by S. Todd Stolp MD
©February 2011
In the 50s, 60s and 70s, the greatest obstacle to the distribution of vaccine to the general population was access to clinical care. For a large proportion of the U.S. population, understanding the benefits of polio and measles immunization, transportation to a clinic site to receive the vaccine and coverage of the cost of vaccination was beyond reach of the poor. During that era, much of the adult population had already developed natural immunity to these conditions due to prior infections or earlier vaccination campaigns, so the illnesses tended to specifically target the young. Therefore, a strategy was implemented to conduct vaccination clinics at schools, since these were sites that were universally attended by children. The outcome was a public health triumph. Polio was eliminated from North America and measles, which in 1920 caused 469,924 infections and 7,575 deaths, was declared eradicated from the United States in 2000. Today, cases and clusters of measles that occur in the U.S. are imported from outside the country.
In 1910, Dr. William Osler expressed his concern for a segment of the population who opposed the use of vaccinations. At that time, there was a considerable movement against the use of the cow pox vaccine that was being distributed to quell widespread outbreaks of smallpox. Today, opposition to vaccination continues to be an issue. While questioning and researching recommendations for maintaining one’s health and the health of one’s family is admirable, an unfortunately large number of unreliable sources of information are now readily available to confuse the picture. Political turmoil in Pakistan has caused populations to be suspicious that the polio vaccination campaign is a political plot, hobbling efforts to eradicate polio in that country, one of only three in the world in which polio remains endemic. We should inquire about the risks and benefits of vaccine, but we should be careful not to make decisions about whether to participate based upon personal convictions that may not be based upon good science.
The science tells us that in nearly every case, it is much safer for a person to receive the whooping cough vaccine during infancy and booster vaccines during childhood, also known as “Tdap,” than it is for us to live in a community that remains unvaccinated. This is well documented by multiple studies conducted by the National Institute of Health, observational studies in the United Kingdom, and confirmed by a study done in British Columbia. For those extremely rare cases in which a person has experienced a prior untoward reaction to a similar vaccination, your healthcare provider can confirm a medical exemption to assure that such a person does not receive the vaccine. In such rare cases involving students, the medical provider must sign a Permanent Medical Exemption form to allow the child to attend school.
The need for a Permanent Medical Exemption from any particular vaccine is such a rare condition that communities will easily be able to vaccinate enough of the population to limit the ability of virtually all vaccine preventable diseases from gaining the advantage during an outbreak, as long as those capable of receiving the vaccine do so. This number – the percent of the population that must be fully vaccinated against a particular infectious disease in order to prevent it from spreading through a community – is different for different diseases. When this percent of the population is vaccinated, the population is said to have achieved “herd immunity.” The following is a brief explanation of why achieving herd immunity works.
Imagine that you are a firefighter and it is your job to prevent a community from being burned up in the event that a lightning storm causes a wildfire. When you look at the record, you notice that one neighboring community has never burned in the forested region around you, and another community has burned to the ground nearly every ten years. As you look more closely, you learn that the community that has never burned has carefully trimmed underbrush, burned grassland every year to prevent it from growing tall, and has kept a small strip of land clear of trees surrounding the town, thereby preventing wildfires from entering the town and getting out of control by moving from tree to tree and bush to bush. The community that has frequently burned has not done so.
This is the same strategy used by vaccinating communities to prevent outbreaks of disease. As long as enough people are vaccinated to prevent the “wildfire” of disease from moving from person to person, the illness will not have enough people who are not immune to the disease to sustain the spread of the epidemic. How thoroughly a community must trim the thick forest and underbrush to prevent wildfire depends upon the kinds of trees and the local weather, just like different infectious diseases require different levels of immunity in a community in order for the community to achieve herd immunity.