The disparity in the number of infectees could have happened because many patients might have already been treated or recovered from the infection during the three to four weeks that passed before they landed in Vietnam, said Dang Hong Hai, deputy head of the General Planning Department at Hanoi's National Hospital for Tropical Diseases.

"Covid-19 has an acute stage of around one to two weeks. For patients whose illness was not severe, the symptoms may subside in two weeks and they would test negative for the novel coronavirus," Hai said, adding that the lower number also makes infections less likely among other passengers and crew members.

The workers had sought help earlier last month from Vietnamese representative agencies in Angola. They were working at the Sendje Hydropower Plant in Equatorial Guinea under labor contracts signed by Duglas Alliance Ltd of the U.K. with three Vietnamese companies. Prime Minister Nguyen Xuan Phuc then ordered relevant agencies to arrange a special flight to bring them home, including those that were infected with Covid-19.

Last Wednesday, 219 Vietnamese workers from Equatorial Guinea returned home on a Vietnam Airlines special flight from the Bata International Airport to Hanoi’s Noi Bai Airport. Authorities then said at least 129 had tested positive for the novel coronavirus.

Upon landing, the passengers and flight crew were quarantined for 14 days at the Hanoi hospital. Patients already at the hospital had either been discharged or transferred to other facilities to reserve rooms for the returnees.

The Health Ministry had confirmed 20 of the returnees infected on July 31. Among them, six showed signs of lung damage and three were also infected with malaria, said Nguyen Trung Cap, deputy director of the National Hospital for Tropical Diseases. Their health is stable and they are being monitored, Cap said.

More than 16,000 Vietnamese have been brought home from many foreign countries on dozens of special flights since early April, according to the Ministry of Foreign Affairs.

Vietnam has recorded 670 Covid-19 cases so far, with 374 recoveries and 288 active cases. Eight people have died, mostly the elderly with pre-existing serious illnesses.