Rates of medical exemptions from vaccination requirements are higher in states where exemptions are easier to get, potentially compromising immunity and posing a threat to other children, according to public health experts at Emory University.
In addition, states with stringent standards for nonmedical exemptions found a higher rate of medical exemptions. In other words, parents may be seeking medical exemptions when it is harder to receive nonmedical exemptions.
The researchers evaluated medical exemptions from kindergarten entry requirements for every state between 2004 and 2010, and came up with a total of 87,631 medical exemptions nationwide. Standards allowing medical exemptions from school immunization requirements were inconsistent from state to state.
The study was published Thursday in the Journal of Infectious Disease.
The investigators concluded that medical exemptions should be monitored and evaluated continually to ensure they are used appropriately.
"The appropriate use of medical exemptions is important to maintaining sufficient herd immunity," Saad Omer, assistant professor of global health, epidemiology, and pediatrics at Emory University Schools of Public Health and Medicine, writes in the study. Herd immunity refers to the resistance to the spread of infectious disease in a group because susceptible members. are few. It explains how nonvaccinated people are protected when a significant portion of a population is vaccinated. "More importantly, they add to existing pockets of susceptibility. It is known that immunizations exemptors cluster geographically, increasing the possibility for local areas of increased disease incidence."
With herd immunity, infections are unlikely to transfer from person to person because most people are immune. This then disrupts the chain of transmission that could infect a person who did not receive a vaccine or did not respond to it.
People with compromised immune systems, such as infants, the elderly, cancer patients or people with other immune disorders, are typically protected by herd immunity. Also protected are those who, for one reason or another, can't, for medical reasons, get vaccinated.
"Children with valid medical exemption need to be protected ... by insuring high coverage rates among the rest of the population," write Daniel Salmon and Dr. Neal Halsey at the Johns Hopkins Bloomberg School of Public Health, in an editorial that accompanies the article.
However, a certain threshold is required to establish herd immunity -- and the exemptions to school immunizations may compromise the ability of a community to attain this level of vaccination.
Most doctors urge parents to weigh very seriously their decision to opt for nonmedical exemptions, especially in light of the agreement among physicians about their safety and effectiveness. The widely discredited theory of an association between vaccinations and autism, though debunked, persists in the culture today.
Many experts worry about the study's finding that parents may be seeking medical exemptions when nonmedical exemptions are difficult to obtain.
"This study for the first time shows us that when nonmedical exemptions are strictly enforced, some parents seem to be gaming the system by seeking medical exemptions," says Dr. William Schaffner, professor and chairman of the department of preventive medicine at Vanderbilt University School of Medicine. "Speaking for myself, I think that nonmedical exemptions should not be permitted."
While experts recognize the autonomy of parents to make decisions regarding their children's healthcare, doctors believe that counseling may alleviate parents' concerns.