In this May 14, 2021 photo, pedestrians cross a road in Hong Kong's Central district. (PHOTO / XINHUA)

The US Centers for Disease Control and Prevention has raised the COVID-19 danger level in Hong Kong, saying trips to the special administrative region has become more dangerous, a move that has puzzled people in both places.  

The CDC raised its advice to Hong Kong one rung to Level 2, citing a “moderate level” of coronavirus. The CDC said unvaccinated travelers with a higher risk of severe illness from COVID-19 should avoid non-essential travel to Hong Kong. The CDC’s highest travel health notice is Level 4.

On the same day, the CDC also said anyone who hasn’t been vaccinated should avoid non-essential trips to Singapore, citing a “high level” of COVID-19.

READ MORE: HK halts quarantine exemption for travelers from Macao

“All travelers may be at risk for getting and spreading COVID variants,” the CDC said, increasing its travel advice for Singapore by one notch to Level 3.

The financial hub has seen fewer than 10 cases a day since late August, and there hasn’t been a locally-transmitted infection in Hong Kong since mid-August, data show. The hub is also yet to experience an outbreak of the Delta variant.

While Singapore has reported almost daily coronavirus caseloads of more than 1,000 since mid-September, the advice is puzzling for Hong Kong. The financial hub has seen fewer than 10 cases a day since late August, and there hasn’t been a locally-transmitted infection in Hong Kong since mid-August, data show. The hub is also yet to experience an outbreak of the Delta variant.

The US, by contrast, is in the midst of another COVID-19 resurgence, as its vaccination rollout stalls. The country added over 180,000 new cases on Friday, and saw more than 2,700 deaths from the virus. 

The primary criteria for determining CDC travel health notices for destinations as large as Hong Kong and Singapore are the number of recent cases and the trajectory of new cases, according to the CDC’s website.

The threshold for Level 2, for example, is 50-99 new cases over the past 28 days, for every 100,000 local residents. The Level 3 threshold is 100-500, the website said.

ALSO READ: Fully vaccinated Hong Kong residents top 4 million

Testing data is also assessed, according to the CDC, but there’s room for less-explicit factors to play a role, too. “Additional information such as hospitalizations and imported case counts may be considered when inconsistencies or other concerns are reported,” the CDC site said.