A new study sheds light on the elusive idea of herd immunity NPR's Asma Khalid speaks with Mahan Gafari, an epidemiologist at the University of Oxford, about the state of the pandemic in Iran and what we can learn about herd immunity.

A new study sheds light on the elusive idea of herd immunity

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ASMA KHALID, HOST:

We've been trying to defeat this pandemic for over a year and a half and keep hearing talk about this elusive idea of herd immunity. Countries around the globe have tried lifting pandemic restrictions in the hopes of somehow getting that herd immunity and hoping that it will stop the spread of the virus, only to see more coronavirus infections. A new study focusing on Iran sheds light on what happens when we try to control the spread of COVID-19 through the notion of herd immunity over other measures, say, like shutdowns or vaccines. One of the researchers working on that study is Mahan Ghafari at the University of Oxford in the United Kingdom. He focuses on virus evolution and epidemiology, and he joins us now. Welcome to the show, Mahan.

MAHAN GHAFARI: Hi, thank you for having me.

KHALID: So how did you go about collecting data for this study? Because my understanding is Iranian authorities - like, frankly, those in many countries - have not provided really detailed, transparent information about cases, fatalities, vaccination rates.

GHAFARI: Iran was one of the, you know, first countries outside mainland China to be hit with a, you know, heavy toll of COVID. And they stopped, essentially, releasing province-level data on the number of cases and deaths, like you said, right from the beginning. So we had to try and be creative and come up with independent sources of information. That included genome sequence data. This is, like, sample of the virus collected from individuals that spent their time, got infected in Iran, went back to their homes - whatever country. We tried to understand, OK, when did the epidemic started in Iran? - which we now know is essentially late December 2019. And then since then, we also tried to look at all-cause mortality data - so this is, like, any deaths that have happened in Iran during a certain period of time - and tried to then compare that all-cause mortality to the years prior to the pandemic to sort of gauge some sort of a baseline level of death and sort of try to attribute anything above that baseline during the pandemic to be the main source of, like, burden of COVID directly and deaths.

KHALID: I see. And so you were essentially sort of, like, retroactively trying to reconstruct this because you have really limited data resources.

GHAFARI: Exactly. Now we know really what happened in each of these provinces. We found out that in 11 provinces, we do see that more than 100% of people have been exposed to the disease. That means, on average, one individual might have been exposed to the disease more than once, in fact, twice. Or even in a particular province - Sistan and Baluchestan - we found 254% percent of people exposed. That means even more than twice being exposed to the disease over a period of 19 months.

KHALID: I mean, that, Mahan, leads me to this idea of herd immunity because there has been so much talk about this. I mean, certainly here in the United States, I've heard this from politicians - this idea that once, essentially, a certain percentage of the population was exposed, that then the United States, let's say, would be able to potentially defeat the virus.

GHAFARI: The problem with that assertion is the immunity does wane over time and the fact that the variants of concerns, like delta, wouldn't impact the - increasing the chance of reinfection. Now, sadly, there's mounting information on both of these problems. We know in this study in India delta variant of COVID-19 has increased the chance of reinfection by 30%. And also, there is waning immunity. Now, we know, again, the duration of immunity lasts as - half as long as the common cold.

KHALID: So what does this data then tell us about the very idea of herd immunity?

GHAFARI: It just simply doesn't stop - you know, the disease doesn't stop where, on average, everyone at least got exposed to the disease once. The bright side of the story, however, was that Iran obviously had a very slow start on its vaccination. But because this is an age-stratified data - so we know, per age group, how many people died in each provinces - we see that, for people above age of, say, 70 to 80, these were vaccinated early on, and we do see a clear drop in the level of excess mortality.

KHALID: How does that all factor into this idea of defeating COVID-19?

GHAFARI: Individuals, you know, in Iran have been exposed more than once to COVID. Nevertheless, they're dying with the same probability as if - they did the first time they got infected. One of the key messages of this paper was to - you know, getting vaccinated is a lot better both in terms of protecting yourself and also the community for having a much lower chance of infection and death.

KHALID: So when you look at what happened in Iran and how the situation unfolded, are there lessons that you feel the rest of the world ought to take - that the United States - that people here ought to take about how the virus is spreading?

GHAFARI: The key message here is that the idea of let it rip through the population - let COVID rip through the population - is just not viable enough. You know, our experience of COVID in general is very - has been very humbling as a researcher because there's so much uncertainty. There's a lot of things we just don't know. So this idea of, again, assuming, oh, COVID is going to turn into, like, seasonal flu - there's so much uncertainty in that. We don't know what potentially could be the consequences of a next variant of concern. If we're not vaccinated, if we're not protecting ourselves from the virus, the chance of getting reinfected and having severe disease is still quite a possibility, so it's best to heed caution.

KHALID: That's Mahan Ghafari, who researches virus evolution and epidemiology at the Zoology Department at the University of Oxford. Thank you so much for your time.

GHAFARI: Thanks for having me.

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