Word on the injections of monoclonal antibodies came from Ivan Hung Fan-ngai, a University of Hong Kong professor and Covid-19 expert.

Hung, head of the infectious diseases division at the university's faculty of medicine, said the long-acting antibody (LAAB) combination is being injected into the 100 patients in two doses, costing HK$10,000 for the course.

The shots were approved by the Department of Health and the Hospital Authority last month, and the treatment has to be repeated after six months.

Pharmaceutical company AstraZeneca's LAAB injections - named Evusheld - was granted authorization for emergency use by the US Food and Drug Administration in December for the pre-exposure prevention of Covid-19 in certain adults and younger people who must be at least 12 and weigh at least 40 kilograms.

The LAAB treatment comprises of two monoclonal antibodies derived from B-cells donated by people who have been infected previously with the coronavirus.

Discovered by the Vanderbilt University Medical Center in Tennessee, the antibodies bind to distinct sites on the coronavirus's spike protein.

In a briefing yesterday, Hung said of the treatment: "It is rather expensive, and you have to give it every six months. That's why we had to prioritize this prophylactic treatment to very special groups of patients."

He said high-risk individuals - people who have undergone bone marrow or kidney transplants, lymphoma or cancer patients, those on chemotherapy, are undergoing dialysists or are on biologics or steroids, plus those born with immune problems - are being prioritized in the SAR.

He estimates that between 80,000 and 100,000 Hongkongers have immune problems, and around 10,000 of them need other ways apart from regular Covid-19 vaccinations to be shielded from the virus.

"We still encourage high-risk individuals to get vaccinated first because probably 50 percent of those we mentioned in the priority [category] are still able to respond to the vaccination despite a lower antibody load," Hung said.

They are also encouraged to "take two or three doses of the vaccine before being considered for this monoclonal antibody treatment."

Hung also explained that the LAAB combination has been modified in the laboratory to extend the half-life of the antibody itself, tripling its durability and providing protection for people for six to 12 months after the two doses have been administered.

"Since the LAAB combination is injected directly into the body it takes effect quickly and immunity can be acquired immediately," Hung added, "while the chance of an allergic reaction will be lower."

Patients injected with the combination within three days of the onset of symptoms had a near-90 percent reduction in risk and experienced few side effects.

Hung also said if people are not vaccinated due to health reasons, or if they are concerned about their antibody levels after vaccination, they can check with public hospitals or government specialists to see if it is appropriate for them to be injected with the LAAB combination.