Eight of the vaccinated monkeys were completely protected.
"The global Covid-19 pandemic has made the development of a vaccine a top biomedical priority, but very little is now known about protective immunity to the SARS-CoV-2 virus", said senior author Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston.
After exposing nine of the animals to the virus, researchers monitored viral levels while the macaques recovered.
"These results support further development of ShaCoVacc as a candidate vaccine for Covid-19", the researchers wrote in the study, which has not yet been peer reviewed. More than a month after the initial infection, the team re-exposed the rhesus macaques to the virus. Instead their viral loads started low and declined rapidly.
In one study carried out by Barouch and other researchers, nine adult rhesus macaque monkeys were infected with the virus.
Researchers around the world are racing to develop a vaccine against the disease that has killed almost 330,000 people, according to Johns Hopkins University data. Following initial viral clearance, they explained, animals were rechallenged with SARS-CoV-2 and "showed 5 log10 reductions in median viral loads in bronchoalveolar lavage and nasal mucosa compared with primary infection".
Additional research will be required to define the durability of natural immunity shown here, the authors noted.
To test their immunity, they were exposed to SARS-CoV-2 again 35 days later for what is called a "re-challenge", and they showed few to no symptoms.
Chen and colleagues examined the safety and tolerability of CanSino's non-replicating Ad5 vectored SARS-CoV-2 vaccine, which aims to induce sustained production of the virus's so-called spike protein antigen in recipients, in a dose-escalation phase I trial.
Vaccinated animals developed humoral and cellular immune responses, including neutralizing antibody titers comparable to those found in convalescent humans and macaques infected with SARS-CoV-2.
In the study, Chinese scientists reported that while people who had high levels of pre-existing immunity to Ad5 responded to the vaccine, the rise in antibodies to the SARS-Cov-2 virus was less robust than among those in the study who had low or no pre-existing antibodies to Ad5.
"While data from humans will require rigorous clinical trials, these data in an animal model increases our optimism that the development of a vaccine for humans will be possible", Dr Barouch said. The problem was more common among subjects at the older end of the age range, among 45 to 60 year olds - a finding that raises questions about how well the vaccine would work in one of the demographic groups that most needs protection from this infection, older adults.