A blood test showing the effectiveness of a vaccine on monkeys has the potential to speed up trials.

A new study in monkeys suggests that a blood test could predict the effectiveness of a coronavirus vaccine — and perhaps speed up the clinical trials needed to get a working vaccine to billions of people around the world.

The study, published on Friday in Nature, reveals telltale blood markers that predict whether a monkey’s immune system is prepared to wipe out incoming coronaviruses.

The finding raises hope that researchers will be able to look for the same markers in people who get vaccines in clinical trials. If the markers are strong enough, they could reveal if the vaccines protect against the coronavirus. And researchers would no longer have to wait for some trial volunteers to get Covid, as they do now.

“It will pave the way for a much more rapid advancement of the Covid vaccine field,” said Dr. Dan Barouch, a vaccine expert at Beth Israel Deaconess Medical Center in Boston and one of the researchers behind the new study.

The new monkey study offers a ray of hope for the next wave of vaccines, dozens of which are in development, suggesting that they could be tested not against the vaccines from Pfizer and Moderna — which reported have extraordinarily high efficacy rates — but using a measurement known as a “correlate of protection.”

“That’s the holy grail of vaccine research,” Dr. Michael said.

Influenza vaccines are already tested this way. Every new flu season requires the design of a new flu shot, but researchers don’t have to run clinical trials comparing it with old versions. Instead, they just check whether the new vaccine triggers a person’s immune system to make enough of a certain kind of antibody against the flu. If it does, then researchers know the vaccine is adequately stimulating the immune system.

If scientists could discover a correlate of protection against the coronavirus, they could follow the example of the flu. “That is an entirely plausible and feasible scenario,” said Dr. Anthony Fauci, the director of the National Institutes of Allergy and Infectious Diseases.

In their study, Dr. Barouch and his colleagues found a correlate of protection in monkeys. They built the experiment on their previous research showing that once monkeys recover from Covid-19, they can resist a second infection. The scientists drew blood from these exposed animals and isolated an array of protective antibodies called IgG.

Until now, scientists relied on circumstantial evidence that suggested IgG antibodies were crucial to clearing coronavirus infections. The new study puts that idea to the test — and determines the threshold of IgG antibodies required to ward off an infection.

“This is the first time, to the best of my knowledge, that we’ve actually proven that antibodies protect,” Dr. Barouch said. “Everything else has been a statistical association.”

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