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A vaccine expert has dashed hopes of an adenovirus vector-based COVID-19 vaccine coming to market in the next few months 鈥� claiming it is unlikely to be effective and, if so, unsafe.

Adenovirus vectors have in recent weeks, after the became one of the first vaccine developers globally to have reached phase 3 clinical trials, with its adenovirus candidate, known as 鈥淐hAdOx1 nCoV-19鈥�.

However, Professor Nikolai Petrovsky from said this class of vaccine 鈥� which works by infecting someone with live virus cells 鈥� is unlikely to be a frontrunner in the race to tackle COVID-19.

鈥淎denovirus vector-based vaccines have never been licensed for human use in more than 25 years of worldwide experimentation,鈥� he said.

鈥淭he problem with this method is that you are treading a fine line. If you don鈥檛 give someone a strong enough infection dose with the vaccine virus it won鈥檛 have much effect 鈥� that is, it won鈥檛 be sufficiently 鈥榠mmunogenic鈥�.

鈥淐onversely, if you give a high dose of virus to try and get sufficient immunogenicity it could generate unacceptable levels of toxicity. Subjects will then experience symptoms of a bad viral infection.

鈥淚f you can鈥檛 find that sweet spot 鈥� which for some viral vectors simply does not exist 鈥� then it鈥檚 not viable.鈥�

This constant trade-off between immunogenicity and toxicity is the main reason adenovirus vector-based vaccines have never made it past large-scale clinical trials to date, Petrovsky argued.

When tested an HIV vaccine based on an adenoviral vector in the STEP trial, those who received the adenoviral vector vaccine had higher numbers of positive HIV cases than a placebo group.

More recently, Oxford University鈥檚 COVID-19 vector-based solution has failed animal trials, with after receiving a vaccine shot.

When asked why researchers continue to experiment with the adenovirus method, Petrovsky said, 鈥淚 guess it boils down to the expertise of that particular research institution. If it鈥檚 a technology they鈥檝e heavily invested in then they are going to do anything possible to promote its use as a potential solution.鈥�

Petrovsky is currently developing a recombinant spike-protein based vaccine for COVID-19 with his team at Vaxine Pty Ltd and Flinders University. He said this method can take longer but it鈥檚, by far, the most likely to be approved for human use.

鈥淩ecombinant spike protein vaccines are tried and tested. They formed the basis of the SARS vaccine we developed in 2005 and a MERS vaccine in 2018.

鈥淵es, protein-based vaccines traditionally take longer to produce and test than other vaccine varieties, but I would say the 鈥榟are and the tortoise鈥� analogy is fitting here. Slow and steady wins the race,鈥� he concluded.

Image credit: 漏stock.adobe.com/au/Andreas Prott