What is herd immunity?

Joshua Krisch
Live Science
Originally published July 2020

Here is an excerpt:

Herd immunity doesn't always work

The ingredients for achieving herd immunity naturally are well understood. "You want a disease that is guaranteed to produce robust immunity with largely asymptomatic spread, and have a low R0," Altmann told Live Science. But even if the R0 is relatively high and most patients are symptomatic, herd immunity is still possible with an effective vaccine, and a vaccine program that immunizes the population en masse. "Think of our big, public-health vaccination success-stories: Smallpox and polio, both entirely due to massive, sustained vaccine programs with simple, highly effective vaccines," he said.

Robust immunity is necessary to ensure that those who become immune stay that way long enough for the pathogen to die out. Asymptomatic spread helps, because it means that fewer people are likely to die while the population waits for herd immunity to take hold — and increases the likelihood that there will be enough survivors to affect herd immunity in the first place. A low R0, of course, lowers the bar for how many individuals need to be immune before we see the infection rate flatten and decline.

Nonetheless, some diseases that are seemingly strong candidates for herd immunity never quite achieve it. Despite widespread infection and vaccination, chicken pox, for instance, has never been entirely eradicated from the population. That's because the virus that causes chicken pox remains latent in the nerve roots of those who are infected by it, even after they recover and acquire immunity to the disease. As once-infected individuals grow older their immune systems weaken and the virus can reactivate, causing shingles, which can, in turn, cause chicken pox.

"You might have eradicated chicken pox in a small island community, but then somebody's granny gets an attack of shingles and, over a matter of weeks, every kid on the island gets chicken pox," Hunter said. "You've achieved herd immunity, and [it appears] the virus has died out, but it's actually waiting to come out." Similar phenomena have been observed with tuberculosis, according to the WHO.

Vaccine-induced herd immunity can also fail when a vaccine results in only short-lived immunity within a population. Pertussis and mumps recently reappeared long after vaccine programs were assumed to have eradicated these diseases, and studies suggest that, while vaccine noncompliance played a role, the outbreaks were in part due to the vaccines losing effectiveness over time. "In the past few years we've had both pertussis and mumps outbreaks, and those have primarily resulted from waning immunity over time," Poland said.

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