by Amy Spangler
June 01, 2010

Why should I vaccinate my children to protect others? Why should I put my children at risk so others are less likely to get sick? These and other questions reflect the ongoing controversy surrounding vaccines—a controversy that has prompted an ever-growing number of parents to forgo vaccinating their children and now threatens “herd immunity.”

Susan Van den Hoff was the first to identify herd immunity and how it affects the spread of infectious disease. In 2002, Van den Hoff found that unvaccinated people in a highly vaccinated community were less likely to get measles than vaccinated people in a community with low vaccination rates. The effectiveness of herd immunity in preventing the spread of a disease depends on several factors—the ability of the disease to spread, the number of people vaccinated or not, and the effectiveness of the vaccine itself.

How easily does the disease spread?
Some diseases, like measles, are highly contagious; others, less so. According to the Vaccine Education Center at The Children’s Hospital of Philadelphia (CHOP), if 10 unprotected individuals got into an elevator with an individual with measles, 9 of the 10 would become infected with measles. Because the measles virus can stay in the air for up to two hours, other unprotected individuals who get on the elevator can become infected as well. Some diseases are highly contagious, such as chicken pox, but infection won’t occur unless there is direct contact with the virus. Experts at the Centers for Disease Control and Prevention (CDC) study family members or other individuals who come in contact with an infected person to determine how contagious a disease is and how easily it is spread.

How many people need to be vaccinated?
The number of people who need to be vaccinated depends on the level of contagiousness and ease of spreading. In the case of measles—a highly contagious disease that is easily spread—an extremely large number of people would need to be vaccinated before herd immunity would take effect. There are some individuals who cannot be vaccinated for medical reasons (individuals with allergies, cancer, or diseases of the immune system), and others who choose not to be vaccinated. The greater the number of unprotected individuals, the less likely the disease will be eradicated.

How effective is the vaccine?
Some vaccines require only a single dose and protect all who are vaccinated, while others require several doses. There are some individuals who never acquire immunity, and remain unprotected despite repeated doses. Although this is extremely rare. For vaccines that have been shown to be less effective, more people need to be vaccinated to make up for those who remain unprotected despite being vaccinated.

Join the herd
Immunization rates in the U.S. have been extremely high for many diseases such as measles, mumps, rubella, Haemophilus influenza type b (Hib), and polio. As a result, our communities have benefited greatly from herd immunity. Many diseases that were commonplace in the 1800s such as cholera, tetanus, and typhoid fever are rarely seen today. But concerns about the safety of vaccines, sparked in 1998 by the release of a study linking autism with the measles-mumps-rubella vaccine (a link that has since been shown to be false), have caused an increase in the number of unprotected individuals nationwide. There have been reports of outbreaks of measles, mumps, Haemophilus influenza, and pertussis (also known as whooping cough) in communities across the U.S. More measles cases were reported nationwide in 2008 than in any other year since 1997. And more than 90 percent of those infected had not been vaccinated, or their vaccination status was unknown. Earlier this year, more than 1,500 New York and New Jersey residents came down with the mumps—the largest outbreak since 2006.

Before you decide as a parent whether to have your child join the herd, educate yourself about vaccines—the benefits and the risks; seek out reliable sources of information such as the CDC and CHOP; discuss your concerns openly with your child’s doctor; and make an informed decision—the health of your child and the herd depend on it.