To study the correlates of protection, scientists are now peering into the blood of people who have recovered from Covid-19 to map the defenses the immune system put up when the virus attacked. In recent weeks, they’ve described the type of antibodies produced, finding that they can have powerful effects against one of the virus’ key proteins, and that almost all patients who had the disease, even those who had mild infections, generated antibodies. Those are positive signs, given that a type of antibody, called a neutralizing antibody, is, in sufficient quantities, expected to offer some amount of protection for at least some amount of time.
The thought is that the defenses the body mounted to vanquish the virus the first time provide clues to what is required to fend off a second attack. To confirm that people who recover from Covid-19 are protected and to determine how long that lasts, scientists have to track people and see what happens to them if they encounter the virus again. That research often focuses on health care workers who are more likely to be exposed repeatedly.
Scientists can’t ethically expose people to the virus again intentionally. But with animals, researchers can “challenge” those that are vaccinated or have had an initial infection to see if they can ward the virus off — which is what recent studies in monkeys demonstrated. Scientists found that the animals generated neutralizing antibodies after they first contracted the virus or when they were given experimental vaccines, and that the higher the level of the antibodies the monkeys had (the higher the “titer,” in scientific parlance), the more protected they were against the pathogen when scientists sprayed a second dose into their noses.
“That is a suggestion that neutralizing antibodies to the virus can protect” against reinfection, said Dan Barouch, the director of Beth Israel Deaconess Medical Center’s Center for Virology and Vaccine Research, who steered that research.
If that finding extends to people, “we will start to be able to use that as a predictor of success,” Barouch said. That is, in experiments with vaccine candidates, researchers can start to see what levels of neutralizing antibodies they are producing, and prioritize those that seem to generate more promising responses.
Scientists often home in on neutralizing antibodies as correlates, but there can be other markers as well. They include other types of antibodies, like binding antibodies; immune cells like T cells and B cells; and cytokines — small proteins released by immune cells that serve as messengers. In the monkey study, for example, Barouch and colleagues also found an association between protection and the level of another type of antibody, though it wasn’t as strong as the correlation between protection and neutralizing antibodies. “There are a whole bunch of other things that people look at for correlates of protection,” said virologist Angela Rasmussen of Columbia University.
One challenge is that people respond differently to infections; some studies, for example, have found people who recovered from Covid-19 actually generated low levels of antibodies. But because the immune system is so complex, having low levels of antibodies does not necessarily mean that a person won’t be safeguarded. All that can make it harder to define exactly what immunity looks like.
“Some people who’ve had this have not had high antibody titers or have had low antibody titers,” said Anna Durbin, a vaccine researcher at Johns Hopkins University. “We still don’t know what’s going to happen to them if they’re re-exposed.”
With Covid-19, immunity — whether from an infection or a vaccine — is expected to wane over perhaps a few years; that is what happens with the four human coronaviruses that cause colds.
Tracking the levels of the different correlates could provide clues to how long immunity lasts, and when a person becomes vulnerable again. It could also indicate when people might need another dose of the vaccine.
“When we’re trying to evaluate an immune response, we don’t only want to see we engage the proper immune responses for protection,” said Scott Hale, a University of Utah immunologist. “We also want to make sure there’s some form of long-lasting immunity in case you’re exposed to the pathogen in a year or five years or 10 years.”