Tuesday, February 10, 2015

Herd Immunity: No, It's Not a Hoax

Contrary to popular belief, getting an immunization isn't a magic shield that will forever keep you from catching that virus. Of course, that is undoubtedly the ultimate goal of vaccines, but we're not there yet. The vaccines we have are very effective, but they're not perfect: most people who are vaccinated are fully immune, but a very small portion are only partially immune and may still be susceptible to disease (though they are able to fight off the disease more successfully if they do get it). Because vaccines are not perfect, some people tend to think that they are a big hoax perpetrated by "big pharma" and "big government." This fear is compounded when people misunderstand concepts like herd immunity and have a limited understanding of how vaccines work.

Photo Credit: NIAID
Because vaccines aren't perfect, and because not everyone can get vaccines, we rely on herd immunity. Herd immunity occurs when a high enough percentage of the population is immune so as to prevent the transmission of a disease. If someone with the disease is surrounded by people who are immune, then they can't pass the disease on to someone else. When we talk about herd immunity, we talk about "chains of transmission" - pathways that the disease can take from infected person to susceptible person.

The required level of herd immunity needed in order to keep a disease at bay is dependent upon the particular disease. Different diseases have different infectious rates (the amount of exposure you need to the disease in order to catch it - a single virus or several milliliters) and contagious rates (the disease's ability to spread from person to person). For example, malaria is not contagious (you can't catch it from someone who has it), but it is very infectious (a tiny mosquito bite can infect you). Diseases also have different "reproduction numbers," denoted by R, that tells us how many new cases of the disease can come from each individual case. Low R numbers mean that it's very hard to spread whereas high R numbers mean that it's very easy to spread.

Photo Credit: Wikipedia: Basic Reproduction Number
In order to prevent an outbreak, you want to infect no more than one additional person for each case. So for a disease with a low R number, say influenza (R=2), you're only going to pass it on to two people, and so as long as half of the population is immune, only one person will be infected per case, and it won't cause an outbreak. That's how we determine how many individuals need to be vaccinated. When the immunity level is high enough, the possible chains of transmission are interrupted, and the disease is contained. This is really important for children who are too young to receive vaccinations, individuals who have compromised immune systems, and those who are too ill to receive vaccines. They rely on broken chains of transmission in order to stay healthy.

Measles has a very high R number, and it's airborne; as a result, the goal for measles vaccinations is around 90-95% of the population. Taking into consideration the number of individuals who can't get vaccinated, that leaves very little room for people choosing not to vaccinate. Unfortunately, due to vaccine hysteria, immunization rates have been falling in different areas as some people are choosing to refuse vaccinations, and herd immunity is breaking down, placing individuals who are already highly susceptible to illness in more danger. This is compounded when people who don't vaccinate tend to cluster into groups, increasing the likelihood that they will pass deadly diseases between them, making it considerably easier for the disease to spread to those who either have incomplete protection through their vaccination or no protection because they can't be vaccinated.

Photo Credit: CDC
The Centers for Disease Control and Prevention (CDC) monitors measles outbreaks. The major measles outbreaks in the United States are caused by American citizens traveling to other countries (usually in Europe, Asia, and the Pacific) and by foreign visitors from these countries traveling to America (not immigrants - I'm looking at you, Rush Limbaugh). They pass the diseases on to those who had not been vaccinated - usually children under 12 months old and individuals who refuse to be vaccinated, but the relatively high population immunity levels in the United States prevented these outbreaks from spreading very far.

That is, until recently. Fueled by a debunked falsified "study" conducted by Andrew Wakefield linking vaccines to autism and the ramblings of celebrities like Jenny McCarthy and Dr. Bob, the United States is now in the throes of vaccine hysteria. Parents are terrified that vaccinations have terrible consequences, regardless of what the actual numbers say about the likelihood of said consequences, and they are forgoing vaccinations in favor of exposing their kids to the actual diseases - ignoring the real harm that these diseases can do - and spreading the disease to people who are susceptible, who want to get the vaccine, but who can't.

As a result, the twenty or so individuals who travel globally and bring measles into the United States each year are now triggering major outbreaks throughout the country. 2014 saw the highest rate of measles in the US since 1994, and these outbreaks are largely occurring in areas where vaccine coverage is significantly below average. There have been over 6,000 preventable deaths since 2007 when the anti-vaccination movement started to gain momentum, and 2015 is looking to be another deadly year.

This has to stop. Unfounded fears perpetuated by celebrities, liars, and politicians need to end. Using fear of severe allergic reactions that occur in one out of every million cases as justification for spreading a disease that can kill 10% of those it infects is not only ludicrous but criminal. The lives of our children depend on it.