An experimental vaccine, made from a mild virus genetically modified to carry a key gene from the COVID-19 virus, is effective at preventing pneumonia in mice infected with the COVID-19 virus, according to a study from Washington University School of Medicine in St. Louis.

The researchers created the experimental vaccine by genetically modifying vesicular stomatitis virus (VSV), a virus of livestock that causes only a mild, short-lived illness in people. They swapped out one gene from VSV for the gene for spike from SARS-CoV-2, the virus that causes COVID-19. The hybrid virus is called VSV-SARS-CoV-2, according to a news release posted on the university's website on Tuesday.

Spike protein is thought to be one of the keys to immunity against COVID-19. By adding the gene for spike to a fairly harmless virus, the researchers created a hybrid virus that, when given to people, ideally would elicit antibodies against spike that protect against later infection with the COVID-19 virus.

As part of this study, the researchers injected mice with VSV-SARS-CoV-2 or a lab strain of VSV for comparison. A subgroup was boosted with a second dose of the experimental vaccine four weeks after the initial injections. Three weeks after each injection, the researchers drew blood from the mice to test for antibodies capable of preventing SARS-CoV-2 from infecting cells. They found high levels of such neutralizing antibodies after one dose, and the levels increased 90-fold after a second dose.

The researchers then challenged the mice five weeks after their last dose by spraying the COVID-19 virus into their noses. The vaccine completely protected against pneumonia. At four days post infection, there was no infectious virus detectable in the lungs of mice that had been given either one or two doses of the vaccine. In contrast, mice that had received the placebo had high levels of virus in their lungs. In addition, the lungs of vaccinated mice showed fewer signs of inflammation and damage than those of mice that had received the placebo.

The researchers are in the process of repeating the experiments in other animal models that are naturally susceptible to the COVID-19 virus. If the vaccine also protects those animals from COVID-19, the next step would be to scale up production under what the Food and Drug Administration refers to as "good manufacturing practice (GMP) conditions" and launch a clinical trial in people.

The experimental vaccine is still in the early stages of development, and is still months behind in the race to develop a pandemic-ending vaccine, the researchers stressed.

The study has been published in the journal Cell Host and Microbe.