With respect to digital contact tracing, success depends on at least four factors—four factors that suggest the Apple and Google system is not likely to be particularly effective in meeting the stated public health goals. Apple and Google have repeatedly emphasized that the use of their contact tracing systems will be opt-in only, meaning that they won’t be used unless people download compatible apps and them on. But in Singapore, only about 20 percent of people are using the country’s Trace Together app, which means that there’s only a 4 percent chance that two people who are exposed to each other will both have the app.