Adenovirus-vectored vaccines like J&J, AstraZeneca, and Sputnik V use infection with adenoviruses, common pathogens that usually cause common colds.

But to increase safety and decrease risk of side effects, most vaccine vectors delete the AdV E1 and E3 genes.
E1/E3 deletions are standard in Ad-vectors. Deleting E1 prevents the virus from replicating and deleting E3 prevents it from interacting with the immune system.
This results in a vaccine vector that causes a transient infection, due to the vector’s inability to complete its replication cycle. When I got J&J it infected my cells, began expressing viral proteins, including SARS-CoV-2 spike, and then hit a block because of the E1 deletion.
Sputnik V is a heterologous Ad-vectored vaccine: the prime dose is an Ad26-vectored vaccine (like J&J), followed by a booster that uses Ad5. This was apparently found in testing the Ad5 booster but I can’t emphasize how sloppy this is.
When making adenovirus-vectored vaccines, once you’ve deleted a region of the genome, it doesn’t just reappear out of nowhere. The presence of replicating virus suggests E1 was either not deleted or it recombined during manufacturing with a full length AdV genome.
And while strong statements from Gamaleya and the RDIF dispute the findings, this isn’t the first observation that the Sputnik V doses submitted to regulators aren’t consistent with the phase 3 trial results published in Lancet.
This is disappointing. Ad-vectored vaccines are easier than mRNA vaccines to store and distribute, and are going to be important for vaccinating people in hard-to-reach places. Despite rare side effects, they are generally considered safe and trial data shows they are effective.
Safety concerns have already reduced enthusiasm for these vaccines, and an unforced error like this further amplifies those concerns and decreases trust in the technology itself. I hope Gamaleya resolves these issues and supports it with data and robust QC/QA.
PS-though I wish it were for different reasons, I’m always thankful for the opportunity to practice my Russian exclamations. Да ты что, Гамалея!
More info here: apparently the Ad5 vector was capable of forming plaques on A549 cells. The explanation is recombination, which begs the question: if it was manufactured using best practices, WTF did the vaccine vector recombine with?
h/t @hildabast
Could be either a wild-type AdV or a packaging vector. To make replication-deficient vaccine vectors, you have to grow the virus in a "packaging" cell line expressing E1 in trans (on a separate piece of DNA called a plasmid).
At this stage recombination could occur with E1 DNA from the packaging cell line, but usually there are QC measures in place to ensure this doesn't happen. Deleting E1/E3 in many Ad-vectored systems is also needed to make "room" in the genome for the insert, SARS-CoV-2, spike.
I don't know enough about the specific AdV system used here, but it's reasonable to think that if E1 recombined w/ the Ad vector, it might replace spike. So that's doubly bad: you not only have a replication-competent Ad5, it's not expressing spike. Which is...the entire point.
One thing is clear: if the vaccine were forming plaques on A549s, that means there's infectious, fully replication competent virus there. A plaque is a "hole" in a carpet (monolayer) of cells on a culture plate caused by cytopathic effect (virus killing cells).
The plaque assay is a classical virological technique used to titrate infectious virus. An individual cell killed by a virus can't be seen without a microscope. To see a plaque with the naked eye, the virus has to spread to surrounding cells, and to do that it has to replicate.
No visible plaque=no spread to surrounding cells=no viral replication.

So if they saw plaques on A549s (a type of lung cell that is convenient for plaquing adenoviruses), that means there was fully replication competent virus.

It's a basic and inexcusable failure of QC/QA.
Here's a GIF that presents an analogous situation of what the Anvisa regulators observed:
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