WASHINGTON (BLOOMBERG) - The United States on Monday (Sept 27) said trips to Singapore and Hong Kong have become more dangerous because of the coronavirus, raising its travel health advice for the Asian financial hubs by one level.

The US Centres for Disease Control and Prevention (CDC) said anyone who has not been vaccinated should avoid non-essential trips to Singapore, citing a "high level" of Covid-19.

"All travellers may be at risk for getting and spreading Covid-19 variants," the CDC said, increasing its travel advice for Singapore by one notch to level 3.

Other countries currently at level 3 include Australia, Germany, Indonesia, Japan, Vietnam and the Philippines.

The CDC also raised its advice to Hong Kong one rung to level 2, citing a "moderate level" of coronavirus. The CDC said unvaccinated travellers 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.

While Singapore has reported almost daily coronavirus caseloads of more than 1,000 since mid-September, the advice is puzzling for Hong Kong.

The city has seen fewer than 10 cases a day since late August, and there has not been a locally transmitted infection in Hong Kong since mid-August, data show. The hub has 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 roll-out 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.

Testing data is also assessed, according to the CDC, but there is room for less-explicit factors to play a role, too.

"Additional information such as hospitalisations and imported case counts may be considered when inconsistencies or other concerns are reported," the CDC site said.