Vaccine Week: A final worry

This proposed vaccination is rapid and global. But what if that has grave unintended consequences?

Welcome to Plugging the Gap (my email newsletter about Covid-19 and its economics). In case you don’t know me, I’m an economist and professor at the University of Toronto. I have written lots of books including, most recently, on Covid-19. You can follow me on twitter (@joshgans) or subscribe to this email newsletter here.


This week I have been looking at vaccines and why they might not bring about a miracle end to the virus. On Monday, I explained that it is doing to take time to produce and distribute the vaccine. On Wednesday, I explained that the vaccine itself may be imperfect in protecting people from Covid-19 and, more worryingly, in preventing the transmission of SARS-CoV-2. Today, I want to raise an issue that might be worse than that. Scientists are actually concerned that the wrong type of vaccine, distributed widely, may create more health issues and the like in the future. While we don’t know if that will occur, it is important to raise these issues because policy-makers can choose to minimise those risks.

I read about this in Debora MacKenzies’ terrific book that you should all read, Covid-19: The Pandemic that should never have happened and how to stop the next one. She examined the fact that viruses such as the coronavirus mutate and this can change their virulence. That evolution is based on whatever is good for the virus which may or may not lead to more virulence.

But we have to ask, what might a vaccine do to virulence?

Andrew Read of Pennsylvania State University has done research with several diseases, including Marek’s disease, a common plague of chicken farms, to see how vaccination affects the virus’s evolution. He found that if a vaccine keeps the virus’s host from getting sick, but still allows the virus to persist and spread—like the poultry vaccine for H5N1 did in China—it can select for a more virulent virus. …

“Leaky” vaccines, though, induce immune reactions in the host that keep the virus at low levels, so an infected host doesn’t die—but the virus doesn’t completely die out either. If that happens, those viruses can become extra virulent, because they don’t have the problem of killing off their host if they do. And changes in the virus that increase virulence—faster replication or faster invasion of the host’s cells, for example—might be helpful for a virus in a vaccinated host, allowing it to persist and spread a bit better despite the host’s immune reactions. Now if that virus reaches hosts who aren’t vaccinated, it will be more lethal than usual. That is exactly what happens with chickens vaccinated for Marek’s.

We could well be producing a ‘leaky’ vaccine. What is more, there is pressure to distribute it fast and wide. In other words, if it is one of those vaccines that can accelerate virulence, we might only find out about it when the damage baked in.

MacKenzie is rightly concerned that we have no brakes in place to prevent this.

Now, imagine we find such a vaccine, but then someone says: you can’t use that, it might cause the evolution of a more severe strain of Covid-19. That might be a hard sell.

She notes that Ebola in West Africa in 2014 showed viruses from animals can become more virulent just circulating in unvaccinated people. There is a widespread, mistaken belief that such viruses always get milder, which might make it even harder for governments to accept that Covid might get worse.

The bottom line is simple. We need to ensure we do our homework.

We will have to keep this in mind as we develop Covid-19 vaccines. The virus is likely to keep circulating, at some level, everywhere until we have a vaccine and use it widely. That means countries will need to keep testing and interrupting chains of transmission to contain it—or if they fail to, they will have more large outbreaks requiring social distancing. Either way, it will be expensive, so there will be enormous pressure to widely use whatever vaccine we develop that seems to work.

Do this wrong and we could have trouble. Do this right and we can minimize risk. But doing the right thing means putting another barrier in the way of a vaccine being a ‘silver bullet.’ It is enough of a risk that it should give us pause in relying on vaccine as our only hope.

[Updated 18th September 11am replacing “She points to the evolution of Ebola as a case in point.” Thanks to Deb MacKenzie for clarifying and showing me how to put this quote in proper context.]


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