(REUTERS) High-profile COVID-19 vaccines developed in Russia and China share a possible shortcoming: They are based on a common cold virus that a lot of individuals are exposed to, possibly limiting their effectiveness, some experts say.
A vaccine developed by Moscow’s Gamaleya Institute, approved in Russia earlier this month despite limited testing, is based on Ad5 and a second less common adenovirus.
“The Ad5 concerns me only because many people have immunity,” said Anna Durbin, a vaccine researcher at Johns Hopkins University. “I am not sure what their strategy is… maybe it will not have 70% efficiency. It may have 40% efficacy, and that is far better than nothing until something else comes together.”
Both developers have years of experience and accepted Ebola vaccines based on Ad5. Neither CanSino nor Gamaleya responded to requests for comment.
Researchers have experimented with Ad5-based vaccines against many different infections for decades, but none are widely used. They employ harmless viruses as”vectors” to ferry genes from the target virus — in this case, the novel coronavirus – to individual cells, prompting an immune response to fight the real virus.
But lots of individuals already have antibodies against Ad5, which might cause the immune system to attack the vector instead of reacting to the coronavirus, making these vaccines less powerful.
Several researchers have selected alternative adenoviruses or delivery mechanisms. Oxford University and AstraZeneca established their COVID-19 vaccine on a chimpanzee adenovirus, avoiding the Ad5 issue. Johnson & Johnson’s candidate uses Ad26, a comparatively rare strain.
Dr. Zhou Xing, from Canada’s McMaster University, worked with CanSino on its first Ad5-based vaccine, for tuberculosis, in 2011. His team is developing an inhaled Ad5 COVID-19 vaccine, theorizing it might circumvent pre-existing immunity difficulties.
“The Oxford vaccine candidate has quite an advantage” over the injected CanSino vaccine,” he said.
Xing also worries that high doses of the Ad5 vector from the CanSino vaccine could induce fever, fueling vaccine skepticism.
“I think they will get good immunity in people who don’t have antibodies to the vaccine, but a lot of people do,” said Dr. Hildegund Ertl, director of the Wistar Institute Vaccine Center in Philadelphia.
In China and the United States, about 40 percent of people have high levels of antibodies from prior Ad5 exposure. In Africa, it could be as high as 80%, specialists said.
Some scientists also worry an Ad5-based vaccine could increase the chances of contracting HIV.
At a 2004 trial of a Merck & Co Ad5-based HIV vaccine, people with preexisting immunity became more, not less, susceptible to the virus which causes AIDS.
Researchers, such as top U.S. infectious diseases expert Dr. Anthony Fauci, in a 2015 newspaper, said the side effect was likely unique to HIV vaccines. But they cautioned that HIV incidence ought to be monitored during and after trials of Ad5-based vaccines in at-risk inhabitants.
“I would be worried about the usage of those vaccines in any country or any population which was at risk of HIV, and I place our country as one of these,” stated Dr. Larry Corey, co-leader of the U.S. Coronavirus Vaccine Prevention Network, that was a lead researcher on the Merck trial.
Gamaleya’s vaccine will be administered in two doses: The first according to Ad26, like J&J’s candidate, and the next on Ad5.
Many experts expressed doubt about the Russian vaccine after the government announced its intention to give it to high-risk groups in October without data from large pivotal trials.
“Demonstrating the efficacy and safety of a vaccine is quite important,” said Dr. Dan Barouch, a Harvard vaccine researcher who helped design J&J’s COVID-19 vaccine. Often, he noted, large scale trials”don’t provide the result that is required or expected.”