A series of studies conducted in mice of Moderna’s COVID-19 vaccine research lent some assurance that it may not hike the risk of more severe disease, and that one dose may provide protection against the novel coronavirus, according to primary data.

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

Scientists have seen this risk as an obstacle to clear before the vaccines can be safely tested in thousands of healthy people.

While the data published by the US National Institutes of Allergy and Infectious Disease (NIAID) and Moderna offered some confirmation, the studies do not fully answer the question.

“This is the barest beginning of preliminary information,” said Dr. Gregory Poland, an immunologist and vaccine researcher at the Mayo Clinic who has seen the paper, which has yet to undergo peer-review.

Poland said the papers were incomplete, disorganized and the number of animals tested were too small.

The authors said that they have submitted the work to a top-tier journal. Moderna’s vaccine is in mid-stage testing in healthy individuals Moderna said on Thursday that it plans to start final-stage trials by enrolling 30,000 people in July.

In the animal studies, mice were given one or two shots of a variety of doses of Moderna’s vaccine, including doses considered to be not strong enough to induce a protective immune response. They then exposed the mice to the virus.

Subsequent analysis suggest that “sub-protective” I'mmune responses do not become the cause of what is known as vaccine-associated enhanced respiratory disease, a vulnerability 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 manuscript, posted on the bioRxiv website.

Further testing showed that the vaccine induces antibody responses to stop the virus from infecting cells.

The vaccine was also seen give protection against infection by the coronavirus in the lungs and noses without a trace of toxic effects, the team wrote.

They noticed that the mice which received just one dose before exposure to the virus seven weeks later were seen to be “completely protected against lung viral replication,” suggesting that a single dose of the vaccination prevented the virus from replicating in the lungs.

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

Poland, who was not involved in the research, said the paper leaves out “important parameters” that could be helpful for the scientists in judging the work.
“The results, such as they are presented, provide interesting data that are reassuring … This needs to be replicated and it needs to be peer-reviewed,” he said.