Pandemic viruses tend to replace the previous influenza A strains although sometimes, they co-circulate with them.
Quote from scienceblogs.com
I've been asked a number of times why I am bothering to get both flu vaccines this year (the seasonal flu trivalent vaccine and the swine flu vaccine when it is my turn). I am in the older age group (last in line for swine flu vaccine) and it is my group that is hit the least hard from the swine flu virus. But there are a lot of us and we're still being hit. I don't know if I will be one of the unlucky few in my age group who draws the short straw or not, and I'd rather get vaccinated with an acceptably safe vaccine than take a chance in winding up having a machine breathe for me or not breathing at all. Moreover, we aren't sure what's going on with the relative lack of impact in my age group. It is possible (although truthfully I don't think it is the most likely thing) that we are still being infected and shedding virus in reasonable numbers but some kind of cross-reactivity obtained in a past infection or a long history of yearly vaccinations is sparing us more serious clinical outcomes. After all, about a third of flu cases are asymptomatic under any conditions. So why care? Because it might still mean I could pass it on to family, friends and co-workers.
Beside the answer that we don't know with confidence what the seasonal influenza viruses A/H1N1 and AH3N2 will do, there is another, very good reason that applies to people of all ages. The seasonal flu vaccine is a trivalent vaccine, meaning it has three components, one of which is influenza B. Influenza B can make you plenty miserable and it co-circulates with the other viruses every winter. Last flu season about a third of flu cases were flu B...
There is no reason to think the swine flu will squeeze influenza B out of circulation, although with all-things-flu anything seems possible. But I think it's unlikely and that means the seasonal flu vaccine will be providing significant protection against influenza B for whoever gets it...
The Editors of Effect Measure are senior public health scientists and practitioners. Paul Revere was a member of the first local Board of Health in the United States (Boston, 1799). The Editors sign their posts "Revere" to recognize the public service of a professional forerunner better known for other things.