This article corrects two misconceptions: that herd immunity can be represented by a single number, and that achieving herd immunity assures safety.




Jul 30, 2020: Vol 20 No 90

It is popularly believed that herd immunity is a fixed number for a disease. It is also believed that once herd immunity has been achieved, the population is safe from that disease. Both beliefs are misconceptions.

Herd immunity against a disease is achieved when so many persons become immune to the disease that too few people remain to contract and spread the disease.

Universal vaccination is the ideal way of attaining herd immunity. However, because a vaccine against Covid-19 will not become widely available for at least the next 6-9 months, the disease will continue to spread.

So what percentage of the population needs to contract Covid-19 and become immune to it to achieve herd immunity. Different authorities state different numbers; most of the numbers lie in the 50% to 80% range. The idea that the percentage is a single number is wrong.

The number associated with herd immunity is based on R0, which is the average number of persons an infected person will infect. The value for herd immunity is calculated using the simple formula (R0-1)/R0; the result is expressed as a percentage.

Using this formula, a few seconds of mental arithmetic tells us that if R0 is 2, then (2-1)/2 or 50% of the population must become immune for an epidemic to subside because herd immunity has been attained. Or, if R0 is 10, 90% of the population must become immune for herd immunity to be attained.

R0 can vary across place. As a simple example, R0 will be higher in densely populated areas than in sparsely populated areas; so the value for herd immunity will be higher in the former than in the latter locations.

R0 can vary across time. For example, if people cannot move around because lockdown has been imposed, the infection cannot spread. So the value of R0 falls, and the number associated with herd immunity becomes smaller during periods of lockdown than during periods of unrestricted movement.

R0 can vary because of behavior. For example, if people wear masks, restrict movement, and practice social distancing, then they will be less likely to contract or spread the disease, and the value of R0 and the number associated with herd immunity will fall.

The fallacy of believing that herd immunity defines safety is that if an infected person moves from a densely populated to a sparsely populated zone, the disease can resume its spread in the sparsely populated zone. If lockdown is lifted, a second wave of disease spread can begin. If people become lax in their precautionary behaviors because they think that the pandemic has ended, those who are not immune become vulnerable to contracting and spreading the disease.

True safety comes when everybody is immune, such as because of universal immunization. Until Covid-19 vaccines become widely available, because lockdown is economically disastrous and no longer feasible, people must realize that precautionary behaviors such as restricting movement, physical distancing, and wearing masks must remain a way of life. We should not count upon herd immunity based on disease spread.


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