(Bloomberg) -- Hong Kong will introduce a tiered health-code system reminiscent of what’s used in mainland China to facilitate a reduction in its deeply unpopular mandatory hotel quarantine.

The new rules, which come into effect on Friday, will mean arrivals at Hong Kong’s international airport must spend three days in hotel quarantine -- down from seven. If they don’t test positive for Covid, they will then undergo four days of health monitoring, underpinned by a yellow health code that restricts entry into a raft of high-risk places. Meanwhile, anyone infected with the virus will receive a red code that means they must isolate.

Read more: Hong Kong Cuts Hotel Quarantine to Three Days to Revive Hub

The new system is a significant easing for a city that once imposed 21 days of hotel quarantine. But experts say the use of yellow codes make little sense if used only by travelers, considering thousands of cases are already being reported locally everyday, and businesses want quarantine to be scrapped entirely.

Here’s how the new health codes will work in Hong Kong:

What is it and what will it do?

The proposed health-code system has three categories and will be housed within the LeaveHomeSafe app everyone in the city must use to check in to places like bars, restaurants and museums.

A red code will be given to people who test positive and must isolate. People who meet conditions, including having individual bathrooms, may be approved to stay home and must wear wristbands to ensure they don’t go out. Others will be taken to a community isolation facility.

A yellow code will be given to inbound travelers for the seven days after they arrive in Hong Kong, which covers their still-mandatory hotel quarantine as well as the four days of surveillance. The code will mean people can’t go to places like restaurants and nightclubs, but they’ll be allowed to commute to work and go to supermarkets and shopping malls.

Once an individual has completed their quarantine and surveillance requirements, their LeaveHomeSafe app will revert to blue. That’s the default for adults in the city who aren’t infected with Covid and meet vaccine requirements, and allows them to enter any venue.

Where’s off limits?

A yellow code will prevent entry into a long list of places that are deemed to be high-risk or that involve an individual removing their mask for an extended period of time.

As well as eating and drinking establishments, gyms and hair and beauty parlors will be off limits. Entry to religious venues, residential care homes, schools and medical facilities like hospitals will also be restricted. 

Staff at those locations can go to work during the period in which they have a yellow code, depending on whether their employer has made appropriate provisions or put restrictions in place. For example, teachers and children can go to schools during this time.

How is it different from China’s health code?

While Hong Kong has studied the colored health code system used in mainland China for more than two years, it features a couple of key differences and is far more limited in scope.

Anyone’s health code in mainland China may turn yellow if they end up being a close contact or have not done PCR tests within a required time frame. As workplaces, public venues and residential compounds usually require a green health code to enter, giving a yellow code effectively restricts the movement of a person until they complete quarantine or receive a negative PCR test result. 

Read more: In China, You May Need This App’s Green Light To Get to Work

Mainland China uses data from public health, transport and immigration departments, as well as information from phone signals, neighborhoods, hospitals and workplaces to determine whether someone poses a health risk. That means that a person’s health code can change to yellow or red suddenly without their knowing.

Hong Kong’s system won’t be so sophisticated. It will rely on test results and immigration records, and doesn’t have a tracking function. 

It will also affect fewer people than China’s system. The red code will only apply to confirmed Covid cases, which are hovering around 4,000 people a day. And the yellow code is only for inbound travelers, currently totaling about 5,000 a day. 

What are the concerns?

The all-encompassing nature of mainland China’s system, which means residents need a green code to do just about anything, initially sparked concerns about whether it will mean Hong Kongers face harsher movement curbs and if it could be used for more than just public health.

Read more: China City Accused of Using Covid Health Codes to Stop Protests

Hong Kong Secretary for Innovation, Technology and Industry Sun Dong said on Monday the LeaveHomeSafe app strictly complies with the city’s security and privacy laws. The public should feel safe in using the app, he said.

The use of a red code for Covid-positive people also risks deterring residents from reporting their result. Currently, local infections are picked up in one of two ways: through PCR tests ordered by the government for high-risk places, or when a resident self-reports a positive rapid-test result and then has to undergo a confirmatory PCR test.

What do experts say?

Siddharth Sridhar, a clinical virologist from the University of Hong Kong, said the main advantage of the health code system is that it keeps potentially infectious individuals out of areas where masks are removed, such as restaurants. If this facilitates the government’s push to open borders internationally, then it’s a reasonable way forward, he said.

Benjamin Cowling, chair of epidemiology at the same university, said he didn’t understand the need for a yellow code in the four days of health surveillance. In the government briefing on Monday, the risk of infection in international arrivals during that period was presented as 0.8%, which is lower than in the general community, he said.

Even if the infection risk from recent arrivals to Hong Kong was the same as the community, there’d be no impact because the number of people traveling into the city is a fraction of the total population, he said.

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