Some coronavirus vaccine candidates currently under development could increase susceptibility to HIV, a group of researchers has warned.

A modified virus being used in four COVID-19 vaccine contenders — called adenovirus 5 (Ad5) — has been shown to increase transmission of the AIDS virus in the past, the researchers wrote in a “cautionary tale” published in The Lancet medical journal Monday.

Adenoviruses — a group of common viruses which can cause a range of illnesses, including the common cold — are sometimes genetically engineered and used to create inoculations.

But the Ad5 strain used in a potential HIV vaccination a decade ago was found to make some men more vulnerable to HIV in placebo-controlled trials, according to Science Magzine.

It is unclear why Ad5 increased the risk of HIV infection in the trial, but following these findings, the National Institutes of Health, led by Dr. Anthony Fauci, recommended against using the strain in vaccines.

The researchers are warning that more needs to be learned about this side effect before COVID-19 vaccinations are rolled out to vulnerable populations.

“If I were in a sub-Saharan African country and making a decision as to what I would want for my country for a general population use of a SARS-CoV-2 vaccine, I don’t see why I would pick an Ad5 vector [vaccine] when there are many other alternative choices,” Professor Lawrence Corey, one of the co-authors of the Lancet warning told Science Mag.

The researchers identify four potential COVID-19 vaccines containing Ad5 that are in development internationally, including in the US.

California-based ImmunityBio received approval from the Food and Drug Administration last week to begin preliminary testing of its COVID-19 vaccine candidate, according to the LA Times. The vaccine uses Ad5 to deliver coronavirus proteins into the body to trigger an immune response which hopefully protects against infection.

However, the company’s CEO Patrick Soon-Shiong insists that ImmunityBio’s Ad5 has four deleted genes that reduce the immune responses it triggers.

“It’s 90% muted,” he told Science Mag.

Two other coronavirus vaccine contenders containing the modified virus are currently in advanced trials in Russia and Pakistan, according to the outlet.

COVID-19 vaccines currently in development by Johnson & Johnson and AstraZeneca use different adenoviruses as vectors to deliver the proteins, it added.

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