Boston in late March was eerily empty, with the stay-at-home advisory in full effect. But cellphone mobility data analyzed by 5 Investigates shows people in Massachusetts are slowly returning to normal.Similar data is being watched closely by a group of experts who fear if not expect the re-emergence of COVID-19 as people more about more."We haven't reached anywhere near herd immunity. So when we start mixing again, we will see the virus start to spread again," said Dr. Caroline Buckee, an epidemiologist at the Harvard T.H. Chan School of Public Health.Buckee is part of a group studying detailed device data to see how population movements impact the spread of COVID-19.We did our own analysis, examining one company's cell phone mobility data -- Descartes Labs -- for some counties in eastern and central Massachusetts. Overall, mobility in Suffolk County is lower, perhaps not surprising given the density of the county. Worcester County, by comparison, is generally higher.The drop in mobility is dramatic in mid-March, when the shutdown began, and continues through much of April. As the end of April approaches, movement increases, a trend continuing through this past Monday."As people have become restless, they're sick of being locked down," Buckee said.The data, which is aggregated by county, does not reflect change from the large numbers of protestors who gathered beginning on Sunday, which is not surprising given the large overall populations of the counties in eastern Massachusetts. The research Buckee is part of produced a preliminary analysis finding higher mobility around commuting times in poorer New York City neighborhoods, suggesting that populations in those areas were not able to work from home to the same extent as other, wealthier areas."That echoes a lot of the epidemiologic findings around the country that poorer communities, communities of color, are being disproportionately impacted by this outbreak," she said.As people in Massachusetts and around the world begin to re-emerge, Buckee and her colleagues are sharing their findings with policy makers to try and identify outbreaks sooner.The Massachusetts Department of Public Health says that mobility data is one of the many tools it's using to monitor the pandemic."This kind of data can give you insights into what's going on, on a neighborhood by neighborhood basis. So, you know, it should be integrated into other platforms which take into account hospitalizations and deaths and test positivity rates and so on, so that the whole of the evidence can be analyzed for decision making," Buckee said.Tracking cellphone data doesn't come without controversy. There are privacy concerns for sure. While Buckee says the data can be an important tool in fighting the pandemic with contact tracing, she also believes the monitoring needs to be rolled back to protect people's privacy once the threat is over.