Could Vaccines make Viruses more Dangerous?

Precise model of the COVID-19 virus Sars-Cov-2 being injected with various vaccine candidates. Fight against the disease. Different versions available. 3d model contains all aspects of this particular virus, including envelope, Spike proteins, M-proteins and HE-proteins.
Yes, imperfect vaccines could make viruses more dangerous, at least in chickens. Researchers studied the effect of “leaky” vaccines —those that don’t make their hosts totally immune to the disease and incapable of spreading it to others — on viruses. (Penn State University)

The vaccines widely used by humans today — especially those used to protect children against mumps, measles, and other potentially deadly ailments — are what we’d call “perfect” vaccines. But new research suggests that imperfect vaccines or “Leaky” vaccines — ones that don’t make their hosts totally immune to the disease and incapable of spreading it to others — might have a surprising downside. For now, this so-called leakiness only exists in vaccines used to treat farm animals. But researchers warn that as humankind tackles bigger, badder diseases, we should keep the potential danger of leaky vaccines in mind.


Virulent Strains for Unvaccined

Andrew Read and his colleagues don’t know whether the vaccines for the disease actually caused more virulent strains of the illness to develop. It’s not a clean-cut evolutionary partnership the likes of antibiotics and antibiotic resistant bacteria.

But according to their research, however those more dangerous strains have developed, it’s the existence of the vaccine that allows them to continue existing.

Imagine for a moment a chicken with Marek’s disease so viral (otherwise known as a particularly “hot” strain of the virus) that it knocks them dead within 10 days. Marek’s disease used to be a disease that didn’t kill, but lingered. It took a while to transmit from one chicken to another. Ten days simply wasn’t enough time, so the hot strain would die with one or two unfortunate chickens.

Now imagine that this super-hot chicken has been vaccinated, and that this vaccine saves its life — but doesn’t keep it from spreading its supercharged Marek’s disease. Suddenly that lucky little chicken is patient zero.

“We had laid out the mathematics in a previous paper, and suggested this might be happening,” Andrew Read told The Post. To prove it, they found themselves some unvaccinated birds and put them in close quarters with infected-but-protected birds to watch the disease spread.

“The experiment shows that strains too hot to exist in an unvaccinated world can actually persist when there’s a leaky vaccination,” Read said.

This isn’t such a big deal for the chicken industry at the moment, Andrew Read explained, since it’s trivial to make sure all of your birds are vaccinated when they live on a farm. And Marek’s disease only affects chickens.

“The problems would start if we weren’t just talking about chickens,” he said. If a imperfect avian flu vaccine was given to chickens, for example, those chickens would still be capable of spreading hot strains to wild fowl — like ducks and geese — who couldn’t be so easily vaccinated.

What about Human?

And the problem gets scarier when you ask what happens if a human gets a particularly hot strain of avian flu.

“It’s just not possible to predict if a virus will get more or less nasty when it jumps species,” Anderw Read said. “It’s not predictable in general, and we just don’t know how that works with avian flu. It’s just not a good idea to create these conditions.”

“In the future, the findings could apply to vaccines that we hope will be developed against generally lethal viruses encouraging scientists to strive towards “perfect vaccines” for them,” Michael Skinner, a virologist from Imperial College London who wasn’t part of the study, told the Science Media Centre.