Why do you call it herd immunity?

Syringes with the Pfizer-BioNTech vaccine are waiting for use at the University Health Service Vaccine Clinic. The idea of ​​herd immunity brought about by my mass vaccination has evolved over the last century. Photo: Jeff Miller

I’ve heard about herd immunity in the news as the world begins vaccination of more and more people against COVID-19. The idea is that if enough people are vaccinated, the virus will not be transmitted, even if it is not the entire population. You can find enough infectious people to maintain its spread.

But where did the term “herd immunity” come from? And how has that meaning changed over the years?

Surprisingly, it has nothing to do with any kind of agricultural herd. According to Richard Keller, a professor of medical history at the University of Wisconsin, the term first appeared in the British Public Health Journal in 1923 regarding bacterial infections in mice. In this study, we investigated how vaccination and some mice outside the group, or “herds,” would begin to prevent bacterial infections between them.

“The authors are talking about a colony of mice they keep in the lab, and they call the group a herd,” says Keller. “That is, they are not talking about what we normally consider to be a herd, such as cows and deer.”

Despite this first appearance, Keller says the term was not widely used until the World Health Organization’s campaign to eradicate smallpox.

Edward Jenner invented the smallpox vaccine in 1798, and it is estimated that billions of people were vaccinated against the disease until the smallpox was eradicated in the second half of the 20th century. Smallpox has a much higher mortality rate than COVID-19 and can kill 1 in 7 infected people or 1 in 3 in a population that has never occurred before.

By the time WHO embarked on an eradication journey in the 1960s, vaccines had already stopped viral infections in many parts of the world. However, some small pockets, usually in isolated villages in places like Africa and Southeast Asia, were still contagious. WHO called their eradication strategy “ring immunity.” That is, they attempted to vaccinate in and around the infected pocket, creating an immunity “ring” around the virus that could not escape. Keller says this idea has evolved into the modern concept of “herd immunity.”

“If you draw a circle around a case of illness and you vaccinate everyone [in that ring], As much as possible, will capture 80-90% of the population, “Keller said. “Achieving 80-90% can dramatically minimize the chances of the disease spreading beyond that circle, which was the official WHO policy of the early 1970s.”

But even today, the term herd immunity is still used in multiple ways, Keller says. While there is the idea of ​​herd immunity from vaccination campaigns, there is also this concept of “wild” herd immunity.

At the beginning of the COVID-19 pandemic, Keller says many people have promoted wild herd immunity as a solution. If a sufficient number of people survive the virus, the infection will stop because some of the population has natural antibodies to the virus from the first fight against the virus.

But that idea is flawed, says Keller. This is because millions of people die when they infect enough people to reach the group immunity threshold (usually 80-90% of the population). Needless to say, new people are constantly being born without innate immunity.

“Especially in the early days of the pandemic, there was a lot of talk about” well, letting it run wild and reaching herd immunity. ” And that’s a really flawed understanding of what herd immunity means, “says Keller. “Thinking that way, without intervention, any disease will eventually reach the level of herd immunity, but new people are constantly being born that are susceptible to this disease.”

From its first appearance in 1923 to the WHO smallpox eradication campaign of the 1970s and today’s various understandings, “herd immunity” has evolved like a virus. However, Keller says that its most common use as a measure of a population’s ability to block viral infections is most relevant to conversations about vaccination.

“As people become more and more satisfied with vaccination, herd immunity begins to collapse,” says Keller.

It is everyone’s responsibility to be vaccinated and contribute to this immunity, as there is inevitably a proportion of the population who cannot be vaccinated for health, access or legal reasons, Keller says. .. Moreover, there is no perfect vaccine. Even if 100% of the population is vaccinated, it is herd immunity that protects them, statistically because there may be some people who are not vaccinated.

“When that general immunity reaches up to about 90%, it means that 9 out of 10 people we encounter become immune,” says Keller. “And it really dramatically breaks the chain of transmission. It’s like burning less fuel in the woods.”