As a resident of Canada who has faithfully obtained the seasonal influenza vaccination nearly every year for the last 5 years, and the son of someone who works in the our provincial healthcare field, let me raise the following points about this study and the way vaccinations work in Canada, because it is entirely different that in the US or perhaps other areas of the world.
In Canada, it can be rather difficult to obtain the season influenza vaccination, since there tends to be a shortage every year. This means that people who are in at risk groups receive it first. These high risk groups include 1. Canadians aged 65 and over, 2. People with immunodeficiency diseases such as AIDS, HIV, etc., 3. People with diseases such as cancer, and 4. People with chronic conditions/diseases such as asthma, etc.
Canadians under the age of 65 with normal immune systems, and no diseases do not as easy access to seasonal influenza. I find it exceedingly hard to believe that he sample space is 12 million people as there are only 33 million Canadians. You can't tell me that an combined number of 12 million Canadians have received the seasonal influenza vaccination and/or received swine flu to allow this study to be possible. I propose the following reasons for people who received the vaccination are twice as likely are for the same reasons some people in Canada get "first dibs" on getting the vaccination in the first place.