Herd (Community) Immunity

You might have heard of herd or community immunity, when a vaccinated portion of a population (aka "the herd") provides unvaccinated people with some protection from a certain disease. That may sound like a pretty safe bet — and a way to keep your child safe from vaccine-preventable diseases without having to subject him to shots — but it's far from a sure thing.

Here's why vaccinating your child is still extra important for his health.

What is herd immunity?

Herd or community immunity is when a high enough portion of the population is immune to a disease and therefore indirectly protects those who are not immune. If the majority of people in a community are immune to an illness, the illness will have a harder time spreading since those who are still at risk will be less likely to come into contact with someone who is contagious.

Herd immunity often occurs through vaccinations. Many diseases that children are regularly vaccinated for today — including measles, mumps, rubella, pneumococcal disease, whooping cough and even chickenpox — were once widespread. But now that those immunizations have become standard, our society has achieved herd immunity against the illnesses, and they are much rarer. 

And in the cases of diseases like polio and diphtheria, herd immunity is so strong that the diseases have almost disappeared completely.

Vaccines aren't the only way to achieve herd immunity. In diseases where a vaccine doesn’t exist, herd immunity can happen if enough people become infected.

Why is herd immunity important?

Herd immunity keeps individuals and communities safe by significantly reducing the spread of infections. In fact, childhood vaccinations have made it possible for the U.S. to achieve herd immunity for many diseases that were once common, and sometimes, deadly. These include:

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• Diphtheria
• Chickenpox
• Mumps
• Rotavirus
• Pneumococcal disease
• Whooping cough
• Measles
• Hib
• Rubella
• Hepatitis A
• Hepatitis B
• Polio

The fact that these diseases are now rare and not easily spread means everyone is better protected against them. That includes those who aren't able to be vaccinated or who haven't yet been vaccinated, like very young infants.

Take Hib (Haemophilus influenzae type b), a potentially deadly virus that can cause meningitis and lead to permanent brain damage. About 20,000 children younger than 5 developed severe Hib disease in the United States each year, and about 1,000 died before a vaccine became available in 1985. Since herd immunity was achieved, the incidence of infection has fallen by 99 percent.

Or look at diphtheria, a lethal bacterial infection that can cause trouble breathing and heart failure. More than 15,000 Americans died from the disease in 1921 alone, before we had a vaccine and were able to achieve herd immunity. But since 2004 and 2014, there have only been two reported cases in the U.S.

Herd immunity can also keep outbreaks at bay, but it works best when everyone (or as many people as possible) are vaccinated against a disease. When the first measles case to hit the U.S. since 2000 struck in 2005, it came from one unvaccinated U.S. resident who had contracted the illness in Europe, where vaccination rates are lower. It wasn’t long before U.S. kids who hadn’t gotten the measles vaccination also ended up getting infected.

Thanks to strong herd immunity, the U.S. avoided a major measles outbreak that year. But as anti-vaccine sentiment has grown in some communities in recent years, measles outbreaks have occurred more frequently. In 2019, 1,282 measles cases were confirmed in 31 states — the highest number since 1992.  

Why is it so important for your child to get vaccinated?

If we now have herd immunity for once-common childhood illnesses, why bother continuing to vaccinate for them? Turns out, there are plenty of important reasons.

Most of the diseases we vaccinate for still exist — and immunizations keep your child safe

Immunizations have helped us achieve herd immunity for many diseases to the point where infections are rare. But many of these illnesses are still common in other parts of the world and can easily be brought back to the U.S., where they can infect your child if she’s unvaccinated. 

It only takes one person to get a disease and spread it to others who haven’t been vaccinated against it. 

Vaccinating your child helps protect others, too

Herd immunity protects people who aren’t able to get vaccinated against a disease, including those with serious allergies or with conditions that affect the immune system — like HIV/AIDS and cancers. 

It also protects newborns who aren’t yet old enough to get vaccinated, along with pregnant women who may not be eligible for some immunizations. If you’re planning to have another baby at some point, vaccinating your child now will help keep your next one safe.

Herd immunity only works when we all opt in 

In the case of measles, 95 percent of the community needs to be vaccinated in order to achieve herd immunity. Fewer immunizations means higher infection rates — and the change can happen quickly. When vaccination rates dropped significantly in Minnesota in 2017, a measles outbreak sickened 73 children under the age of 10.

Herd immunity doesn't apply to every vaccine

Community immunity doesn't apply to every disease, specifically tetanus, which is a vaccine-preventable disease that's not contagious. (Instead, tetanus bacteria enters the body through a wound or cut.) 

Why vaccinate your child with the tetanus shot? If your child hasn't been immunized, it doesn't matter if everyone else is — he still won't be protected.

Herd immunity makes for a safer future 

Vaccinating your child to preserve the herd immunity we currently have increases the odds that herd immunity will still exist for your grandchildren, their grandchildren and beyond. 

What would happen if we stopped giving children vaccines?

The simple answer is that outbreaks would become more common. Before long, herd immunity could start to erode and might eventually disappear altogether — leading to many, many more people getting sick.

Remember, the herd immunity that we currently have against many diseases in the U.S. only exists because people continue to get vaccinated. Many of these vaccine-preventable diseases are still widespread elsewhere, and it only takes one non-immunized person to bring a disease to the U.S. And once it’s here, it could easily spread to anyone who isn't vaccinated. 

If large enough numbers of kids stopped getting vaccines, that could lead to major disease outbreaks. It’s upsetting to think about, but stopping vaccinations would ultimately put many, many more children at risk.

Is herd immunity possible with COVID-19?

At the beginning of the COVID-19 pandemic, you may have heard the phrase "herd immunity" in reference to a possible end point. Experts now believe it's unlikely that we'll achieve community immunity against COVID-19 in the near future for several reasons:

• Vaccine hesitancy. Many people in the U.S. have not gotten vaccinated, even though it's recommended and safe for everyone 6 months and older.
• Uneven distribution. In other parts of the world, many people do not have access to the COVID-19 vaccines at all. 
• Reinfection risk. Getting infected with COVID-19 can lower the risk of another infection with a similar variant for several months, but it is possible to get infected with COVID-19 again. 
• Breakthrough infections and new variants. The COVID-19 vaccine offers strong protection against severe illness, but it isn't 100 percent effective at preventing infection, especially as new variants evolve and spread.

While it's frustrating to hear that continued COVID-19 outbreaks will likely occur, getting vaccinated (and boosted) can help slow the transmission of the virus. More importantly, it greatly reduces the risk of serious illness, hospitalization and death. 

In short, a future without vaccines would be dangerous and bleak. The good news is that routine shots have made it possible for us to achieve herd immunity against many diseases and protect our kids and communities.