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What is herd immunity and can it stop COVID-19?

27 May 2020
by Paige Dorkin

You may have heard that the pandemic will end if enough people develop antibodies through exposure to the virus. But – convincing as that might sound – the experts agree: in the case of the coronavirus, this is not a good outcome. In fact, it’s something we should actively avoid.
 
In recent months, the term ‘herd immunity’ has been bandied about on social media and in the news. Simply put, it describes what happens when enough people in a population have been exposed to a certain virus and develop immunity – the ability to fight off infection – thus stopping its spread.
 
According to professors Gypsyamber D’Souza and David Dowdy at John Hopkins University, between 70 and 90 per cent of a people need to be immune for a population to achieve herd immunity status, depending on how contagious a virus is.

How does immunity work?

On an individual level, when our bodies encounter a pathogen, or germ (such as COVID-19), our immune systems start to produce specialised proteins called antibodies. Every virus has a unique protein, called an antigen, on its surface. The antibodies we make attach themselves to these antigens in order to destroy the virus.
 
It’s often explained as a key (the antibody) fitting into a lock (the antigen) because the antibodies that grant us immunity are specific to the virus. When our bodies know how to make a certain type of antibody, they also make memory cells. If we come into contact with the virus again, the memory cells immediately trigger the production of the right antibodies to fight off infection before it takes hold.
 
In the case of herd immunity, so many people have antibodies that the virus is unable to spread. There are two ways that this critical mass can be reached – either as a naturally occurring phenomenon or via vaccination (because vaccines train your body to produce antibodies without you having to become infected).

Why natural herd immunity is not the answer

Gideon Mayerovitz-Katz, an epidemiologist from Australia, argues that herd immunity without a vaccine is not a valid preventative measure. That is to say, while it would eventually halt the spread of the disease, the cost – in terms of serious illness and death – would be inordinately high.
 
In the case of COVID-19, about 70 per cent of people would need to catch the virus before we hit herd immunity, explains Mayerovitz-Katz. So far, the best estimates we have put the fatality rate for COVID-19 at between 0.5 and 1 per cent of infections. In South Africa, which has a population of close to 58 million, this translates to between 203 000 and 406 000 deaths before we reach herd immunity.
 
But, even that is likely an underestimation because when the healthcare system reaches capacity, people who would likely recover if they had access to medical care die for lack of it, pushing the fatality rate up even higher. Since about 10 per cent of COVID-19 cases require hospitalisation, we’d be looking at over four million South Africans needing a hospital bed before the virus runs its course. Seen in such stark terms, it’s very clear why herd immunity as a natural phenomenon is not something for which we should aim.
 
It’s only once an effective vaccine is developed and given to the majority of the population that we can think about herd immunity as the light at the end of the coronavirus tunnel. And while scientists around the world are racing to come up with this vaccine, it might take up to 18 months after they do for it to become widely available.
 
Until then, our best defence against the spread of the disease is a combination of social distancing, hand-washing and respiratory hygiene (including masks).
 
If you suspect you might have the COVID-19 virus, call your healthcare provider. Calling ahead prevents you from spreading – or being infected by – the coronavirus and allows your healthcare provider to direct you to the right facility.

Call the Department of Health’s 24-hour coronavirus hotline: 0800 029 999

IMAGE CREDIT: 123rf.com