As governments around the world wrestle with questions about how and when to reopen their economies, they must rely on predictions or weeks-old data to make informed decisions. Cornell University says that a Cornell researcher is among a team of engineers, statisticians and computer and data scientists who have developed a potential solution: a digital “virus” that could piggyback on contact-tracing apps and spread from smartphone to smartphone in real time. This would allow policymakers to gauge the impact of various social distancing measures without waiting two weeks or longer to learn how coronavirus has actually spread, said Shane Henderson, professor of operations research and information engineering and a co-author of “Safe Blues: A Method for Estimation and Control in the Fight Against COVID-19.”