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In the early days of the pandemic there was the idea that we should try to get “herd immunity” as soon as possible. It was a terrible, terrible, not a good, very bad idea because of it meant giving up and letting millions of people die. And yet, now that we have a vaccine, herd immunity is suddenly a good thing. What gives?

In short, herd immunity from vaccination protects people from disease. Herd immunity to natural infections is what you get after not protecting everyone from disease.

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“Herd immunity” describes how the community protects people

Let’s talk about what herd immunity actually means. It is also known as “community immunity”.

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When most of the people in a community are immune to a contagious disease, the few who are not immune are less at risk of developing the disease. For example, if almost everyone in your town is immune to measles – either from the vaccine or from surviving as a child – babies who are not old enough to get the MMR vaccine are protected simply because not many People nearby are able to transmit measles to them.

Herd immunity is not a perfect protection, but it does mean that there aren’t many people who are prone to the disease and that these people are unlikely to be in contact with one another. It would be very difficult for an outbreak to begin in such a community.

Illustration for article titled Why Is Herd Immunity Suddenly A Good Thing?

Herd immunity is a goal of vaccination

Vaccinating a person protects that person. Vaccinating most people protects vulnerable people across the community.

The definition of “most” depends on how contagious the disease is. For measles you would need it at least 92% of people be immune. For COVID-19, it is often estimated that 70% of people must be immune to achieve herd immunity.

Herd immunity has historically kept disease at bay. Smallpox vaccination campaigns resulted in smallpox being eradicated from the earth – the only human disease that was ever completely eradicated. Measles is still there Outbreaks in the US are disappearing quickly Thanks to our good vaccination coverage and aggressive contact tracing (although the pandemic has affected this work).

Illustration for article titled Why Is Herd Immunity Suddenly A Good Thing?

Infection-related herd immunity is not the same

In theory, it is possible to achieve herd immunity through natural infection. But that’s not a smart plan for dealing with deadly diseases for two reasons:

First, if there is no vaccine, we will only achieve herd immunity after many, many people become sick. If you had a community where 70% of the people are COVID-19 survivors, the community would have buried many of its residents, and many of the remaining people would have long-term complications. (Notwithstanding the collateral damage of an overstretched health system and the economic consequences of an uncontrolled pandemic that would have caused even more death and misery.) Even with the remaining 30% protected, it came at dire costs.

Second, if the only way to gain immunity is to infect yourself, then there is no way to give newcomers – like babies – that immunity without exposing them to all the dangers of disease. Herd immunity without vaccination is temporary.

The reason vaccines are so valuable is because they give us immunity without having to face the threat of disease. Herd immunity to natural infections is the result of tragedy, while vaccines allow us to achieve herd immunity safely.