The spike protein of an Omicron version of SARS-2 was fused to a virus of the Wuhan strain, the original version that emerged from China in 2020. The goal was to determine if the mutations in the Omicron spike protein were responsible for this variant’s increased ability to evade the immunity to SARS-2 that humans have built up, and whether the changes led to Omicron’s lower rate of severity. The testing actually showed, though, that the chimeric virus was more lethal to a type of lab mice than Omicron itself, killing 80% of the mice infected. Importantly, the original Wuhan strain killed 100% of mice it was tested in. The conclusion is that mutations in the spike protein of the Omicron variant are responsible for the strain’s ability to evade immunity people have built up via vaccination, infections, or both, but they are not responsible for the apparent decrease in severity of the Omicron viruses.