The doctor said through his social media account that the a safe zone is always a place which is free from a disease for two times the incubation period. The concept was used in the time of Sars, Mers and also Ebola. 
For example in the case of Sars, a safe zone is an area where a Sars case is not found for 20 days, since the incubation period is 10 days. 
In the case of Covid-19, the safe zone must be areas where Covid-19 is not found for 28 days, since its incubation period is 14 days.

Expert virologist spells out main criteria for Covid-19 safe zone

Only a place that has been free from a Covid-19 case for 28 days can be considered a safe zone, Dr Yong Poovorawan, expert virologist at Chulalongkorn University, said.