This is an influenza virus. It is different from viruses like polio or yellow fever, where the vaccine lasts 10 years. Influenza viruses mutate several times each year. That is why for the seasonal flu, we adjust according to the virus that is out there. Why is it so important for so many Canadians as possible to be immunized against H1N1? Because we expect it to be around for many years to come. By giving immunization today, we are probably investing for the years to come. Bear in mind that that is a hypothesis.
The fact remains that for any vaccine, for this virus, we know that less than 10 days after the first dose, immunity is over 90%. We do not achieve that percentage for seasonal flu. Now, they want to ensure that at some point, for more than 21 days, it can be maintained in future years. That is part of the normal aspects considered by pharmaceutical companies. They always assess how long a vaccine will protect us.