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You’re on a packed light rail train, trying not to fall on the person in front of you as you round another corner. Then you hear it — the unmistakable hack of a cough or muffled sneeze somewhere close by. Heads swivel, people scoot away, and everyone hopes that it’s not something super contagious.
Yes, dealing with germs is a part of everyday life in our society. But what if, instead of just worrying about yourself on that train, you could help fight illness as a community? Enter the concept of herd immunity.
What is herd immunity?
Herd immunity occurs when enough people in an area have immunity to an infection so that if a person with that infection enters the area, it’s unable to spread easily.
“Herd immunity is also called community immunity, probably because some people might take offense at being thought of as being part of a herd,” says Anna Wald, MD, a UW School of Medicine professor of Medicine, Epidemiology and Laboratory Medicine & Pathology.
Herd immunity can be achieved when enough people have been infected and developed immunity — or are vaccinated against it — that the infection can no longer spread. Vaccines protect the person who got the vaccine, but if enough people in an area get vaccinated, it makes it very difficult for the infection to spread, which helps protect the rest of the community.
Herd immunity generally applies to infections for which you develop immunity after you recover, and that are spread from person to person. It has been achieved in the U.S. in the past for measles, mumps, polio, diphtheria, rubella and chickenpox.
What percentage of the population needs to be immune to achieve herd immunity? The exact number varies depending on the pathogen, but 95% seems to be a sweet spot often mentioned by health professionals, though it can often be much lower for some infections.
Even after these infections have been eliminated (the spread has been stopped in a specific location), they can still come back, as the U.S. has seen recently with measles outbreaks happening across the country. It’s only if they’ve been eradicated (which means the disease has been destroyed worldwide) that they’re gone for good. Surprising fact: The only human disease that’s been eradicated worldwide is smallpox.
“Now we don’t have smallpox, even though we’re not vaccinating for it, because we’ve eradicated it,” says Wald. “The last known case of smallpox was in 1977.”
Unfortunately, viruses like COVID-19, RSV and influenza can mutate and re-infect you, meaning you can’t really achieve herd immunity against them.
“For COVID or influenza, we’ll never have herd immunity, because you can get infected more than once, although the infection is much milder if you’re vaccinated and you are less likely to transmit it,” says Wald.
Which means it’s still important to get your COVID-19 and influenza vaccines every year because you’re lowering your chances of catching and spreading the illness, and you’re likely to get less sick if you’re vaccinated.
The effects of herd immunity
So, why is herd immunity so important? According to Wald, herd immunity has two important consequences:
Stopping transmission
Even if you’re infectious, but you’re surrounded by people who are all vaccinated, you can’t transmit to them. That means that they can’t get infected and transmit to other people who might be susceptible.
And why is that important?
Protecting vulnerable community members
There have always been people in the population who cannot be protected with a vaccine. One example? Babies. Infants can’t get certain vaccines until they’re at least 1 year old, so they need to be protected differently.
The list of vulnerable people also includes those who are undergoing cancer treatment, those who are immunocompromised because of advanced age, transplant recipients, or folks with specific chronic conditions. Those individuals may not respond to a vaccine, and it may not be safe to give them a live vaccine. This means that having herd immunity in a population can help protect people who cannot directly benefit from vaccination.
“There’s sort of a ring around them, or an umbrella, of people who are vaccinated and who won’t let potentially infected people close to them,” says Wald.
How you know you achieved herd immunity
This may seem obvious, but the clearest way to determine that you’ve achieved herd immunity is when the disease stops spreading. And, as mentioned earlier, this doesn’t always mean that everyone has been vaccinated; it can sometimes mean that enough people have been infected to limit the spread.
For example, rubella is a contagious illness caused by a virus. Though usually harmless to healthy people, it’s very dangerous for pregnant people and can cause miscarriage or life-threatening conditions for the baby.
“Before we had the measles, mumps and rubella vaccine, there were regular rubella parties to spread the disease among young people, because rubella in children is a very benign illness that doesn’t cause a lot of complications,” says Wald. “But you really want to be protected later in life when you get pregnant, because having rubella during pregnancy results in spontaneous abortion or miscarriage.”
Fortunately, the MMR vaccine, which children in the U.S. started getting in the 1970s, protects against rubella, and the disease has been eliminated in the U.S. for over 20 years (though it sometimes spreads in other countries). As mentioned earlier, though it’s eliminated, it won’t be gone forever until it’s eradicated, meaning that if vaccine rates drop, it could start spreading again.
Some of the challenges
Of course, there are some notable challenges in the effort to reach herd immunity, and then maintain it, since herd immunity isn’t always permanent, as measles has proved:
Dealing with vaccine confusion
People (often on social media) get caught up in the fact that many people who get vaccinated still get the disease.
“If you have three unvaccinated people, maybe 100% of them get sick, but there are only three of them,” says Wald. “On the other hand, you may have 3,000 people who are vaccinated, and maybe only 10 of them get sick, but since 10 is more than three, people say, ‘Well, the majority of people who got sick were vaccinated, so the vaccine doesn’t work.’”
Creating effective vaccines
Even in a vaccinated population, some people will still get the illness. And some vaccines are more protective than others. For example, the measles vaccine is very effective (you almost certainly won’t get the infection), whereas you may still get the flu after the influenza vaccine, but you’re much less likely to be hospitalized or die.
Assuming vaccinated people are evenly spread
Some vaccine research assumes that vaccinated and unvaccinated people are spread uniformly throughout the population, which is usually not true.
“The unvaccinated people are often all hanging out together, so within their community, the risk is much higher,” says Wald.
For example, when you see measles outbreaks happening in certain communities or states, it usually means that people in that area aren’t getting vaccinated as much for measles, and those lower vaccination rates are dropping local coverage below the threshold needed to maintain the herd immunity for that area.
In conclusion
Even if you aren’t too worried about getting sick, you should care about herd immunity and protecting the most vulnerable people in your community.
Plus, there’s always the hope of disease eradication, after which there wouldn’t even be a need to vaccinate because there’s no possibility of transmission. That is inspiring and gives some hope that, in the future, communities can continue to work together to keep the most vulnerable members safe and everyone healthier.
