Severe Infections 5 Times More Likely Among Sinovac Than Pfizer COVID-19 Vaccine Recipients: Study

A health worker administers the CoronaVac vaccine, developed by China's Sinovac firm, to a woman from an at-risk group at Saeng Thip sports ground in Bangkok, Thailand, on April 7, 2021. Mladen Antonov/AFP via Getty Images

The China-made Sinovac-CoronaVac vaccine is the least effective in preventing infection or severe symptoms among COVID-19 patients compared to the Sinopharm, Moderna, and Pfizer-BioNTech/Comirnaty vaccines, according to a recently published study from Singapore.

“We observed a lower relative effectiveness of two inactivated whole virus vaccines (Sinovac-CoronaVac and Sinopharm) against COVID-19 infection compared with two mRNA vaccines (Pfizer-BioNTech/Comirnaty and Moderna),” reported the researchers in a study published in Clinical Infectious Diseases on April 12.

Researchers said their findings imply Sinovac-CoronaVac and Sinopharm are 54 percent and 84 percent effective against severe cases respectively, “assuming the vaccine effectiveness against severe disease for Pfizer-BioNTech/Comirnaty of 90 percent as suggested by systematic reviews.”

Meanwhile, the study suggests that the Moderna vaccine is 96 percent effective in preventing severe symptoms among COVID-19 patients.

The researchers looked into infections and severe cases among a cohort of over 2.7 million people aged 20 and above, from 14 to 120 days after they had been vaccinated with two doses. Seventy-four percent of the cohort was vaccinated with the Pfizer-BioNTech/Comirnaty vaccine, 23 percent received the Moderna vaccine, while only 2 percent and 1 percent received the Sinovac-CoronaVac and Sinopharm vaccine respectively.

The study was conducted based on observations between Oct. 1 and Nov. 21, 2021, when daily cases in Singapore reached a peak, driven by the spread of the Delta variant. Ninety-six percent of the eligible population had received at least two doses of any vaccine by the end of last year.

The researchers found that people vaccinated with the Sinovac-CoronaVac were 2.37 times more likely to be infected by the CCP (Chinese Communist Party) virus, also known as the novel coronavirus, and 4.59 times more likely to develop severe symptoms compared to people who received the Pfizer-BioNTech/Comirnaty vaccine.

“Singapore study that showed five times the risk of severe COVID with Sinovac vs Pfizer. Thankfully only 2% vaccinated with Sinovac,” commented David Lye in a Twitter post on April 13. Lye, a coauthor of the study, is the director of the infectious disease research and training office at the National Center for Infectious Diseases (NCID).

Sinopharm, another China-made inactivated whole virus vaccine, was also found to be less effective than the mRNA vaccines. Infection and severe disease were 1.62 times and 1.58 times more likely among Sinopharm recipients.

“Sinopharm demonstrated a higher relative effectiveness against COVID-19 infection than Sinovac-CoronaVac in our study, although lower than the mRNA vaccines,” the researchers wrote.

“To date, there has not been any study directly comparing these two inactivated virus vaccines,” they added.

However, the researchers noted that previous studies have found the Sinopharm vaccine to be less effective in inducing antibody and T-cell response, including a published Hungarian study of 57 adult healthy volunteers who received either the Sinopharm or the Pfizer-BioNTech vaccine.

The Moderna vaccine was found to be more effective than the Pfizer-BioNTech/Comirnaty vaccine in Singapore. People who received the Moderna vaccine were 0.84 times less likely to be infected by the CCP virus, and 0.42 times less likely to develop severe symptoms.

“Our findings suggest that the Moderna vaccine is more effective than Pfizer-BioNTech/Comirnaty against severe disease, which is supported by other studies,” the researchers for the Singapore study wrote.

“In a report by the U.S. Centers for Disease Control and Prevention, vaccine effectiveness against COVID-19 hospitalization was slightly higher after two doses of Moderna than Pfizer-BioNTech, likely attributed to higher mRNA content in the Moderna vaccine and longer time interval between doses,” said the authors.

The researchers acknowledged several limitations to the study, including the assumption that all four vaccines did not experience differential waning of immunity. “Restriction to individuals who completed their second dose within 120 days was done to mitigate any potential impact,” said the authors.

The authors also noted potential confounding factors such as the individuals’ comorbidities, their health behaviors, and factors that influence their choice of vaccine in the first place.

Singapore started to vaccinate its population against the CCP virus on Dec. 30, 2020 with the Pfizer-BioNTech/Comirnaty vaccine. It subsequently approved the Moderna and the Sinovac-CoronaVac vaccine on Feb. 3 and Oct. 23 last year, while the Sinopharm vaccine was available through a special access route at private health care institutions from Aug. 30, 2021.

However, recipients of the Sinovac-CoronaVac and the Sinopharm vaccine are only considered to be fully vaccinated after they have received a third dose since the beginning of this year. They were also recommended to receive an mRNA vaccine for the booster dose.

The most recent figures suggest 93 percent of the total population in Singapore have received at least one dose of a COVID-19 vaccine, and 73 percent have received booster shots.