From the WebMD Archives

Dec. 3, 2018 --As flu season gets rolling, you’ll keep hearing health authorities urge everyone age 6 or older to get vaccinated.

A main reason doctors want as many people as possible to get a flu shot is that it protects more than just you. It also cuts the risk for your family, co-workers, and everyone else around you.

When lots of people in an area are vaccinated, fewer people get sick. Then fewer germs are around to spread from person to person.

This concept is called herd immunity, or community immunity.

"The whole principle is if you give a vaccine to somebody, you protect them from getting infected, but you also prevent them from transmitting the disease to other people," says Michael Brady, MD, associate medical director at Nationwide Children's Hospital in Columbus, OH, and a member of the hospital's Division of Infectious Diseases.

Herd immunity protects people who can't get vaccinated because their immune system is weak and vaccines might make them sick. This includes babies, people with vaccine allergies, and anyone with an immune-suppressing disease like HIV or cancer.

"Often the people we need to protect with herd immunity are most vulnerable to serious disease," says Amanda Cohn, MD, a pediatrician and senior adviser for vaccines at the CDC’s National Center for Immunization and Respiratory Diseases.

Why Vaccines Are So Important

To have herd immunity and protect lots of people from disease, a very high percentage of people in any one area need to be vaccinated. This is called the threshold.

"The more contagious a disease is, the higher percentage you need," Brady says. To get herd immunity against measles, for example, 93% to 95% of people in a community have to be vaccinated. In other words, about 95 out of every 100 people have to get the vaccine to prevent the disease.

When too few people get vaccinated, diseases that disappeared from the United States years ago can make a comeback. That's because diseases that are no longer here still spread in other countries.

Rubella was wiped out in in the U.S. in 2004, but it's still common in Mexico. Measles stopped spreading in America in 2000, but outbreaks happen today in Europe, Africa, Asia, and the Pacific.

People from other countries carry diseases here when they travel. And those diseases can spread in areas where vaccination rates are low. In 2014, a measles outbreak that started at Disneyland in California led to 147 cases in seven states, along with Mexico and Canada.

"If we lower vaccine coverage in the U.S. to those diseases, there's a high chance of those diseases being reintroduced," Cohn says.

Herd immunity is also important because vaccine protection can fade with time. The pertussis vaccine starts to become less effective 2 years after you get it. The meningococcal conjugate vaccine that protects against meningitis also doesn't work as well over time.

When people's immunity drops, it can lead to new disease outbreaks unless more people in the area keep getting vaccinated.

Herd Immunity and the Flu

Some vaccines are better at producing herd immunity than others. The measles, mumps, and rubella (MMR) vaccine is 97% effective at preventing measles. So when lots of people in a community get this vaccine, protection rates stay high.

The flu vaccine is a little different. It's only about 40% to 60% effective in any given year. That's because sometimes the virus strains in the vaccine don't exactly match the virus that spreads.

Even if the flu vaccine isn't perfect, it's still worth getting, Brady says. In any one flu season, the flu vaccine prevents millions of people from getting the flu. "That's particularly important when the people who don't get the flu are around people who are over 65, or have [other illnesses], or are young," he adds.

The flu vaccine is also good at protecting small groups of people -- such as in your home, office, or school. When you get vaccinated, you help an older adult relative who has a chronic disease, or a baby who is too young to get vaccinated avoid getting sick, Cohn says.

Vaccines are especially important for people who work in hospitals and other health care centers. The sick people they care for are more likely to get flu complications, and they need more protection.

Don't Rely on the Herd

You might think, "If herd immunity protects me, why do I need to get vaccinated?" Vaccines are still the best way to protect yourself, Cohn says. And you may one day travel to a place where vaccine coverage isn't so high.

"While herd immunity is an amazing benefit to having high vaccination coverage, direct protection if you can get vaccinated is the best way to protect your child and yourself from vaccine-preventable diseases," Cohn says.

Show Sources

Michael Brady, MD, associate medical director, Nationwide Children's Hospital, Columbus,  OH; member of the hospital's Division of Infectious Diseases.

Amanda Cohn, MD, pediatrician; senior adviser for vaccines at the CDC’s National Center for Immunization and Respiratory Diseases.

CDC: "Frequently Asked Questions About Measles in the U.S.," "Measles Vaccination," "Meningococcal: Who Needs to Be Vaccinated?" "Pertussis Frequently Asked Questions," "Rubella in the U.S.," "Six Things YOU Need to Know about Vaccines," "Vaccine Effectiveness: How Well Does the Flu Vaccine Work?" "Who Should NOT Get Vaccinated with these Vaccines?" "Year in Review: Measles Linked to Disneyland." "Vaccines Protect Your Community."

World Health Organization: "Critical Immunity Thresholds for Measles Elimination."

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