One of the foundational principles of access control is the realization that the people might fail. No defence is ever going to withstand every attack mounted against it. Therefore, the people rely on a fundamental concept referred to as defence in depth. The people must prepare another layer of defence since they have to assume that the travel limitations will not be a perfect protection. Once the virus has become established in their populations (pretty much inevitably), they have other safeguards, such as advice or guidance (or, in some cases, directives) about the need for physical isolation and limitation of contacts. So there is constant handwashing to kill and remove the virus when the people touch those surfaces they must when they go out to obtain supplies. When a test for a virus tells the people a virus is present, they need to validate that this identification is true..