Vaccination is one of the most cost-effective and successful public health interventions. Each year, vaccines save an estimated 6 million to 9 million lives worldwide, including the lives of 3 million children. In the United States, vaccinations have decreased most vaccine-preventable childhood diseases by more than 95 percent. Vaccines have minimized or eliminated outbreaks of certain diseases that were once lethal to large numbers of people, including measles and polio in the United States and smallpox worldwide. But because the bacteria and viruses that cause diseases still exist, the public health gains achieved through vaccines can only be maintained by ensuring that vaccination rates remain high enough to prevent outbreaks.
Vaccines are effective not only because they protect individuals who have been vaccinated but also because they confer a broader protection for communities by establishing “herd immunity.” When sufficiently high proportion of a population is vaccinated against communicable diseases, the entire population can obtain protection. As the number of vaccinated people in a given population increases, the likelihood that a susceptible person will come into contact with an infected person decreases; it ultimately becomes difficult for a disease to maintain a chain of infection. Although the vaccination rate required to achieve herd immunity varies by vaccine, it typically ranges from 80 percent to 95 percent of a given population.
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