CanSino Biologics' vaccine, approved for military use in China, is a modified form of adenovirus type 5, or Ad5. The company is in talks to get emergency approval in several countries before completing large-scale trials, the Wall Street Journal reported last week.

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 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.”

Vaccines are seen as essential to ending the pandemic that has claimed over 845,000 lives worldwide. Gamaleya has said its two-virus approach will address Ad5 immunity issues.

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

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.

Several researchers have chosen alternative adenoviruses or delivery mechanisms. Oxford University and AstraZeneca based their COVID-19 vaccine on a chimpanzee adenovirus, avoiding the Ad5 issue. Johnson & Johnson’s candidate uses Ad26, a comparatively rare strain.

Read: Mexico to Get 2,000 Doses of Russia's Sputnik Vaccine to Test

HIV RISK

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

In a 2004 trial of a Merck & Co 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.

“I would be worried about the use of those vaccines in any country or any population that was at risk of HIV, and I put our country as one of them,” said Dr. Larry Corey, co-leader of the U.S. Coronavirus Vaccine Prevention Network, who was a lead researcher on the Merck trial.

Gamaleya’s vaccine will be administered in two doses: The first based on Ad26, similar to J&J’s candidate, and the second on Ad5.

Alexander Gintsburg, Gamaleya’s director, has said the two-vector approach addresses the immunity issue. Ertl said it might work well enough in individuals who have been exposed to one of the two adenoviruses.

Many experts expressed skepticism about the Russian vaccine after the government declared its intention to give it to high-risk groups in October without data from large pivotal trials.

“Demonstrating safety and efficacy of a vaccine is very important,” said Dr. Dan Barouch, a Harvard vaccine researcher who helped design J&J’s COVID-19 vaccine. Often, he noted, large-scale trials “do not give the result that is expected or required.”

REUTERS