Snowball Fights & Herd Immunity
What do snowballs have to do with immunity? The shield you may not know you have, and why it matters
Hi friends!
I have been seeing lots of comments circulating similar to the post below, asking why anyone cares if others are vaccinated if their vaccine works. This question is missing important context – particularly regarding what herd immunity is and why it matters. So, let’s dig into that topic today!
Snowballs and Shields
It snowed here in Maine the other day, so indulge my winter analogy. I know spring will come eventually!
Let’s imagine you’re in the middle of a neighborhood snowball fight. The two sides of the street are flinging snowballs at each other, hoping one side will give up before the other. At first, it’s complete chaos. Snowballs are flying left and right, whacking people in the back, face and eyes. A few toddlers get hit in the face and run screaming to their parents.
Then, some older kids get a brilliant idea. They begin grabbing sleds and nearby trash can lids to use as makeshift shields. Now, if the kids are fast enough, the snowballs bounce off the shields, leaving them untouched behind them. Not all of them are fast enough, so plenty of kids still get covered in snow.
Pretty soon, other kids catch on, and more and more makeshift shields enter the game. Soon, there is a wall of shields protecting those who are standing behind them. Now, the snowballs stop hitting their targets. Parents usher the littlest kids behind these shields so they can be included without getting an eyeful of snow. Eventually, there are so many shields that the game peters out, and the kids go inside to warm up with some hot chocolate.
In this story, the snowballs are pathogens—viruses or bacteria that can make us sick. The shields are vaccines that protect against those pathogens. This story is an illustration of herd immunity in action.
When enough people are protected against a pathogen, it has fewer and fewer places to spread until it slows down or completely dies out.
July 2025 update: here is a video version of this analogy for those of you who prefer audio!
The Source of the Shields
Natural immunity can play a role in generating protection against a specific pathogen you’ve survived, but it comes at a cost. Pathogens carry risks of long-term complications, hospitalizations and death. For example, measles erases some of your immune system's memory of prior pathogens it has fought off and causes other complications. Vaccination is, therefore, the best way to build your shield. It gives us the benefits of immunity without the risks of the disease itself.
When Herd Immunity Breaks Down, People Get Hurt
Right now, the measles outbreak is an example of what happens when there aren’t enough shields and herd immunity fails. Measles is serious. It is one of the most contagious infections—9 out of 10 unvaccinated people exposed will become infected. Out of 1,000 infections, one to two people will die and others can have complications. But the shield works – the vaccine is highly effective, it provides 97% protection with two doses!
As of March 28, the CDC reports there have been 483 confirmed measles cases in 20 states. Most cases are in Texas. Of these cases, 97% occurred in those who are unvaccinated or have an unknown vaccination status.
This outbreak has consequences. Two individuals have died—one was only six. Children are going to the hospital with vitamin A toxicity due to parents giving them inappropriate amounts of vitamin A, which is not a substitute for vaccination. Vitamin A should only be given by medical providers in response to measles.
We are also hearing about newborns who were exposed in the hospital after birth. This is scary for those parents because babies are not eligible to get the measles vaccine until they are 12 months old (though they can receive it at 6 months if an outbreak area or traveling. Discuss with your doctor!). These babies are examples of people without a shield that depend on herd immunity.
Why Should You Care?
Many people view vaccination as a personal protection measure, like umbrellas, seat belts or sunscreen. There is no doubt that vaccines DO offer personal protection, but they are different from these things because vaccines also protect those around you through herd immunity.
This is critically important because some people cannot get vaccinated. These people rely on herd immunity to avoid the severe consequences of infection. Here are some examples:
Newborns and infants who are too young to get certain vaccines and therefore are at risk.
People with weakened immune systems, such as those undergoing chemotherapy or those with immune disorders who may not develop strong immunity.
People who are high risk, who have other health conditions that make infections more dangerous.
Pregnant people, who are at higher risk for many infectious disease complications.
Older adults, whose immune responses can weaken over time.
For all of these people, even an illness you think is mild may turn into something deadly for them. They don’t have a shield in the snowball fight, and they can’t make their own. The only way they stay safe is if the rest of us block the snowballs before they reach them.
No Shield is Perfect
No vaccine is 100% effective. Let’s go back to the shields in the snowball fight. Depending on the situation, your cardboard shield may start to weaken. If you’re surrounded by people who also have shields, your chances of getting hit drop significantly no matter how good or bad your own shield is. This is because their shields also can help protect you.
Even if a vaccine doesn’t prevent 100% of infections, it still offers major benefits. It can:
Make the illness less severe if you do get sick.
Reduce the risk of complications and hospitalization.
Lower the chance of passing the disease to others.
So, getting vaccinated protects you and your community—two benefits in one!
How Many Shields Do We Need?
The number of people who need immunity to stop a disease depends on how contagious the disease is. The more contagious, the higher the percentage of the population that needs immunity. This is known as the herd immunity threshold. Here are a few examples:
Measles: Around 94%.
Polio: Around 80%.
Diphtheria: Around 83%.
The Bottom Line
Herd immunity is about stopping the snowballs before they can hit someone who can’t protect themselves. It’s one of the best ways we keep both ourselves and our communities safe. Let’s keep those snowballs from hitting our neighbors!
Take care!
Until next time,
Liz
That is such a good way of explaining herd immunity! Brilliant 👏 ❤️
What a beautiful analogy!