At present, most scientists agree that in order to limit the spread of sars-cov-2, we need to take strict control measures, including effective contact tracking, detection and isolation, as well as maintaining social distance and wearing masks. Moreover, countries such as South Korea, China and New Zealand have successfully used this method to suppress the spread and spread of the virus. Some have even called for the clearance of covid-19 patients to try to eliminate the virus, rather than curb its spread. New Zealand seems to have succeeded in this regard, but after 100 days of no new cases, new infections have emerged, from international travel and other unknown sources. Although the above control measures can flatten the number of infected counties, it seems not feasible to achieve zero infection of covid-19. For some island countries, it may be possible. In New Zealand, for example, it must prevent virus re-entry. Of course, this may require long-term and strict travel restrictions...