For the first time, one of the many COVID-19 vaccines in development has protected an animal, rhesus macaques, from infection by the new coronavirus, scientists report. The vaccine, an old-fashioned formulation consisting of a chemically inactivated version of the virus, produced no obvious side effects in the monkeys, and human trials began on 16 April.

Researchers from Sinovac Biotech, a privately held Beijing-based company, gave two different doses of their COVID-19 vaccine to a total of eight rhesus macaque monkeys. Three weeks later, the group introduced SARS-CoV-2, the virus that causes COVID-19, into the monkeys’ lungs through tubes down their tracheas, and none developed a full-blown infection.

The monkeys given the highest dose of vaccine had the best response: Seven days after the animals received the virus, researchers could not detect it in the pharynx or lungs of any of them. Some of the lower dosed animals had a “viral blip” but also appeared to have controlled the infection, the Sinovac team reports in a paper published on 19 April on the preprint server bioRxiv. In contrast, four control animals developed high levels of viral RNA in several body parts and severe pneumonia. The results “give us a lot of confidence” that the vaccine will work in humans, says Meng Weining, Sinovac’s senior director for overseas regulatory affairs.