China has sold and donated its Sinovac Covid-19 vaccine far and wide in Southeast Asia. Photo: Xinhua

Health officials and medical experts in Hong Kong will soon discuss if people need to have a third jab of Covid-19 vaccines if their antibody levels fail to meet minimum requirements.

Lau Yu-lung, chairman of the Scientific Committee on Vaccine Preventable Diseases, the Centre for Health Protection, said the government had started giving BioNTech jabs to teenagers who have chronic diseases including cancer and kidney and blood diseases last Saturday.

Lau said they would be given a third dose if their bodies did not generate enough antibodies after two shots.

The Scientific Committee will hold a meeting within two weeks to see whether anyone needs to get a third shot and whether a different kind of vaccine can be used, Hong Kong’s newspapers reported.

Citing some initial findings, an unnamed source told the media that those who received the Sinovac vaccine might need a third jab, which would be available as early as 2022.

On Monday, Thailand announced it would use the AstraZeneca vaccine as a second dose for people who received the Sinovac vaccine as their first dose in a bid to increase protection.

Health Minister Anutin Charnvirakul said such a move would help the country build a high level of immunity against the disease, particularly the Delta variant.

Starting from July 1, people who have been inoculated with one of four vaccines – BioNTech, Moderna, AstraZeneca and Johnson & Johnson – and those who have recently recovered from Covid-19 can enter the member states of the European Union without quarantine.

The jury is still out in Hong Hong on getting a third shot. Photo: AFP/Daniel Roland

That means that people who are vaccinated with Chinese vaccines, including Sinovac and Sinopharm, will still have to be isolated after arriving in EU countries.

On July 7, Singapore also announced that people who received the Sinovac shots were excluded from its count of total vaccinations against Covid-19 due to the Chinese vaccine’s inadequate efficacy data.

In Hong Kong, about 2.62 million people had received their first dose as of Monday, while 1.83 million people had received their second dose. Of the fully vaccinated people, 1.08 million or 59% had the BioNTech vaccine, while the remaining 750,000, or 41%, had the Sinovac shots.

According to Roche Elecsys’s total antibody test, the Covid-19 antibody level in a person should lie on a scale between 0.4 units per milliliter (U/ml) and 2,500 U/ml. A result below 0.8 U/ml means that the lab did not find antibodies in a person’s sample.

At present, there has not been a global standard of what constitutes a high, low or average level of antibodies.

On June 29, Hong Kong Sanatorium & Hospital published research after it collected blood samples from 457 medical staff who had received at least one dose. Of them, 48% received the Sinovac vaccine, while 52% had the BioNTech one.

About 99.5% of people who received two Sinovac jabs tested positive in Roche Elecsys’s total antibody test, while 100% of those vaccinated with the BioNTech vaccine tested positive.

The average antibody level was 141.7 U/ml for those with Sinovac and 244.1 U/ml for people with BioNTech. Two of 289 fully-vaccinated people tested negative for anti-spike IgG. They were both immunocompromised, one with psoriatic arthritis and the other with chronic lymphocytic leukemia, according to the research.

According to Abbott’s test, the positive threshold of IgG antibody level against the SARS-CoV-2 spike (S) protein is 50 arbitrary units (AU) per milliliter, while the upper limit is 40,000 AU/mL.

The first consignment of the BioNTech vaccine is unloaded from a Cathay Pacific plane in Hong Kong in February. Photo: Handout

On July 2, Chan Kin-por, a pro-Beijing lawmaker representing the insurance sector, said in a video he found that his antibody level was below 100 AU/ml, which was considered as low, in a recent Roche Elecsys’s total antibody test.

He said he received the Sinovac vaccine in February and March. He said he had once been worried about this so he sought medical advice from a doctor.

He was told he would be protected as long as he tested positive for the Covid antibodies. He said the T-cell-mediated immune responses, triggered by the inoculation, would also help control the virus. He said people vaccinated with Sinovac should not be over-worried.

Priscilla Leung, a legislator of the Business and Professionals Alliance for Hong Kong, told Hong Kong media that several vaccinated people said their antibody levels did not meet or only barely met the minimum requirements and they were worried they would be unable to go to mainland China without quarantining.

Leung declined to say which vaccine these people had used, but she said the government should provide a guideline to the public on whether anyone should get the third jab.

Currently, the Hong Kong government does not recommend people to mix vaccines. When it suspended the BioNTech injection in March due to a minor product defect, a 32-year-old man who had BioNTech as his first dose was mistakenly given Sinovac as his second dose.

After the BioNTech shot was resumed, he went to the vaccination center and had the German vaccine again as his third dose. The government later fixed this loophole to prevent people from getting two different vaccines.

Visitors queue at a community testing center in Hong Kong. Photo: Miguel Candela Poblacion / Anadolu Agency via AFP

In May, health officials found the man had not shown any side effects, but in fact had generated an extraordinarily high amount of Covid antibodies, which would protect him from the coronavirus for at least one year.

Government adviser on the pandemic David Hui Shu-cheong said it was too early to say whether all vaccinated people should get a third jab. Citing some latest research, Hui said only those who had organ transplants might need it. He also warned that drug makers’ recommendations about the third shot might involve commercial considerations.

On June 4, Sinovac Biotech chairman and chief executive Yin Weidong said in a TV interview that the third shot could boost antibody level by 10 times in one week and 20 times in two weeks. Yin said the company would do research in the coming six months to one year to see whether the third dose was necessary. 

Read: HK should aim for zero infections, experts say