Herd immunity protects those too young or frail to be vaccinated, a group that includes infants, chemotherapy patients, organ transplant recipients, the elderly, and people with immune diseases such as AIDS.
To achieve herd immunity, the number of vaccinated individuals in a population must reach a critical mass. The Centers for Disease Control sets different vaccination goals for each disease depending on how contagious it is and how effective the vaccine for it is. For example, whooping cough is highly contagious and the vaccine is not very good, so the goal is to have 92 to 94 percent of the population vaccinated. In contrast, mumps is less contagious so only 75 to 86 percent of the population needs to be vaccinated in order to protect unvaccinated individuals.
Whooping cough vaccination in the United States has met or exceeded the minimum herd immunity levels since 1994. But localized outbreaks of the disease show what happens when immunization drops below critical mass. In the last year, the number of whooping cough cases in California exploded to four times that of the previous year. A healthy adult can usually survive the “100-day cough” typical of the disease, but young or unhealthy people risk death. As of October 2010, ten infants have died in California from this vaccine-preventable disease.
The primary cause of these outbreaks, experts say, is a trend toward fewer childhood vaccinations and the failure of adults to get their booster shots. In California, vaccination rates fell to a level that compromised herd immunity. The vulnerable population, including infants too young to be vaccinated, was left unprotected.
Health officials say that fear over vaccine safety is a leading cause of the drop in vaccinations. A 1998 study in The Lancet linked the measles-mumps-rubella (MMR) vaccine to autism. Although the study was widely criticized, it wasn’t officially retracted until 2010. In the meantime, some parents refused to vaccinate their children against MMR and other vaccine-preventable diseases. This has led to outbreaks of whooping cough, measles, and bacterial meningitis in communities across the U.S.
Thanks to Dr. Christopher Gill, an infectious disease specialist with the Boston University School of Public Health, and to Thomas Skinner, Press Officer specializing in immunity and vaccinations with the Centers for Disease Control and Prevention.