But in one study, monkeys developed immunity against the new SARS-CoV-2 coronavirus after receiving experimental vaccines. The challenges in the development of a COVID-19 vaccine are unprecedented, and the ability to trigger these immune responses does not necessarily indicate that the vaccine will protect humans from COVID-19.
At least 100 vaccines against COVID-19 are now in development, with at least eight starting clinical trials, the authors noted, among them Moderna's mRNA vaccine, and the University of Oxford's non-replicating chimpanzee adenovirus-vectored vaccine, which was recently shown to be protective in a small preclinical trial.
These studies, which have been peer reviewed, do not prove that humans develop immunity or how long it might last, but they are reassuring.
"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 Dan H. Barouch, MD, PhD, professor of medicine, Harvard Medical School, director, Center for Virology and Vaccine Research, BIDMC, and senior author on both studies.
In one study carried out by Barouch and other researchers, nine adult rhesus macaque monkeys were infected with the virus.
They then proceeded to expose these monkeys and ten control animals were also exposed to the virus. After they recovered, the team exposed them to the virus again and the animals did not get sick.
The disease has infected more than 4.9 million people worldwide and killed over 324,000, according to the USA -based Johns Hopkins University.
In all cases, as the production of immune system antibodies rose, viral load declined. Upon second exposure, the animals demonstrated near-complete protection against the virus. Further trials are needed to determine whether the vaccine effectively protects against infection - rather than just triggers an immune response to the virus. In follow-up tests, the monkeys that were vaccinated with the coronavirus showed "dramatically lower viral loads" compared with the control group.
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.
In the second study, Barouch and colleagues tested 25 monkeys with six prototype vaccines to see if antibodies produced in response were protective.
While noting a higher reactogenicity profile in the higher dose, and potential viremia caused by Ad5 vector infection, the authors said they chose the low dose and middle dose to be further assessed in a phase II clinical trial.