Saturday, August 21, 2010

Vaccines and the Value of Herd Immunity

Many vaccine preventable diseases are all but unheard of here in America. When was the last time you saw a child with polio? Unless you work in epidemiology or travel overseas a lot, you probably haven't seen a child with polio or any of a number of other vaccine preventable diseases. The reason is that for decades our vaccination rate was 90% or more. Although those diseases did (and do) exist overseas, America had very few outbreaks of those diseases. Our herd immunity was great enough that even most of those who didn't or couldn't get vaccinated were still protected.

The problem is that because people aren't seeing these diseases as any kind of real threat, the perceived danger of not vaccinating is considered negligible. If no one in the community has had measles in recent memory, why bother with the vaccination, is probably a common thought. Add into the equation that the anti-vaccination movement has made very real inroads into public consciousness and we can see why some parents weigh the risks as they perceive them and opt out of vaccines.

If one or two families out of a few million decide not to vaccinate, it is really no big deal. Herd immunity will protect them. But as the percentage of unvaccinated children rises, that herd immunity is threatened. Especially if we realize that the unvaccinated aren't equally distributed throughout the country. They tend to clump together geographically to some extent as some whole communities have dangerously low vaccination rates. These are the places where epidemics can catch hold. And where epidemics probably will run amok before people again see these diseases as real dangers and start taking vaccinations seriously again.

So a few people get sick? Big deal, right? Well, yes, actually. Many vaccines cannot be given to infants or some people with compromised immune systems. So at any one time we cannot have a 100% vaccinated population. Even if their parents will vaccinate them as soon as they can or the adults would get vaccines themselves if they could. All of those babies and adults rely on the herd immunity that the rest of us who get vaccinated provide for them. When our societal vaccination rates drop, we endanger them. And this is a major contributor to what is happening right now in California with the babies dying of whooping cough.

We as a people are far more mobile than at any other time in history. When it comes to disease transmission we really are a hair's breadth from being a truly global community. So the diseases that do run amok in other countries are a real threat right here in our own communities. Because we are probably never more than five degrees of separation from most diseases in the world.

Traveled overseas recently? Been to an airport recently? An international airport? Been on an airplane, or a bus, or a train? Interact recently with someone who has? What about the people they know? Those are all potential vectors of transmission that can bring what we might think of now as exotic diseases home. So it is actually more important than ever to keep our societal vaccination rates up. And disheartening that we aren't.

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