Testing whether a drug or vaccine inhibits a virus can be especially difficult if the virus is dangerous and thus requires cumbersome biosafety level-3 conditions. A now common alternative is to create a pseudovirus, a harmless, non self-replicating virus core that has been modified to exhibit the same surface proteins as the dangerous virus. In the case of SARS-CoV-II, for example, pseudoviruses were created with the same spike proteins as the real virus. In addition, the pseudovirus is given a Luciferase gene. The Luciferase gene (aka light bearer gene) is a gene for creating the kinds of enzymes which light up fireflies and it is only expressed after entering a cell. Thus, scientists can test whether a virus has been successfully neutralized by measuring how much light virus-exposed cells exhibit. Neat!
Here’s a nice picture of the idea from Berthold Instruments who sell luminometers which measure the intensity of Luciferase activity.