Monkeys that were injected with a prototype vaccine for the coronavirus were protected from the disease, an optimistic sign that an effective vaccine for humans is possible, researchers said on Wednesday. Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston, and his research team conducted two separate studies on monkeys and published the results on Wednesday. In one study, researchers infected monkeys with the coronavirus and found that they developed protective neutralizing antibodies after recovering. “If we did the re-challenge study and it didn’t work, the implication would be that the entire vaccine effort would fail,” said Dr. Barouch. “That would have been really, really bad news for seven billion people.”

In the second study, the researchers injected the monkeys with prototype vaccines and exposed them to the coronavirus. Barouch said they saw “a substantial degree of protection” in the vaccinated animals. The World Health Organization has recently warned that there is “currently no evidence” that an individual who recovers from the coronavirus cannot be reinfected, however the new findings offer reassurance that the coronavirus vaccine effort could prove to be successful. Scientists around the world are testing whether candidate vaccines are safe for humans—the next group of trials will test their effectiveness. “I think that overall this will be seen as very good news for the vaccine effort,” Barouch said of the findings.