There are two ways to become immune to an infectious disease: to catch it and fight it off so that the body forms antibodies, and to get a vaccine, which introduces a small amount the pathogen so the body can form antibodies without getting sick. People who are immune to a viral or bacterial infection protect not only themselves but the community at large. Herd immunity prevents the disease from spreading into vulnerable populations, limiting outbreaks.

Herd Immunity and the Spread of Disease

When a community is not immune to a disease, germs spread very quickly. If enough people contract the infection, it leads to an outbreak that affects large portions of the community. When enough people are immune to a disease, however, either through natural immunity of vaccination, the germs have a difficult time spreading, which slows down transmission. This is herd immunity. If someone does get sick, the chances of passing it on are lower since there are fewer people to contract it.

Spread Germs Herd Orbon Alija / Getty Images


Who Benefits from Herd Immunity

People who cannot be vaccinated or are more vulnerable are directly protected by herd immunity. If a disease is not spreading because there are enough immune or vaccinated people to prevent it, the odds of vulnerable people catching the disease are less than they would be otherwise. On a larger scale, herd immunity protects the entire community because it slows or stops diseases from spreading.

At Risk Immunocompromised FatCamera / Getty Images


Vulnerable Populations

Some conditions prevent people from getting vaccines, which means they cannot participate in creating herd immunity for legitimate medical reasons. For example, it is unsafe to vaccinate people with serious allergies or diseases that cause a weakened immune system, like HIV/AIDS or cancer. Some vaccines cannot be administered until babies reach a certain age, so newborns and infants may be particularly vulnerable.

Newborn Infant Vaccines Catherine Delahaye / Getty Images


Stopping Vaccinations

Even if there are only a few cases of a disease, continuing to vaccinate is essential to maintaining herd immunity. If people stopped vaccinating, the disease would eventually come back in full force. Worse, as it is the younger generation would be unvaccinated, the return of these illnesses could result in a lot of children falling ill.

Vaccines Children Schedule FatCamera / Getty Images


Japan and Pertussis

In the past, vaccination rates plummeted with tragic results. In 1974, close to 80 percent of Japanese children received vaccinations for pertussis or whooping cough. By 1976, this number fell to 10 percent. Just three years later, in 1979, a pertussis epidemic occurred. There were more than 13,000 cases that resulted in a total of 41 deaths. In 1981, the Japanese government began using the vaccine again, ending the epidemic. This case shows the importance of vaccinations in maintaining herd immunity.

Pertussis Coughing Whooping Cough Hailshadow / Getty Images


Examples in the US

There are obvious benefits to herd immunity in the US. In the early 1920s, about 200,000 Americans contracted diphtheria every year. In 1923, the vaccine was introduced, establishing herd immunity. Per the CDC, there were only two cases of the disease between 2004 and 2014.

Between 1964 and 1965, the US experienced a rubella epidemic that killed more than 12 million people, including 2,000 babies. The vaccine was introduced in 1969, and, since 2012, there have only been 12 cases of rubella.

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Disease Brought Under Control by Herd Immunity

In addition to pertussis, diphtheria, and rubella, vaccines, and the resulting herd immunity, have brought several other diseases under control. Chickenpox, mumps, pneumococcal disease, rotavirus, Haemophilus influenza type b or Hib, hepatitis A, hepatitis B, influenza, polio, and tetanus are all controllable today. Some of these vaccines are given to children as they grow, while others, like the flu shot, must be administered yearly to maintain effective herd immunity.

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Ineffective Herd Immunity

The percentage of people who need vaccinating for herd immunity to work varies depending on the disease. It also only works for diseases passed from one person to another. For example, measles is extremely contagious, so more than 90 percent of the population has to be vaccinated for herd immunity to work. On the other hand, polio is less contagious, so vaccination of more than 80 percent of the population establishes effective herd immunity.

Herd Immunity Peopls Bernhard Lang / Getty Images


Measles: An Example

In 2000, measles was declared eradicated in the US, which means there had been no sustained transmission of the virus for more than a year. Due to a recent decline in the number of children receiving the MMR vaccine, though, measles cases have resurged. There is fear that, without enough vaccinated people to stop the spread and maintain herd immunity, the disease will have a resurgence.

MMR Vaccine Immunity sefa ozel / Getty Images


Vaccination Rates and Outbreaks

Vaccines are the most effective way to establish herd immunity. If vaccination rates fall, outbreaks become more frequent. One 2016 study shows that of the 1,416 people diagnosed with measles since 2000, more than half were unvaccinated. Of those, about 70 percent were eligible for vaccination. This study also shows that children with legitimate vaccine exemptions were 35 times more likely to contract measles than vaccinated individuals. Researches are certain that higher rates of unvaccinated people is linked to a greater incidence of measles because herd immunity is not properly maintained.

Measles Outbreak Rash CHBD / Getty Images


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