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Vismita: Hello and welcome to Science in 5, WHO's conversations in science. I'm Vismita Gupta-Smith. And this is a series where WHO experts will explain the science related to COVID-19, so that you can protect yourself and your loved ones. In today's show, we are talking to WHO's Chief Scientist, Dr Soumya Swaminathan and she is going to explain herd immunity. Welcome, Soumya.

Soumya: Thank you, Vismita.

Vismita: Soumya, we are hearing a lot about herd immunity. So let's start with, please tell us what is herd immunity?

Soumya: So, let's take an example, of a disease like measles, which is a very common childhood infectious illness caused by measles virus, for which there is a very effective vaccine. Now, we say that to achieve herd immunity in the population, for measles, you need about 95% of the people to have immunity or antibodies. Even if you have 5% of children not vaccinated, these others actually have enough protection in the population to prevent the measles virus from actually going from one person to the next. So it's really like having a barrier of people who are protected, who break that chain of transmission. So, you don't need every single person in the area, in the population, to necessarily be protected.

Vismita: Is it the right way to think about herd immunity in the context of COVID to say the vaccine is far away, why don't we just let everyone get infected? 

Soumya: So, the SARS-CoV-2 virus is a highly transmissible virus. We think it needs at least 60 to 70% of the population to have immunity to really break the chain of transmission. If you allow this to happen naturally, it will take a long time, of course, but more importantly, it's going to do a lot of collateral damage. So even if 1% of people who get infected are ultimately going to die, then this can add up to a huge number of people, if we look at the global population. And that is why we believe it's not a good idea to try to achieve herd immunity by just letting the infection run wild in the population and infect a lot of people and that we should talk about herd immunity in the context of a vaccine.

Vismita: So, let me come to the vaccine now. So, our strategy is to vaccinate enough people rather than just letting people get infected. Is that where the science is now?

Soumya: That's right. Because with a vaccine you can achieve immunity and herd immunity safely. Through natural infection, we could also achieve it at some point, but it would be at great human cost. And so naturally, the better choice is doing it through a vaccine. 

Vismita: Those are preventable deaths, right? We can actually, even though we don't have a vaccine, we don't have therapeutics right now, we can actually slow down and save those lives rather than just let it spread, right? Speak to us about that.

Soumya: Exactly! So there are actions now that we can take, which can help to slow down transmission, to control it, to try to contain it. And, also we know how to manage people better. The other measures that are effective are, of course, the public health and social measures that we talk about, the physical distancing, making sure you're wearing a mask when you're in crowded settings, washing your hands frequently. Those are the modes by which you can actually prevent the virus from transmitting from one person to the next. And then on the side, be able to detect rapidly those who are infected in the community, making sure that enough testing is available so that you're able to detect and diagnose people, be able to isolate them, then test their contacts and quarantine them. These are the measures that have been shown to be successful. They are hard work, they are difficult to implement but it's worth doing, because then you're saving lives until the time that we have more effective medicines to treat this disease and, of course, a safe and effective vaccine.

Vismita: Thank you, Soumya. There you have it, WHO's Chief Scientist explaining herd immunity. Join us next time in our series Science in 5. Until then stay safe and stick with science. Bye bye.


Speaker key

Vismita   Vismita Gupta-Smith

Soumya   Dr Soumya Swaminathan