CHICAGO (Reuters) – A study of Moderna Inc’s COVID-19 vaccine in mice lends some assurance that it will not increase the risk of more severe disease, and that one dose may provide protection against the novel coronavirus, according to preliminary data released on Friday.

Prior studies on a vaccine for SARS – a close cousin to the new virus that causes COVID-19 – suggests vaccines against this type of virus might have the unintended effect of causing more severe disease when the vaccinated person is later exposed to the pathogen, especially in individuals who do not produce an adequately strong immune response.

Scientists have seen this risk as a key hurdle that must be cleared before vaccines can be safely tested in thousands of healthy people.

While the data released by the U.S. National Institutes of Allergy and Infectious Disease (NIAID) and Moderna was encouraging, mouse data is no guarantee of what will happen in humans.

The vaccine is currently in midstage testing in healthy volunteers. Moderna said on Thursday it plans to begin final-stage trials enrolling 30,000 people in July.

In the new study, six-week-old mice received one or two shots of a variety of doses of Moderna’s vaccine, including doses considered not strong enough to elicit a protective immune response. Researchers then exposed the mice to the virus.

Subsequent analyses looking for signs of disease enhancement suggests that “sub-protective” immune responses do not cause what is known as vaccine-associated enhanced respiratory disease, a susceptibility to more severe disease in the lungs.

“Subprotective doses did not prime mice for enhanced immunopathology following (exposure),” Dr. Barney Graham of the Vaccine Research Center at NIAID and colleagues wrote in the not yet peer reviewed manuscript, posted on the bioRxiv website.

Further testing also suggested that the vaccine induces potent neutralizing antibody responses – the type of response needed to block the virus from infecting cells.

The vaccine also appeared to protect against infection by the coronavirus in the lungs and noses without evidence of toxic effects, the team wrote.

They noted that the mice that received just one dose of the vaccine before exposure to the virus seven weeks later were “completely protected against lung viral replication,” suggesting that a single vaccination prevented the virus from making copies of itself in the lungs.

“At first glance, it looks promising in inducing neutralizing antibody protection in mice,” Dr. Peter Hotez, a vaccine researcher at Baylor College of Medicine said in an email. He had not yet reviewed the paper in detail.