Could a Blood Test Show if a Covid-19 Vaccine Works?

A new study in monkeys suggests that a blood test could predict the effectiveness of a Covid-19 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 Covid-19. And researchers would no longer have to wait for some trial volunteers to get the disease, 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.

Last month brought the stunning news that clinical trials of two new coronavirus vaccines, one from Moderna and the other from Pfizer and BioNTech, showed efficacy rates around 95 percent.

The strength of these two vaccines is, paradoxically, bad news for the dozens of others in earlier stages of development. Many of them will most likely have to be compared against the robust front-runners rather than a placebo shot. Because that is a high statistical bar to clear, their trials will need a lot more volunteers, time and money.

“You’d have to follow millions of people for a long time,” said Dr. Nelson Michael, the director of the Center for Infectious Diseases Research at Walter Reed Army Institute of Research, where a protein-based vaccine is being prepared for clinical trials in early 2021. “It’s just fantasy.”

For some smaller companies, these comparison trials may be deal-breakers. “You’re going to see a lot of dropout,” said Dr. Gregory Poland, a vaccine expert at the Mayo Clinic.

That’s a big problem, because Pfizer and Moderna won’t have nearly enough doses to give to everyone in the United States, let alone the world. And the next wave of vaccines may turn out to be superior to the first, in one way or the other. They could cost a lot less, for example. Some might come in just one dose instead of two, and won’t need a deep freeze. Some might offer protection that lasts a lot longer.

“We’d rather not have to revaccinate the world every one or two years,” Dr. Michael said.

The new monkey study offers a ray of hope for these next-generation vaccines, suggesting that they could be tested not against older vaccines, 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.

A spokeswoman for the Food and Drug Administration said that basing clinical trials on correlates of protection — if they turn out to exist — “possibly could be considered in the future.”

In their new 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.

The researchers set out to see if there was a level of IgG that reliably protected monkeys from Covid-19. If the IgG antibodies produced by vaccines were above that level, the vaccines could be judged effective.

To find that line, the researchers gave monkeys varying doses of antibodies, and then exposed them all to the coronavirus and watched how well they fought off the infection. In the monkeys with the weakest dose, the viruses multiplied much as they would in an ordinary animal.

But the monkeys that got a medium dose produced far fewer viruses. Some of them were able to wipe out the viruses altogether. At the highest dose, the monkeys were completely protected.

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.”

The experiment revealed that a modest amount of IgG antibodies turned out to be enough. That could be heartening news for vaccine developers, because even so-so vaccines may be able to cross the threshold.

Researchers are now starting to review the results of vaccine clinical trials to see if a correlate of protection like the one identified by Dr. Barouch and his colleagues in monkeys exists in people. “The study bodes well for the upcoming immune correlates studies,” said Holly Janes, a biostatistician at the Fred Hutchinson Cancer Institute in Seattle who was not involved in Dr. Barouch’s study.

Although it will take some time for those studies to produce solid results, Dr. Janes said preliminary hints made her optimistic.

“The emerging data do suggest that we could be gleefully surprised,” she said.

Rebecca Robbins contributed reporting.