In a study published earlier this month, researchers at Boston University sought to find out why the omicron variant of the COVID-19 virus has proven relatively mild. They wanted to know if all the mutations in omicron’s spike protein—the structure that juts from the core of coronaviruses—made the variant less virulent than others. In a highly secure lab known as a BSL-3 facility, the Boston team tested chimeric (hybrid) viruses on experimental mice. While the combination of the spike from the omicron strain and other viral components from an older variant could have been “mild” like omicron, that didn’t prove to be the case. The new virus killed 80 percent of the mice in an infection experiment, while none of the mice subjected to omicron died. After publishing their results in a pre-print study, the researchers, Boston University, and the US National Institutes of Health had a PR crisis on their hands.