Dutch health authorities said on Friday they were worried that some passengers arriving from South Africa in the past week were testing positive for COVID-19 on arrival despite having been vaccinated and testing negative before their flight departure.
"It shows that the virus is spreading easily and that is worrying," said Bert van de Velden, director of the regional health authority for Kennemerland, which includes Amsterdam's Schiphol Airport.
"It is of the utmost importance that travellers are properly tested before they fly and that they let themselves be tested after arrival."
Hundreds of passengers arrive on flights from South Africa at Amsterdam's Schiphol Airport weekly, and health authorities said about 9% of those who agreed to undergo voluntary testing on arrival - only a third of passengers agreed - turned out to have COVID-19.
Health authorities began conducting post-flight tests after a complete investigation into two flights from South Africa on Nov. 26.
Those flights were mid-air when the Dutch government introduced new travel restrictions in light of worries about the newly detected Omicron variant.
The more than 600 passengers on the flights were kept isolated and were carefully tested, and it emerged that 62 had COVID-19, of whom 14 had the Omicron variant.
The U.S. government on Thursday announced rules requiring a preflight test for all international travellers that goes into effect on Monday.
