A Simple Tool Illustrating the Protective Shield of Vaccines

A screen shot showing the Pittsburgh area with simulated measles cases at 95% and 80% vaccination rates

When discussing the topic of highly contagious infectious diseases such as measles that require a the population to maintain a high vaccination rate to be kept at bay I invoke concepts like herd protection. Herd protection, sometimes also described as herd immunity, refers to the ability of an unvaccinated or vaccine non-responding individual to rely on the fact that others are adequately immunized so the contagion's ability to reach them is severely restricted. 

When I was an infectious disease fellow one of my attending physicians would illustrate this point with a graphic of a herd of cows with various degrees of vaccination within the ranks of their herd. 

The point he was concretizing is that the more contagious a microbe is the higher the vaccination rate must be. In mathematical and epidemiological terms the level of vaccination must be 1 - (1/Ro). Ro represents the contagiousness of the disease and you can see just by plugging in an approximate number of say 20 for measles, how high the vaccination rate must be (95%).

If that all seems too abstract and the cow example seems too simplistic, I have a solution.

The Graduate School of Public Health (GSPH) at the University of Pittsburgh has a fun tool to help illustrate these points. Affectionately known as FRED, this epidemic stimulator allows one to see a measles outbreak unfold in various locales with disparate vaccination coverage rates. One can see how a measles outbreak is stopped when vaccination rates are at the 95% threshold and how the spread when below. 

Such a tool, coupled with the data and graphics of another of GSPH's great projects, is a welcome addition to the armamentarium when trying to educate the public and policymakers (some of who defend vaccines like PA State Rep. Corbin and US Reps. Dent and Marino) of just how important the measles vaccination has been in eliminating this disease from the US--a status that is now severely threatened. 

Treat Your Children Like Puppies: Vaccinate Them!

The origin of the measles virus, currently capturing headlines across the country with 121 cases in the US (and 147 in the region of the Americas) so far this year, is likely zoonotic. Like almost every infectious disease of humans, animals likely played a part and the virus jumped into humans. The virus that measles evolved--I wanted to use that word on Darwin Day--from is canine distemper. 

Canine distemper virus is a member of the same viral family as measles (paramyxovirus) and was first described in the early 20th century. The symptoms it causes include fever, nasal discharge, and eye inflammation (sound familiar?). Vomiting and diarrhea, lethargy and loss of appetite, labored breathing and/or coughing, and hardening of footpads and nose, and other symptoms can also occur. It is vaccine preventable and remains a leading cause of infectious disease death in dogs. 

There is good reason to believe that canine distemper virus (or the related and now eradicated rinderpest virus of cattle) jumped into humans and evolved into measles when human populations reached the threshold population density needed to sustain human-to-human transmission of the virus. Indeed measles vaccination protects canines against distemper.

It's puzzling to me that we don't hear about an anti-vaccine movement amongst dog owners yet amongst parents of human children we have no such luck.

Children deserve to be treated as well as their puppies.

Measles at Disneyland: Donald Duck Can Give You More than Bird Flu

When I try to quantify the contagiousness of diseases, I always use measles as an example of extreme contagiousness. On stage giving lectures, I tell the audience that if I had measles they would all be exposed. Fortunately many were lucky enough to be vaccinated as children and grow up in an era with high levels of immunization, conferring the protection of the herd (i.e. herd immunity). 

However, largely due to the efforts of the anti-vaccine movement, there are pockets of this nation in which immunization levels have fallen to levels that have fostered the return of this dread disease that causes pneumonia in 1 of 20 who contract it and kills at a rate of 1 per 1000. 

One of these pockets happens to be California which is currently in the midst of its worst measles year in two decades. Making matters much worse for California is the report that approximately one dozen cases of measles have been reported to be linked to exposures at Disneyland. Of the original cases reported, half were unvaccinated despite being eligible for vaccination.

To measles, Disneyland is a viral-exchange center in which it will gain access to individuals from all over the world some of who may not be vaccinated because of their parents' (deadly) choices, some because they are too young, and some whose immune system precludes it. From Disneyland, the virus can be carried to other states and nations.

The anti-vaccine movement has now made Disneyland the happiest place on earth--not for the children of the world--but for a wretched virus.

The Real Crazy Train: Measles

Yet another large measles exposure occurred in this country as the result of an unvaccinated individual contracting the illness while in Asia. 

The patient, who is a college student in California, rode a public transportation train and potentially exposed thousands of individuals to the virus, which can be deadly in select cases.

This incident has several aspects that worth highlighting:

• Though measles has been nearly eradicated from the US that is not the case in the rest of the world
• Unvaccinated individuals, who may have a relatively low (but not zero) risk of contracting the disease in the US, are at a high risk of infection when traveling to areas in which measles is still present and human populations are not immunized to a high degree, i.e. herd immunity has not been achieved
• Measles is extremely contagious and if exposed individuals were not adequately vaccinated, for personal reasons or because they are less than 1 year of age, they may contract measles

The bottom line: Measles is too contagious of a disease for the human population to become lax about.