Epidemiology Night School: Introduction to Outbreaks (or “Don’t expect Dr. Jay to understand all this stuff”)
Posted Aug 23 2011 3:00am
With EpiRen offline, I feel a bit of a void. In his blogging he was taking on an educational project: using current topics to discuss important topics in epidemiology. EpiWonk is also offline, but his body of work remains. EpiWonk also took on describing topics and terminology in epidemiology. Since I find these efforts valuable, I’ve decided to lift some of EpiRen’s “Epidemiology Night School” posts to preserve here. Below is one from March 25, 2011. I picked this somewhat at random, so don’t read too much into the selection.
From here on out, it is in EpiRen’s voice. As he is offline, don’t expect him to respond to comments directed at him.
If you’ve been reading me for a while, you know that I absolutely detest having to attack someone personally. Sure, I may point out the stupidity in some statements by people like Christina England , some homeopath here and there, and even Ms. Jennings was a subject of several postings. But I try not to inject my personal opinions about a person (much) because discussions of science should leave personal feelings out of it. (Sorry it was too late for you, Galileo .) Doing this avoids all that background noise. Know what I mean?
Still, there are those times when someone just somehow manages to get under my skin with comments so outrageous (in my opinion) that I am forced to think ill of them. (Not wish them ill, though. Even I am not that big of a bastard so as to wish others ill .) So I’m going to weave in some comments from the twitter feed of one Dr. Jay Gordon, just to show you how someone who doesn’t understand epidemiology can come off as crass and uncaring (in the opinion of many). All, of course, after the jump…
Traditionally, an outbreak has been defined as “one case over the expected rate (or number) of cases for a given location in a period of time.” In Minnesota, they have seen 22 cases over the last 14 years (22/14=1.6 cases per year in all Minnesota). Rounding up, we can say that two cases per year is what is expected. Three cases in 2011 would mean an outbreak. What was that in 2010, you ask? Well, 19 cases in 13 years give us a rate of 1.5 cases per year. It would also be an outbreak situation, especially if the three cases were epidemiologically linked. That information is not yet available from the MDH, but it will be interesting to read later on.
YES. YES, IT’S AN OUTBREAK. IT’S A FRIGGIN’ OUTBREAK
When two or more people develop a disease or condition, and they have similar exposures, they are said to be epidemiologically linked. For example, if two people ate at the same place in the hours before their onset of the same illness, then they are epidemiologically linked regardless of whether or not the food they shared is found to be the culprit. Some links are stronger than others, but this concept is not lost in outbreak investigations. During the outbreak of what is now known as Legionnaires’ Disease in Philadelphia in 1976 , the fact that all those men were coming down with pneumonia raised some flags… The fact that they were all staying at the same hotel AND were all members of the American Legion was a cause for alarm. (It would be a while before the bacteria that caused the outbreak was discovered, but their epidemiological link proved to be an enormous clue.)
BACK TO MINNESOTA
So, eleven cases, all epidemiologically linked, is that an outbreak?
YOU LOST ME AT PORN.
I learned in grade school that 11 cases (to date) in the current outbreak is 9 cases over the expected 2. I also learned that it’s over 5 times the expected rate. I then learned in epidemiology school (a master’s level degree) that the fact that all these cases are somehow related to each other pretty much makes this an outbreak. Am I – or anyone working on that outbreak – being obsessive about “a few extra cases of measles”?
If it means stopping a disease that can do this to a child, then YES, YES I AM OBSESSED.
I want to emphasize the fact that they are all related to each other. If they had absolutely nothing to do with each other and were found in different corners of planet Earth, I wouldn’t obsess. But they’re all in one region of Minnesota. There’s nothing random about that, is there?
You can lead the horse to the water, but… RANDOM, I DO NOT THINK YOU UNDERSTAND WHAT THAT WORD MEANS
As I’ve stated before, there are clearly defined guidelines on what constitutes an outbreak and what doesn’t. If you look at the definition, there are factors of person, place, and time. How many people, where, and when? As I’m writing this, the answer is 11 people, in one region of Minnesota, in the last two to three weeks. That’s an outbreak, my friends. It’s a clean and clear situation.
What the hell does Noro have to do with measles? REMEMBER YOUR TRAINING
Pop quiz. What is the definition of incidence? Yep. You got it. It’s the number of new cases divided by the population at risk. Vaccine coverage for measles is estimated at 85% in Minnesota (maybe lower or higher, but certainly not enough for herd immunity now). That means that about 780,000 people in Minnesota are at risk for measles because they’re not considered immune. (Others who have been immunized, but whose immune system didn’t “take” the vaccine are too low in number to make an impact. The vaccine is really quite good, giving immunity to 99.7% of those who get their two doses and to 95% of those who get at least one dose .) That little quip about 15% norovirus? It’s a GUIDELINE on when to call an elevated number of cases of norovirus symptoms on a cruiseship an outbreak. (It appears that Dr. Gordon wants to go with one guideline but not another.)
Not a dangerous epidemic? Would he say that to the mothers of those sick children?
TELEVISION AND REALITY
I don’t know about you folks, but I have never based an epidemiological decision or observation on a television show. I am yet to hear anything on television (or in any other media) and take it as gospel. What I have done is look at books about epidemiology that build upon a couple of centuries of knowledge and scientific experimentation. Maybe the Brady Bunch didn’t succumb to measles because – and I’m only taking a wild guess here – THEY WERE A FICTIONAL FAMILY! Moreover, they were a fictional family that caught measles in 1969 and then mumps in 1973 . I would NEVER take my medical or epidemiological advice from such a careless bunch.
The guy isn’t even wearing gloves!
I’d never use them as an example for such a serious situation. Heck, if I used stuff on television to justify my thinking on epidemiological matters, I would be laughed out of a profession… I mean, imagine if I advocated to call in the US Army to wipe out a town because they had an outbreak of hemorrhagic fever? (“ Outbreak “, 1995)
Alright, so we learned that an outbreak is defined by person, place, and time. (If you remember one of the first lessons of the night school was descriptive epidemiology, and the example there was learning to recognize an outbreak.) There are certain situations where you have a group of people who are sick at the same time, but they were never in the same place. We call that a cluster in time.
Not to be confused with a cluster OF time.
There are other situations when a group of people who live in the same area get sick from the same pathogen (or condition) but at different times. We call that a cluster in space.
We call this “Lost in Space”
To be an outbreak, you need to establish associations between person, place, and time. Sure, the cluster in time may turn out to be an outbreak once you find out through questionnaires and interviews that they all bought the same brand of milk, albeit from different retailers. Or the cluster in space may turn out to be an outbreak once you notice that something has been leaking into the environment over a long period of time.
So, when trying to determine if something is an outbreak, consider how many cases you’re looking at compared to previous years (or other periods of time), consider their geographic spread, consider common exposures of attributes (like gender, race, ethnicity, social status, grade in school, etc.), and consider them all human beings worthy of your caring and best honest effort to bring the outbreak under control and prevent it from ever happening again – like with vaccines and stuff.
Thanks for this. Sad to see Rene's employers not back him in his endeavours.
David N. Andrews M. Ed., C. P. S. E.:
@Sharon ... agreed.