Flu shots! They help a lot?!

Nowadays, many people are taking flu shots each year to prevent themselves from getting flu. Indeed, as I am going on the job market, one piece of advice I got is taking a flu shot. Well, the reasoning is looking for a job as an economist takes a lot of time and energy, and endurance -- It is a marathon, not a sprint -- and the least thing you want is getting sick during that long demanding process. Sure, I'll take a flu shot.

However, have you ever wondered, is it really effective to take a flu shot? I haven't thought about this question, as I have taken it for granted, a flu shot, of course, should prevent you from getting a flue, isn't it clear enough? Until the other day, I talked to a labor economist, and he started to mention how people usually take a correlational relationship as a causal relationship, and then he brought up the issue of the flu shot.

Economists (well, at least some of them) are known for being suspicious. They cast doubt on a lot of seemingly obvious conclusions: going to college brings you a higher salary? Not necessarily. People who go to college tend to earn a higher salary anyway, even without going to college. Did better economy or better policy cause the crime rates to fall by 50% in the early 1990s? No. It was because of Roe vs. Wade in 1973 and birth control (if you don't believe this, check the  book Freakonomics for more information. And still, you don't have to buy this. It is just something shown by analyzing the data).

How about flu shot? Well, what we observe from the data is that people taking flu shots are on average healthier. Is this better health, however, caused by the flu shot? It is really hard to say. Data also shows that people taking the flu shot tend to have fewer car accident, tend to have better marriages, tend to have better salary... If we just look at the data, you would claim that flu shots are the best invention ever -- it is the panacea for all troubles!

The main issue is, the relation shown by the data is just correlational. We cannot conclude that A causes B simply because A happened before B, or A and B happened together. People taking flu shots are those who take better care of themselves, and probably more responsible, and hence have better marriages, and fewer car accident. The labor economist claim that, without an experiment or a good instrumental variable*, we can never prove the effectiveness of flu shots. 

Again, several days later, I talked with an instructor from the department of health and kinesiology about flu shots, and asked what he thought about them. He then mentioned how experiments in the lab show the strain of viruses can be killed by the vaccine contained in the flu shots. I didn't ask for the details about how the experiments are conducted, and whether that results can be carried over from the lab to the human body, but it dawned on me that medical (natural) scientists do have a different approach in tackling a problem. Instead of straining their brains looking for a good instrumental variable or randomly assigned people to take the flu shots and check the effectiveness, they go directly to controlled experiments to study the virus. Nice and easy.

Well, that said, I will continue taking flu shots, if not for their effectiveness, then at least to prove that I am a responsible person.

BTW,  the health and kinesiology instructor did tell me the most effective way to prevent disease: wash your hand, thoroughly. He also mentioned a statistic that you may not be comfortable to hear: about one third of the people do not wash their hands after using the bathroom.

I will not continue to argue whether people who wash their hands are more responsible people, but I do have  the gut feeling that washing your hands might be more effective to prevent disease than taking a flu shot. 

*An instrumental variable  is something used to establish a causal relationship from a correlational relationship. Come and take my econometrics class if you want to know more (https://sites.google.com/site/huanrenzhang/teaching/econ360sp13)