China abruptly ended its strict zero-covid policy in December, when demands for mass testing, lockdowns and special quarantine in authorities' custody were dropped.
Rapid spread
At the same time, the coronavirus is spreading rapidly.
The outbreak is expected to worsen during the Chinese New Year, which is celebrated at the end of January, when millions of Chinese are expected to travel from big cities to the countryside to visit relatives.
Beijing has urged restraint in travel ahead of the weekend, but barriers between domestic and foreign travelers have been removed at the airport.
The staff in white protective clothing are not visible either.
Up to 50,000 Hong Kong residents are now allowed to cross the border daily after registering their journeys online.
A further 10,000 are admitted via plane or boat without registration requirements.
Condemned foreign claims
The large spread of infection has caused many other countries, including Sweden, to introduce requirements for presenting a negative covid test when entering from China.
Even travelers between Hong Kong and China must still present a negative covid test taken within the last 48 hours, AP writes.
China has condemned the foreign demands for covid tests and has threatened countermeasures against the countries that introduced them.