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High-profile COVID-19 vaccines developed in Russia and China have one potential flaw: they are based on a cold virus that many people have been exposed to, which may limit their effectiveness, some experts say.
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 obtain emergency clearances in several countries before completing large-scale studies, the Wall Street Journal reported last week.
A vaccine developed by Moscow’s Gamaleya Institute, which was approved in Russia earlier this month despite limited testing, is based on Ad5 and a second, less common adenovirus.
“The Ad5 only affects me because a lot of people have immunity,” said Anna Durbin, a vaccine researcher at Johns Hopkins University. “I’m not sure what your strategy is … maybe it’s not 70% effectiveness. Maybe it’s 40% effectiveness, and that’s better than nothing until something else is added.”
Vaccines are seen as essential to ending the pandemic that killed over 845,000 people worldwide. Gamaleya has announced that its two-virus approach will resolve Ad5 immunity issues.
Both developers have many years of experience and approved Ebola vaccines based on Ad5. Neither CanSino nor Gamaleya responded to requests for comments.
Researchers have experimented with Ad5-based vaccines against a wide variety of infections for decades, but none are widespread. They use harmless viruses as “vectors” to transport genes from the target virus – in this case the novel coronavirus – into human cells, which triggers an immune response to fight the actual virus.
However, many people already have antibodies to Ad5, which can cause the immune system to attack the vector instead of responding to the coronavirus, making these vaccines less effective.
Several researchers have selected alternative adenoviruses or delivery mechanisms. Oxford University and AstraZeneca based their COVID-19 vaccine on a chimpanzee adenovirus to avoid the Ad5 problem. The Johnson & Johnson candidate uses Ad26, a comparatively rare strain.
Dr. Zhou Xing from McMaster University in Canada worked with CanSino on his first Ad5-based vaccine against tuberculosis in 2011. His team is developing an Ad5-COVID-19 inhaled vaccine that could theoretically bypass pre-existing immunity problems.
“The Oxford vaccine candidate has quite an advantage” over the injected CanSino vaccine, he said.
Xing also fears that high doses of the Ad5 vector in the CanSino vaccine could trigger a fever and fuel skepticism about the vaccine.
“I think they will get good immunity in people who don’t have antibodies to the vaccine, but who have a lot of people,” said Dr. Hildegund Ertl, director of the Wistar Institute Vaccine Center in Philadelphia.
In China and the United States, around 40% of people have high levels of antibodies from previous exposure to Ad5. In Africa it could be up to 80%, experts said.
Some scientists also fear that an Ad5-based vaccine could increase the chances of getting HIV infection.
In a 2004 study of a Merck & Co Ad5-based HIV vaccine, people with pre-existing immunity became more, not less, susceptible to the AIDS-causing virus.
Researchers, including leading U.S. infectious disease expert Dr. Anthony Fauci, said in a 2015 paper that the side effect is likely only to occur with HIV vaccines. However, they cautioned that HIV incidence should be monitored during and after trials of all Ad5-based vaccines in at-risk populations.
“I would be concerned about the use of these vaccines in any country or population at risk of HIV, and I would call our country one of them,” said Dr. Larry Corey, co-head of the U.S. Coronavirus Vaccine Prevention Network, who was the lead researcher on the Merck Process.
Gamaleya’s vaccine is given in two doses: the first is based on Ad26, similar to J & J’s candidate, and the second is based on Ad5.
Alexander Gintsburg, Gamaleya’s director, said the two-vector approach addressed the immunity problem. Ertl said it might work well enough for people exposed to either of the two adenoviruses.
Many experts expressed skepticism about the Russian vaccine after the government announced its intention to distribute it to high-risk groups in October with no data from major pivotal studies.
“Demonstrating the safety and effectiveness of a vaccine is very important,” said Dr. Dan Barouch, a Harvard vaccine researcher who helped develop J & J’s COVID-19 vaccine. Often, he noted, large-scale experiments “do not give the expected or required result”.
(Except for the headline, this story was not edited by GossipMantri staff and published from a syndicated feed.)