hundreds of millions of dollars to build laboratories where the vaccine will be produced even before it’s approved. The Food and Drug Administration on May 12 promised an accelerated review of Moderna’s vaccine, which works by injecting pieces of synthetic viral RNA into the body to stimulate an immune response to the virus.

The speed in developing vaccines for widespread testing this summer is impressive, certainly compared with the nation’s inadequate, delayed response to providing coronavirus testing and personal protective equipment to health care providers.

Still, many scientists have expressed skepticism at the breakneck timetable put forward by some Trump administration officials, who say that  of a vaccine could be available by November. Even the normally sober Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases,  a Senate committee on May 12 that a vaccine could have proven safety and efficacy by then.

Running a trial of the size and speed contemplated by the NIH will be an immense undertaking. Just setting up trial locations and getting common consent and data-entry forms into shape usually take months. Enrolling 30,000 people for a single vaccine trial is a big challenge.

In addition, defining success in a vaccine against COVID-19 will be no simple matter. As scientists design vaccine trials, they first have to set the “endpoints” that determine success or failure. Death? Length of illness? Hospitalization? Number of days in which a subject is infectious?

If there is little virus circulating where a trial is being run, even a vast study won’t prove anything. On the other hand, if a vaccine trial had started in early April in New York City, where roughly 10,000 cases a day were reported for weeks, 30,000 participants would have been plenty to show whether the vaccine protected against the disease.

In all likelihood, the big NIH trials will focus on rates of infection as well as clinical symptoms such as fever and cough.��To discover whether the vaccine prevents severe disease, which is relatively rare, is harder. COVID-19, according to , kills about 0.6% of those it infects, while perhaps  that many require hospitalization.

People who take part in a trial will be given clear instructions to protect themselves against infection through social distancing, face masks, frequent hand-washing and so on. That will lower the number of people infected during the study.

“You’d have to ask all the people enrolled in a trial to practice good hygiene,” said Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “You don’t want them to get infected — but you do.”

When Jonas Salk announced the successful trial of his polio vaccine in 1955, the nation celebrated a vaccine that could virtually eliminate a deadly infectious disease overnight. A new coronavirus vaccine may not provide that kind of overnight success. Instead, it may be more akin to the flu vaccine, which reduces the risk or severity of the illness but requires a new shot each year.

Vaccinating 20,000 people in a trial can reveal whether a vaccine is clearly dangerous to a general population. But when 200 million receive the same vaccine, less common side effects could still affect thousands. Botched batches of polio vaccines released after Salk’s trial permanently  and killed 10.�� caused tens of thousands of cases of grave illness in the 1960s.

Maurice Hilleman, the vaccine pioneer who developed successful vaccines against measles, mumps, hepatitis A and B and other diseases, once said he never breathed a sigh of relief “until the first 3 million doses” had been delivered.

Unexpected problems naturally bedevil quick rollouts, as this one will almost certainly be as the nation searches for a way to check a pandemic that is killing tens of thousands of Americans and paralyzing the economy. But as Gregory Poland, the leader of Mayo Clinic’s vaccine research, told me, “There is an irresolvable tension of speed versus safety.”

Allen is an editor for California Healthline, produced by Kaiser Health News, and the author of “Vaccine: The Controversial Story of Medicine’s Greatest Lifesaver.”

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