What’s Driving A Rise In Mumps Cases In The United States?

KATHLEEN DAVIS: This is Science Friday. I’m Kathleen Davis.

SHAHLA FARZAN: And I’m Shahla Farzan. Next we’re going to take a step back in time to 1971, the year the US rolled out a revolutionary new vaccine for measles, mumps and rubella. The MMR vaccine almost eradicated all three of those viruses– for a while at least. You’ve probably seen headlines about this over the last few years. Measles cases popping up across the country, especially among unvaccinated kids. But what about mumps, that second M in MMR? Well, since 2006 there have been mumps outbreaks too. But unlike measles, most of the people getting the mumps are vaccinated and they’re older too, mostly teens and young adults.

So what’s going on here? To help answer that question and more is my guest Dr. Deven Gokhale, who co-authored a paper on the topic. He recently completed his PhD from the University of Georgia’s Odum school of Ecology based in Athens, Georgia. Dr. Gokhale, welcome to Science Friday.

DEVEN GOKHALE: Hi thanks for having me.

SHAHLA FARZAN: We’ve been seeing these big cyclical swings in mumps cases in the us since about 2006. Is there something in particular about that year that could help explain why?

DEVEN GOKHALE: So there are multiple reasons why we have seen these cases re-surge during that period of time. One story about this particular year is to do with there might be importations of college students who had gone back for a vacation and then they come back already infected and carrying the virus. And since the vaccine effectiveness has been altering over a period of time, you start seeing this resurgence as infected individuals come in contact with susceptible individuals within the population.

SHAHLA FARZAN: Right. So I’m curious about the effectiveness of the mumps vaccine. Does that protection from being vaccinated wane over time?

DEVEN GOKHALE: Yeah, it’s interesting because the real ineffectiveness is not really very well understood. There have been these competing hypotheses that we investigated in this research article that we recently published. And so one of the hypotheses is that the effectiveness just wanes over a period of time. So you get vaccinated and then after some period the vaccine is just ineffective against the virus. And the other hypothesis is that it’s always just imperfectly acting against the circulating strain. What we found in our article was that waning is the more probable reason of this delayed resurgence, as you can see, in the early 2000.

SHAHLA FARZAN: So not necessarily that there’s a mismatch between the vaccine strain and the circulating strain, but more so that protection from the vaccine really does decline over time then

DEVEN GOKHALE: That is definitely the case. And in a sense, that is a good thing because if it is just that the vaccine wanes over a period of time, the only thing you have to do is then boost the immunity at the population level by introducing boosters within population. Whereas, if the vaccine was leaky or was imperfectly working, then you may have to change the vaccine altogether. So that is a good news not a bad news.

SHAHLA FARZAN: So what’s the time scale that we’re talking about here when we’re talking about waning immunity? Like is it a couple of years? Is it longer?

DEVEN GOKHALE: Yeah so our analysis has shown that the immunity derived to the vaccine lasts on an average for around 34 to 36 years, with a mean of around 35 years. So it’s a very tightly bounded interval that we found and which has been reflected in many other studies, one which was predominantly featured in another journal, Science Translational Medicine, in m who reported an average time span of around 27 years. So it pretty much checks out.

SHAHLA FARZAN: So you had mentioned the study that you co-authored earlier this year. And in that study you estimated about one in three vaccinated people might lose their immunity to the mumps by about age 18. So we’re potentially talking about a pretty large swath of the population here who could be not immune to the mumps, is that right?

DEVEN GOKHALE: Yeah. That might actually be the case. And the worst part being because we do not have a schedule for boosters for adults, a major proportion of adults could have lost their immunity and might be susceptible to the disease now.

SHAHLA FARZAN: Why are vaccinated people still protected against the measles and rubella but not mumps? Is there something different about mumps then?

DEVEN GOKHALE: So the main reason why the mumps vaccine acts a little differently from its other counterparts in the MMR vaccine is because we don’t really understand what constitutes immunity for mumps, as opposed to measles or rubella for that matter. So what happens is when you get vaccinated you get an immune response. So you get like a surge of antibodies in your system.

