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Science and Innovations

Weekly Update With Dr. Paul Keim: Herd Immunity


Public health experts say the end of the coronavirus pandemic is tied to an idea called “herd immunity.” That’s when a large number of people are able to resist the disease, because they’ve already recovered from it or because they’ve had a vaccine. In KNAU’s weekly update on the science of COVID-19, Melissa Sevigny speaks with infectious disease expert Dr. Paul Keim about the world’s progress toward that goal.

Melissa Sevigny: So if you get COVID-19 once, are you immune to getting it in the future? Is it possible to get it twice?

Paul Keim: There’s definite evidence from animal studies in particular that once an animal is infected they’re immune against reinfection. We don’t know how strong that protection is and we don’t know how long it lasts. All of this is based upon what we call “immune memory.” When you get exposed to a disease like COVID-19 for the very first time, your immune system has a way to respond, but it’s having to do in a way that is untrained, it’s the same way it would respond to any disease. But if you’ve had the disease once, your immune system then sets up what we call memory. It stores away immune cells, and those immune cells are sitting there and waiting for the next time you see that virus, and they kick into action really fast.

What percentage of the population do you think would have to get the disease and then recover for us to have herd immunity?

There’s a relationship between the transmissibility of a pathogen and the herd immunity that’s needed to stop it. So for example, there’s this metric that we call the R naught. That’s the number of people on average you transmit the disease to. They’ve been estimating the R naught for this novel coronavirus between 2.5 and 3.5, and with those numbers the prediction is we need to have herd immunity on the order of 40 to 60 percent of the population. That’s a lot of people. That means that something between 3 to 5 billion people globally have to be infected to get that herd immunity. Or of course we have the alternative, and that is to get a vaccine.

Right, so tell me about what’s happening with vaccine development.

This is unprecedented, but there are over 100 different vaccines in development right now. The approaches that are being taken are really diverse…. Let me jump forward to the most advanced vaccine platforms that we’re talking about today, and those are called “shuttle vector viruses.” These are viruses that have nothing to do with the coronavirus that can be used to deliver a coronavirus protein to your immune system. One of the most promising of those is being developed by the Jenner Institute at Oxford University in the United Kingdom… and that particular vaccine is already in its initial clinical trials… This really accelerated development of vaccines is very expensive… The average length of time it takes to get a vaccine developed is more than 10 years normally, and right now at least Tony Fauci is saying we could have a vaccine in 12 to 18 months.  

Twelve to eighteen months is still a pretty long time. What should we all be doing in the meantime to protect ourselves from this disease?

There’s really three approaches, I would say, to controlling this disease. The vaccination approach we’ve talked about, and that’s the long term hope that we can use that. A second approach is with therapeutics, drugs….So there’s a lot of therapeutics under development but none really right now that are proven to work really well. What does that leave us with? It leaves us with public health measures and hygienic behavior. Social distancing is really important, it’s going to be really important for the next year or two I think. Washing your hands, covering your cough, being socially responsible… If you’re feeling poorly you need to be cognizant of that and try not to get other people sick along with you.

Is it a given that we can develop a vaccine? Should we be taking that for granted?

The failure rate of vaccines in clinical trials is very high. There is no guarantee.  Now, I think there’s some signs that COVID-19 might be an excellent virus to attack with a vaccine. It doesn’t mutate as fast as HIV so it’s not going to avoid the immune system like that. We know that at least in animal studies that these vaccines are protective…and if that holds up in human trials, and those [vaccines] are safe, this could be a critical approach to controlling this disease… Under the current emergency scenarios that we have, you can imagine that people will start getting vaccinated almost immediately after the Phase 3 trials come in. It’ll be a few years before we know how long the protection lasts. And we don’t know that we need protection long-term either. If we can drive this virus to extinction, perhaps in five years or ten years we won’t be getting vaccinated at all.

Do you think that’s still a possibility, that we could actually drive this coronavirus into extinction?

I’m always the optimist, I still think it’s possible that we can drive this virus into extinction. It’s not evolving that fast and so it’s not going to be able to evade our immune systems or our vaccines the same way that influenza does or HIV does. If we can approach this in the right way with herd immunity I think we can drive it to extinction. 

Paul Keim, great to talk to you as always.

You bet, Melissa.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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