A leading Covid-19 vaccine under development in China showed inconclusive results about its level of protection, although scientists remain optimistic that the candidate can be among a roster of effective vaccines used to fight the pandemic.
In early-stage clinical trials, Sinovac Biotech Ltd.’s CoronaVac vaccine was shown to induce antibodies in the human body within 28 days of the first immunization, according to results published in the Lancet medical journal this week. The level of antibodies, however, were lower than that seen in people who were previously infected with Covid-19.
By contrast, the levels of antibodies in results from vaccine trials by
were roughly on par with those in people who previously contracted the virus. This week, Pfizer said its experimental vaccine was 95% effective at protecting people from Covid-19, while Moderna said its vaccine was 94.5% effective based on an early look at late-stage results.
The results from Sinovac are for Phase 1 and Phase 2 clinical trials, which aren’t designed to measure whether the vaccine works but can indicate whether it provokes an appropriate immune response. The firm hasn’t published late-stage, or Phase 3, results, which measure efficacy.
“One would have liked the antibody levels to be comparable,” said William Schaffner, professor of preventive medicine at Vanderbilt University School of Medicine. However, he said, the finding “may just mean the duration of protection may not be quite as long.”
Scientists don’t know what absolute level of antibody is needed to protect against the new coronavirus disease. It is possible that Sinovac’s vaccine induces enough antibodies to confer protection.
The level of antibody often correlates with the duration of protection, although Dr. Schaffner said that Sinovac’s results don’t rule out its vaccine being able to protect people for a similar period of time as the Pfizer and Moderna vaccines.
China is behind four of nearly a dozen Covid-19 vaccines that are in the last phase of clinical trials, according to the World Health Organization, having recruited tens of thousands of volunteers around the world.
Scientists say the Chinese candidates may ultimately prove less effective than Western counterparts that use advanced gene-based technologies; the effective rates of more than 90% reported by Pfizer and Moderna surprised the scientific community, where many were hoping that the experimental Covid-19 vaccines would be at least 50% effective.
Both Sinovac and state-owned Sinopharm, the two leading Chinese vaccine developers, are using inactivated viruses for their vaccines. That technique involves growing viruses in a petri dish and then weakening them for vaccine purposes.
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China has begun using vaccines by Sinovac and Sinopharm on hundreds of thousands of its citizens under an emergency-use program that started in July.
The Chinese drugmakers and government officials have declared the vaccines as safe to use, even as global public-health experts warn of the dangers of using vaccines so broadly and before clinical trial results are published.
Sinopharm has said that none of the 56,000 people who have received its vaccines, including Chinese workers overseas, have gotten infected. The firm’s chairman, Liu Jingzhen, at a recent conference shared an example from an office of Huawei Technologies Co. in Mexico, where none of the 81 employees who were inoculated got infected; 10 employees who didn’t get the vaccine ended up getting infected, he said.
The Huawei example didn’t appear to be part of a clinical trial by Sinopharm, which is conducting formal studies in the United Arab Emirates, Bahrain, Egypt and other places.
Sinovac hasn’t disclosed outcomes from people who have been inoculated with its vaccine and then traveled abroad.
Zhu Fengcai, who is an official at the Jiangsu province branch of the China Center for Disease Control and Prevention and a lead author of the Sinovac study laying out the result of the early-stage trials, said the quick antibody response the company’s vaccine induced could make it suitable for emergency use. “However, further studies are needed to check how long the antibody response remains,” he said.
Sinovac researcher Zeng Gang, another of the study’s authors, said CoronaVac could be an attractive option because it can be stored in a standard refrigerator between 2 and 8 degrees Celsius, which is typical for flu vaccines. Pfizer’s vaccine must be stored at minus 70 degrees Celsius, though it can be kept at standard refrigeration temperatures for about five days. Moderna said its vaccine can be kept in long-term storage at a below-freezing temperature comparable to that of most home or medical freezers.
The Sinovac vaccine may also remain stable for up to three years in storage, which would offer some advantages for distribution to regions where access to refrigeration is challenging, Mr. Zeng added.
Sinovac conducted its early-stage trials in Jiangsu province. It is conducting late-stage clinical trials in Brazil, Indonesia and Turkey.
Health experts say having a vaccine is just one front in a two-front battle against Covid-19. The other is effective treatments for those who are already sick with the disease. WSJ breaks down the three most promising types in development. Photo Illustration: Jacob Reynolds/WSJ.
Write to Chao Deng at Chao.Deng@wsj.com