Dr. John Cahill explains that though Infectious Disease specialists would like to vaccinate everyone, getting a large amount of people vaccinated can provide a barrier for most of the group. If enough of a population gets vaccinated then the potential for disease to travel across the globe, or even within U.S., is greatly diminished. In the event of a serious outbreak, we would start by vaccinating the health care providers. We would then vaccinate people serving sectors valuable to society infrastructure like police officers and firemen.

Herd immunity is a "firewall" that could slow a viral outbreak down enough to stop a large scale pandemic. Polio is a good example of herd immunity. In parts of the third world, vaccines are still not available to the majority of the population. However, even in societies where only 50% of the people have been vaccinated for polio, we don't see large outbreaks of polio anymore.

Dr. John D. Cahill, M.D. is a physician and medical educator who founded the Center for Global Collaboration & Health Initiatives. While studying abroad in medical school and after his postgraduate medical training at Brown University, Dr. Cahill spent time in Southeast Asia & Africa.

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