I just read this article by Dr. Brownstein which very clearly explains the truth.
What I found very helpful and important to understand is the difference between relative risk and absolute risk reduction. We have to understand these numbers so that when we see reports in the media we can distinguish the truth of what they really mean.
Simply put relative risk as explained in Patient.info is:
Relative risk compares the risk in two different groups of people. For example, the groups could be smokers and non-smokers. Research shows that smokers have a higher risk of developing heart disease compared to (relative to) non-smokers.
So, for example: If men have a 2 in 20 risk of developing a certain disease by the time they reach the age of 60 and research shows that a new treatment reduces the relative risk of getting this disease by 50%; the 50% is the relative risk reduction, and refers to the effect on the 2 not the 20. Fifty percent of 2 is 1. So this means that the relative risk is reduced from 2 in 20, to 1 in 20. If you didn’t understand that, you might think it reduced the risk from 20 to 10. If that isn’t clear – it can be very misleading.
Absolute risk is: Absolute risk of a disease is your risk of developing the disease over a period of time. We all have absolute risks of developing various diseases such as heart disease, cancer, stroke, etc. The same absolute risk can be expressed in different ways. If you have a 1 in 10 risk of developing a certain disease in your life this can also be said to be a 10% risk, or a 0.1 risk.
Using the example from Dr. Brownstein’s post on the flu vaccine, the true reduction of the flu vaccine in preventing influenza-related medical visits across all age groups is around 1%. A 50% relative risk reduction means that the absolute risk reduction is probably around 1%. This means that the flu shot failed in 99% who took it. So,100 people must be injected to prevent one case of the flu, leaving the other 99 unprotected. A very different picture from the one the media portrayed. (Finish Reading here)