Like clockwork at the beginning of flu season, the banners go up at local drugstores: “Protect yourself from the flu — get your shots today.” But that proclamation leaves out a key message: by receiving a vaccine, there’s a good chance you’re helping more people than just yourself.
The Wisdom of the Herd
The reason for that is a phenomenon known as herd immunity, or community immunity. In any given population, not all people are able to receive or benefit from vaccines because their immune systems are not mature or strong enough. “With herd immunity, the vaccinated individuals in a population don’t become infected or get sick, so they avoid the risk of passing the disease to those that are more vulnerable, like infants, the elderly, or individuals who are unable to receive vaccines for other medical reasons,” says Kathrin Jansen, Pfizer’s Senior Vice President and Head of Vaccines R&D. The proportion of vaccinated individuals in a population that is needed for this effect to take hold — the herd immunity threshold — depends on the disease but in general has to be very high. For example, not all diseases spread at the same rate or by the same mechanism because of the basic biology of the pathogens involved.
At the same time, a population’s “mixing” pattern, or the way that people interact among themselves, can make a disease pathogen more or less transmissible. For example, a pathogen that’s spread by human contact can spread more easily across a population if the people in it are very mobile, or if they live in close proximity to each other.
Another key factor that determines the herd immunity threshold is the way vaccines are designed in a pathogen-specific manner, and the type and level of immunity they elicit. Says Jansen, “The way a vaccine is designed may elicit different types of immune responses, so the number of vaccinated individuals required to provide effective herd immunity may vary. For example, only with the design and development of polysaccharide conjugate vaccines was effective herd immunity established for meningococcal and pneumococcal diseases.” So if the threshold level of vaccination for a disease is not reached in a population or subpopulation, diseases can pop up again where they’d previously been suppressed, as is often seen with measles outbreaks among unvaccinated individuals.
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But don’t our immune systems naturally build immunity anyway? And wouldn’t a vaccine interfere with that somehow?
Turns out that natural process isn’t quite so simple. Because of the biology of different diseases, the way that pathogens spread, and the way that people mix, there are a lot of factors that go into a “natural” response to a pathogen — and it varies widely depending on the disease. For example, while people can build natural immunity to influenza, the dominant strains of the flu circulating from year to year are highly variable, and that’s why people need to get the shot every year.
But perhaps the best way to demonstrate the positive effects of herd immunity is with results. Recent successful vaccine programs have shown both direct (among the vaccinated) and indirect (via herd immunity) success in disease prevention by pneumococcal and meningococcal conjugates, HPV, and rotavirus vaccines. Examples of successful vaccines with direct protection and proven herd effect include Haemophilus influenza type B, measles, mumps, rubella, pertussis and influenza vaccines. And finally the classic success story is that of smallpox vaccines, which illustrates just what can happen with an effective vaccine whose widespread use also induces herd effects: recorded during the early settlement of America and even as far back as 10,000 B.C., smallpox brought down entire empires — and now it’s essentially gone from the planet.