Current guidance from the US Centers for Disease Control and Prevention says fully vaccinated people can refrain from routine testing. "I think now we should revisit this policy with the Delta variant and determine if the current recommendations hold up," Hotez wrote in an email to CNN on Wednesday. Plus, the CDC is only reporting data on "breakthrough" infections that cause severe disease. That could mean scientists and health officials will not know how many vaccinated people have mild or asymptomatic infections -- and it will be very difficult to track whether a new variant such as Delta is causing more vaccine failure. "We need to design studies. Assuming this is underway, then the question comes, do we wait for those studies or change recommendations now and reconsider regular testing for asymptomatic vaccinated individuals? Given how disruptive this is, I would probably be inclined to wait for additional data before going backwards," Hotez wrote.