• A study in China found that SinoVac and SinoPharm were 70% effective in preventing COVID-19 infection
  • A preprint study in Brazil found that SinoVac was only 54% effective in preventing coronavirus infection
  • Singapore had previously refused to count SinoVac jabs in the country's tally of vaccinations

The China-made COVID-19 vaccine, SinoVac, has been found to produce significantly lower levels of antibodies compared to the vaccine developed by Pfizer-BioNTech, prompting residents in Singapore to get booster shots.

Antibody levels in people who have received two doses of the Pfizer vaccine were normally between 1,300 and 2,000 international units per milliliter. However, the numbers are much lower in individuals who received two shots of the SinoVac vaccine.

“For Sinovac, it is zero to 40. We have a few with 200 to 300,” Dr. Leong Hoe Nam, an infectious diseases specialist at Rophi Clinic in Singapore, told South China Morning Post.

Dr. Leong noted that the lower antibody levels in SinoVac recipients have prompted an increasing number of individuals to get Pfizer as a booster shot.

“They took the Sinovac shots, did the blood test and saw low antibody levels, then opted for Pfizer as the third dose,” he added.

A study conducted by researchers in China found that the SinoVac and SinoPharm vaccines had a combined efficacy of 70% in preventing an infection caused by the more contagious Delta variant in the city of Guangzhou. The study, which has yet to be peer reviewed, also noted that the shots were 100% effective in preventing severe infections and deaths.

However, a Brazilian preprint study comparing SinoVac to the AstraZeneca COVID-19 vaccine found that recipients of the Chinese-made vaccine had lower protection levels against the virus.

SinoVac recipients were 54% less likely to contract COVID-19 and 74% less likely to die of the virus compared to the unvaccinated. The efficacy waned in the older population, reducing the risk of death by only 35% in people over the age of 80. In comparison, AstraZeneca reduced the risk of infection by 70% and the risk of death by 90%.

The Singaporean government had previously excluded people who received the SinoVac shots from the country’s total count of vaccinations. Health Minister Ong Ye Kung cited inadequate efficacy data as the reason for the move.

"We don't really have a medical or scientific basis or have the data now to establish how effective SinoVac is in terms of infection and severe illnesses on Delta," Ong said during a July media briefing, according to Reuters.

Singapore’s Ministry of Health later announced it will consider people who received vaccine shots included in the World Health Organization’s emergency use list, such as SinoVac, Sinopharm and AstraZeneca, as fully vaccinated, in an effort to be more “inclusive.”

“What is important now is the difference between those who are vaccinated and not vaccinated and less so between different vaccines,” Ong said at a press conference in early August.

Despite this, the Singaporean government is still offering third doses of vaccines to residents who had taken the SinoVac jab as part of a “heterologous vaccination strategy.”

"We have not stopped them although data is still lacking considering what the effectiveness is of this strategy using two different vaccines – we call this a heterologous vaccination strategy," Director of Medical Services Kenneth Mak said during a virtual doorstop interview.

The trial will test the efficacy of combining an 'inactivated' vaccine made by China's Sinovac with a DNA-based one
The trial will test the efficacy of combining an 'inactivated' vaccine made by China's Sinovac with a DNA-based one AFP / Lillian SUWANRUMPHA

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