Is herd immunity the solution?

If we have not been able to contain the COVID-19 pandemic we can at least wait for that point where we have all passed the disease and become immune. This is the strategy of the Government of Boris Johnson (United Kingdom) ―different from that of other countries―, to combat COVID-19.

Although this is a tempting idea, it doesn’t work in practice. Herd immunity will not save us from disease. Only the vaccine will.

When a life-threatening communicable disease spreads through a population it leaves only two things in its wake: dead or survivors. In many diseases it happens that these survivors become immune to the pathogen that made them sick. However, this is not certain in the case of COVID-19, since there are no accurate data regarding how this immunity works and for how long it works.

Herd immunity is precisely that. If everyone is immune to the disease, a person who is not can be protected because they are surrounded by people who are, and the disease is not transmitted.

Herd immunity is precisely the goal of vaccination campaigns. Let’s say 95% of the population is immune to measles. If a person with measles falls into that group, the pathogen has little chance of being transmitted to a person who is not immune. 5% of people who are not vaccinated are protected because the disease cannot infect enough people to reach them.

The percentage of immune population necessary to achieve herd immunity depends on the disease. In the case of COVID-19, which is not nearly as contagious as measles ―each infected person can infect 12-18 people in a population where no one is immune―, it is estimated that in order to stop its expansion, we only need 60% of the population to be immune.

However, the idea of achieving herd immunity to return to normal does not usually take into account one detail, and that detail is that in order to achieve it we all have to get sick.

It is true that it is possible to get sick from COVID-19 and that its symptoms are not serious, but there are people who contract the coronavirus and become very ill and/or die. Even if they survive, many patients must spend weeks in hospital with constant medical care. On the other hand, some overcome the disease but with important long-term consequences such as damage to certain organs. Also, we don’t even know what the long-term sequelae of COVID-19 are yet because the first patients in history to survive it have only done so a few months ago. And that without counting that to achieve herd immunity many people will die (the percentage of fatalities is still the subject of scientific debate because it depends on each person and how you count the victims, but it is believed that it is around 3%).

Herd immunity works against measles because there is a vaccine. To become immune to measles, you just need to get the vaccine, you don’t need to get the disease. With COVID-19 the only way to gain immunity is to get sick.

In summary, establishing herd immunity against COVID-19 without a vaccine will not only be ineffective, but will also cost very much.

Editorial Disclaimer: information published during the 2020 COVID-19 pandemic may be updated frequently to reflect the dynamic nature of current understanding.

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