But what translates to protection given this amount of an immune response is not very well understood for mumps. So that’s one principal reason why there isn’t a good explanation behind why the immunity wanes. Because you kind of see that the antibodies decay over a period of time. But how does that map to protection against the disease is not very well understood. So that’s one reason why this vaccine, or why this component within the MMR vaccine, works differently from the measles and rubella.

SHAHLA FARZAN: Yeah, you mentioned that the immune response to mumps is still a bit of a mystery which is honestly kind of surprising to me just given how long this virus has been around and how long the vaccine has been around.

DEVEN GOKHALE: Yeah, but that’s because the issue of protection is far more complicated than like an active immune response. Because you can have an active immune response as a– it could just be a function of having the virus in your system. So your body reacts to it or your immune system reacts to it. But what it is protecting against is a very complicated and multi process? Does it does it only protect against transmission or does it protect against establishment of an infection? Or does it protect against disease of that infection? And it is a very complicated problem for mumps, and as much as I would want to answer this, there isn’t a very good answer. We just don’t understand it yet.

SHAHLA FARZAN: Right. So when we’re talking about protection, the majority of kids in the us, about 95% or so, have gotten the MMR vaccine. But overall vaccination rates have gone down during the pandemic. And the CDC estimates somewhere around a quarter of a million kindergartners are vulnerable to getting these viruses now. And at the same time, we’ve also seen this uptick in anti-vaccine messaging in recent years. Do you think that either of these could have anything to do with the increase in mumps cases that we’re seeing now?

DEVEN GOKHALE: Yeah, there might be a conjoint effect of a bunch of these factors that might be acting simultaneously for sure. But at the same time, until 2018 at least, we don’t see a drop that is precipitous enough within the vaccine coverage at least to lose the immunity that we had acquired for at least four children. So since the pandemic began, everything has gone off the norm of things as we would like to call it. So these very recent mumps transmissions might be associated with changes in the vaccine coverages. But overall, until 2018 at least, we don’t really see a big drop in vaccine coverages.

What might be happening though is that you get a cohort of individuals who were previously vaccinated and who kind of lose their immunity by the age of 18, and then they get exposed and hence you get the spike and a shift of this age of infection to a higher age. That explanation seems to be more in tune with the dynamics in incidence that we see recently than the hypothesis about oh there might not be enough vaccine going on.

SHAHLA FARZAN: And most people getting the mumps are already vaccinated and older, right?

DEVEN GOKHALE: Exactly.

SHAHLA FARZAN: If someone wants to know whether they’re still immune to the mumps are there things that they can do or things they can check?

DEVEN GOKHALE: Yeah. As I said, unfortunately there might not be good tests to check for immunity against mumps. But what one could do is they could ask their doctors to boost them against mumps. Because it’s a very commonly available vaccine, I would recommend people getting booster rather than waiting to see whether or not they were immune to it. So during the 2006 epidemic, which was in a university setting, nearly all students in the university were boosted against mumps, even if they were infected. So it’s a very accessible vaccine and everybody should just get vaccinated.

SHAHLA FARZAN: And what age would you recommend that folks consider getting a booster?

DEVEN GOKHALE: I think any anybody who’s an adult. As I said, on an average the immunity lasts for around 33 years, but that is an average characteristic so there is a distribution associated with it. So there might be individuals who lose their immunity faster than the others.

SHAHLA FARZAN: So it’s still relatively rare to get mumps in the US, but it can cause some pretty serious symptoms for adults. Things like hearing loss and infertility. Should we be concerned about waning immunity on a population level?

DEVEN GOKHALE: Yeah, so that is the main problem. What happens is what we’re currently seeing are these transients as more and more people start losing their immunity, you start seeing these spikes of epidemics within the population. If the immunity isn’t restored, however, you’d start seeing bigger waves of mumps and most of those will happen in adults given these patterns. Given that there is a severe disease associated with adulthood mumps disease, I would recommend people who have reached 18 years of age to get vaccinated.

SHAHLA FARZAN: That’s all the time we have. I’d like to thank my guest Dr. Deven Gokhale, who recently completed his PhD from the University of Georgia’s Odum School of Ecology based in Athens, Georgia. Thanks so much.

DEVEN GOKHALE: Thank you.

SHAHLA FARZAN: This is Science Friday from WNYC studios.

Copyright © 2023 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/