For two years, Hong Kong successfully insulated most of its residents from COVID-19 and often went months without a single locally spread case. Then the omicron variant showed up.

Patients wait at a temporary treatment area outside Caritas Medical Centre in Hong Kong, Feb. 26, 2022.

The fast-spreading mutation breached Hong Kong’s defenses and has been spreading rapidly through one of the world’s most densely populated places, overflowing hospitals and isolation wards and prompting measures to test the entire 7.4 million population and hastily build six isolation and treatment centers.

An elderly woman receives a dose of China's Sinovac COVID-19 coronavirus vaccine at a community vaccination center in Hong Kong, Feb. 25, 2022.

The surge shows what happens when COVID-19 strikes a population unprotected by immunity from previous infections, and has exposed a low vaccination rate among elderly citizens who are bearing the brunt of the crisis.

A medical worker prepares a dose of China's Sinovac COVID-19 coronavirus vaccine at a community vaccination center in Hong Kong, Feb. 25, 2022.

Only about 30% of Hong Kong residents over the age of 80 and around 58% of those in their 70s are fully vaccinated, lagging behind younger populations by a large margin.

An elderly woman wearing a face mask works out at a housing estate in Hong Kong, Feb. 26, 2022.

This is despite the fact that vaccines have been widely available in Hong Kong since early 2021.

A man wearing a face mask walks across a street In Causeway Bay, a shopping district of Hong Kong, Feb. 25, 2022.

The city has reported about 150 deaths in the past three days, many among the unvaccinated elderly.

Hui Ngai-seng, 75, receives his first dose of China's Sinovac COVID-19 vaccine at a community vaccination center in Hong Kong, Feb. 25, 2022.

Health authorities said that the vaccine reluctance among the elderly is an unfortunate side-effect of Hong Kong’s success in warding off the virus for months.

Customers wearing face masks purchase COVID-19 antigen test kits at a market in Hong Kong, Feb. 26, 2022.

Many people thought that the risk of getting COVID-19 was virtually nil since there were no cases, and senior citizens were led to believe that the risk of vaccination was greater than not getting vaccinated, said Karen Grepin, a public health expert at the University of Hong Kong.

Workers build rooms of a new makeshift COVID-19 hospital and isolation facilities, in Tsing Yi of Hong Kong, Feb. 26, 2022.

Hundreds of millions of jabs have been given to people around the world and few serious risks have been identified after intense safety monitoring. But early reports of a few adverse reactions to the vaccine in Hong Kong created a false perception that people needed to be perfectly healthy to get vaccinated.

An aerial view shows a construction site of a new makeshift COVID-19 hospital and isolation facilities, in Tsing Yi of Hong Kong, Feb. 26, 2022.

“Hong Kong is the world’s guinea pig, when it comes to omicron,” said Grepin.

Patients on hospital beds wait at a temporary holding area outside Caritas Medical Centre in Hong Kong, Feb. 28, 2022.

The Hong Kong experience may also hold lessons for mainland China and its decision on when to re-open its borders and eliminate a two- to three-week quarantine requirement for anybody entering the country. Only a small proportion of the population has been infected, thanks to the Communist Party's strict zero-COVID approach of mass testing and lockdowns.

An aerial view shows a construction site of a new makeshift COVID-19 hospital and isolation facilities, in Tsing Yi of Hong Kong, Feb. 26, 2022.

The Hong Kong government's response has been to ramp up a zero-COVID approach similar to the mainland. Chinese officials have urged Hong Kong to stick to the approach, despite grumbling from residents, with even leader Xi Jinping weighing in to make sure the message got through.

People, including current hospital patients, showing COVID-19 symptoms wait at a temporary holding area outside Caritas Medical Centre in Hong Kong, Feb. 16, 2022.

Under the zero COVID-19 policy, everyone who tests positive in Hong Kong needs to be isolated. While this worked in the past, Hong Kong, unlike the mainland, does not have the beds to isolate so many people in a large outbreak.

Residents line up to get tested for the coronavirus at a temporary testing center in Hong Kong, Feb. 17, 2022.

Construction teams from the mainland are rushing to build two permanent and four temporary isolation and treatment centers to handle more than 20,000 patients in an effort reminiscent of the early days of the virus when China threw up two temporary hospitals in the city of Wuhan in a matter of days.

A family line up to get tested for the coronavirus at a temporary testing center in Hong Kong, Feb. 17, 2022.

Authorities also launched a vaccine pass on Thursday, requiring vaccination to enter shopping malls and other premises, and it is driving some to get the shot.

A woman looks out of the window of an isolation unit at the Penny's Bay Quarantine Centre on Lantau Island, in Hong Kong, Feb. 24, 2022.

“If I don’t get vaccinated, I can’t even go to restaurants,” said 73-year-old Yu Mui as she lined up for her first dose Friday. "So I have to come here today even though I am worried about the side effects.”

A woman looks out of the window of an isolation unit at the Penny's Bay Quarantine Centre on Lantau Island, in Hong Kong, Feb. 24, 2022.

Scientists believe that the omicron variant is milder than the delta version of the virus. But Hong Kong’s situation is nearly unique. In other nations where the omicron variant spread, people had immunity from vaccines or previous infections, and this blunted the severity of the disease.

Workers wearing protective suits, clean the isolation units at the Penny's Bay Quarantine Centre on Lantau Island, in Hong Kong, Feb. 24, 2022.

With many people unvaccinated and vulnerable, Michal Head, a global health expert at University of Southampton, fears that it may have a “worryingly high burden of severe COVID-19 in coming weeks.”

People line up to get tested for the coronavirus at a temporary testing center despite the rain in Hong Kong, Feb. 21, 2022.

“Omicron has been described by some as ‘mild’. But it’s certainly still severe enough to have a high mortality rate, far higher than flu or other similar respiratory infections,” he warned.

A general view shows a construction site where facilities for isolating the COVID-19 patients are built in Kai Tak Cruise Terminal in Hong Kong, Feb. 23, 2022.