What is herd immunity?
When a person gets vaccinated or gets natural infection that individual develops immunity – in other words, that person is now protected from getting infected with the same virus again, and there’s a threshold at which those who do not have immunity are also protected because enough people within the population have immunity. We call that “herd immunity”.
What is the required threshold for COVID-19?
Unfortunately, there isn’t a single answer to that question. The level of herd immunity might be higher in a setting where it is very dense and the risks are high, it may be lower in a farm setting where people are not in close contact all the time. A key factor in determining whether herd immunity is achievable is to establish whether those individuals who have developed immunity, either through natural infection or through vaccines, what’s the rate at which their family members, their household members, their work, colleagues are getting infected. So, in other words, are they transmitting it to people they work closely with.
So, the secondary attack rate gives us empiric information about whether individuals who become infected, despite being vaccinated, do they transmit or not? It is also not easy to establish what that herd immunity level is, until you get to the end of the epidemic because it’s the end of the epidemic that helps you determine what that number is.
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Is there a way to estimate the threshold?
You can use a very simple formula which is based on the reproductive rate of infection for every person who has the virus, how many new people are going to get it from that first. If we are dealing with a reproductive rate of, let’s say, three, then we can calculate the herd immunity by simple formula. That subtracts one from the reproductive rate and divides it by the reproductive rate. In other words, (3 – 1) / 3 that gives you 2 / 3. So that gives you 67%. There are many assumptions made in that simple calculation, which I won’t go into, but it’s just illustrative. So, if we say that, we need, 67% immunity to achieve herd immunity.
Then that means two out of every three people in a community must not transmit the virus if they get infected. In other words, they have to have the protection against this virus. So that if they become exposed, they don’t pass it on when we’ve achieved that you’ve achieved herd immunity.
How does that compare with other viruses?
A highly infectious virus like measles has a very high level for herd immunity. You have to achieve somewhere in the order of around 95% of the population that has been vaccinated or has had natural measles in order to protect everybody. For other diseases, the level you have to achieve is lower because they are not as infectious. So, the more infectious a virus, the higher the level of herd immunity.