Two doses of China’s Sinovac vaccine only offered older people a moderately high level of protection against serious illness and death from Covid-19, but a third dose significantly boosted their defenses, according to a new study by scientists in Hong Kong.
The study, based on patients infected during the current devastating Omicron wave in Hong Kong, serves as a warning to mainland China, where Sinovac is a mainstay of the country’s vaccination program. Many older people there have not yet received booster shots.
For people 60 and older, two doses of Sinovac were 72 percent effective against severe or fatal Covid-19 and 77 percent effective against Covid-related death, the study found. Those levels of protection were lower than two Pfizer-BioNTech doses. The same study found that they were 90 percent effective against severe or fatal Covid and 92 percent effective against death among Hong Kong residents of the same age group.
A Sinovac booster shot helped significantly and was found to be 98 percent effective against severe or fatal Covid in people at least 60 years old, the study found.
Yanzhong Huang, a global health expert with the Council on Foreign Relations, said the results highlighted the urgency for mainland China to accelerate its lagging booster campaign. “There is a lot of work for the government to do to ensure that this segment of the population gets the booster injections,” he said.
The study authors, who are scientists at the University of Hong Kong, noted that the city’s booster program recently began, making it difficult to determine how long the protection against a third dose would last.
Because people with underlying health conditions in Hong Kong were more likely to resist vaccination, they said, it was also possible that those who chose to get vaccinated or boosted were healthier in the first place, pushing estimates of how protectively the vaccines were initially inflated.
Sinovac, a privately owned Chinese company that makes the vaccine, is one of two manufacturers of Covid shots available in China. Vaccines that use mRNA technology, such as those from Pfizer-BioNTech and Moderna, are not available there.
The new study highlights the potential impact on China, which has been heavily dependent on Sinovac and is facing the largest Covid outbreak in two years. More than 87 percent of the Chinese population has been vaccinated. But just over half of people 80 and older have had two injections, and less than 20 percent of people in that age group have received a booster, Zeng Yixin, a deputy minister for the National Health Commission, said recently.
The new Hong Kong study received funding from the China Center for Disease Control and Prevention as part of what one of the study’s co-authors described this week as an effort to understand the comparative effectiveness of vaccines. It has been posted online as a preprint, but has not yet been reviewed by fellow scientists for publication in a scientific journal.
Sinovac’s vaccine performed similarly to Pfizer’s in younger people, even without a booster dose, the study found. In people under the age of 60, two doses of Sinovac were about 92 percent effective against severe or fatal Covid, while two doses of Pfizer were about 95 percent effective.
Neither vaccine offered much protection against mild or moderate Covid, although Pfizer’s offered more than Sinovac and a booster dose significantly increased levels of protection. During the latest wave, people in Hong Kong have been largely infected by the sub-variant of Omicron known as BA.2. Like other versions of Omicron, BA.2 has infected many vaccinated people.
The wave in Hong Kong is killing people at a rate faster than almost any country since the coronavirus outbreak – largely a result of low vaccination rates among older residents. Nearly 90 percent of people who died in the latest wave were not fully immunized, suggesting that getting shots to the most vulnerable is more important than the specific brand of vaccine.
Like Hong Kong, mainland China had largely succeeded in curbing the transmission of the virus before Omicron, leaving the population with very little immunity to previous infections.
Outside of China, Sinovac vaccines have also been critical in protecting people from severe Covid, especially in poorer countries. The vaccine is used in 49 countries, including South America and Africa.
But concerns about the protection it provided had already led the World Health Organization to recommend in October that recipients age 60 and older receive a third dose.
dr. Andrew Morris, an infectious disease specialist at Sinai Health and University Health Network in Toronto, who was not involved in the Hong Kong study, said the results are consistent with lab studies suggesting that Sinovac generated lower levels of neutralizing antibodies than mRNA vaccines, such as that of Pfizer.
“I think we will see in countries that have relied heavily on Sinovac, if they don’t have boosting – especially with an mRNA booster or even with Sinovac – they are likely going to struggle with high infection rates with this latest BA.2 wave,” he said.
dr. Morris said the results in Hong Kong, like those of other vaccine studies, also depended heavily on how long it had been since people had the injections. Protection tends to weaken over time.
The results of the latest study on the effectiveness of third doses of Sinovac could be taken by Chinese leaders as an encouraging sign that vaccines made in China could remain the focus of their immunization campaign, said Dr. Huang of the Council on Foreign Relations.
“Now, for the Chinese leaders, they don’t have to face strong pressure to approve BioNTech’s vaccine,” he said.