Antibodies From China's Sinovac Decline After Only Six Months, Say Beijing Researchers

A new study has found that neutralizing antibodies from Sinovac Biotech's COVID-19 vaccine CoronaVac declined below a key threshold from around six months after the second jab for most people. A third dose, in the form of a booster shot, however, could result in a remarkable increase in antibody levels, a group of Chinese researchers unveiled on Sunday.

Findings from a study of blood samples from healthy vaccinated adults aged between 18 to 59 were published on medRxiv on Friday for peer review. It also found evidence that a two-dose schedule appears to generate good immune memory, according to the Global Times.

Researchers say that among the fully vaccinated participants who got the doses two and four weeks apart, only 16.9 percent and 35.2 percent still had neutralizing antibodies above the detectable threshold level, respectively. The assessments were made six months after the second shots were administered to recipients.

However, it remains unclear how the decline in antibodies could affect the effectiveness of the CoronaVac because scientists are yet to precisely determine the desired threshold of antibody levels for a vaccine to prevent COVID-19.

The proponents of the study also noted that other components in human immune systems, such as T cells and B cell memory elicited by the vaccine formula, may also contribute to protection apart from durable antibodies.

"In the short-to-medium term, ensuring more people complete the current two-dose schedule of CoronaVac (Sinovac's vaccine) should be the priority," the paper said.

Meanwhile, the 540 participants who got inoculated with a third Sinovac dose about six months after the second exhibited around a threefold to fivefold increase in antibody levels after a further 28 days.

Siti Nadia Tarmizi, a senior health ministry official in Indonesia, told Reuters on Tuesday that the immunization advisory board recommends a booster vaccination 12 months after the second dose of the Sinovac vaccine.

She then noted that clinical data from Indonesia showed the reduced antibody level is still enough to protect people against the virus without going into the details.

Researchers cautioned the study did not test the effects of the antibodies against more transmissible variants like the Delta strain, suggesting further scrutiny to assess the antibody duration after a third shot of Sinovac.

Channel News Asia noted that Singapore has recently announced the CoronaVac shot to be administered to eligible COVID-19 vaccine recipients who are reporting allergic reactions after receiving the first dose of mRNA COVID-19 vaccines such as the Pfizer-BioNTech and Moderna formulas.

Senior Minister of State for Health Janil Puthucheary told Parliament on Monday that people who got the jabs in such a manner will be deemed fully vaccinated.