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Vaccines developed by Russia and China are based on adenovirus type 5, or Ad5, which might reduce the effectiveness when injected (Photo: Aviralbox)

According to Reuters, both CanSino Biologics’s vaccine, approved for military use in China, and Moscow’s Gamaleya Institute’s vaccine, approved in Russia earlier this month despite limited testing, are based on adenovirus type 5, or Ad5.

Both developers have years of experience and approved Ebola vaccines based on Ad5. Neither CanSino nor Gamaleya responded to requests for comment.

“The Ad5 concerns me just because a lot of people have immunity,” said Anna Durbin, a vaccine researcher at Johns Hopkins University. “I’m not sure what their strategy is ... maybe it won’t have 70% efficacy. It might have 40% efficacy, and that’s better than nothing, until something else comes along.”

Dr. Zhou Xing, from Canada’s McMaster University, worked with CanSino on its first Ad5-based vaccine, for tuberculosis, in 2011, worries that high doses of the Ad5 vector in the CanSino vaccine could induce fever, fueling vaccine skepticism.

“I think they will get good immunity in people that 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% of people have high levels of antibodies from prior Ad5 exposure. In Africa, it could be as high as 80%, experts said.

Some scientists also worry an Ad5-based vaccine could increase chances of contracting HIV.

In a 2004 trial of a Merck & Co (MRK.N) Ad5-based HIV vaccine, people with pre-existing immunity became more, not less, susceptible to the virus that causes AIDS.

Researchers, including top U.S. infectious diseases expert Dr. Anthony Fauci, in a 2015 paper, said the side effect was likely unique to HIV vaccines. But they cautioned that HIV incidence should be monitored during and after trials of all Ad5-based vaccines in at-risk populations.

(Photo: Focus Washington)

Researchers’ experiments on Ad5

Researchers have experimented with Ad5-based vaccines against a variety of 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 - into human cells, prompting an immune response to fight the actual virus.

But many people already have antibodies against Ad5, which could cause the immune system to attack the vector instead of responding to the coronavirus, making these vaccines less effective.

Researchers turn to other alternatives

Several researchers have chosen alternative adenoviruses or delivery mechanisms. Oxford University and AstraZeneca (AZN.L) based their COVID-19 vaccine on a chimpanzee adenovirus, avoiding the Ad5 issue. Johnson & Johnson’s (JNJ.N) candidate uses Ad26, a comparatively rare strain, as reported by Reuters.

Xing’s team is developing an inhaled Ad5 COVID-19 vaccine, theorizing it could circumvent pre-existing immunity issues.

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