While a strain of H1N1 virus is one of the three flu viruses contained in the annual flu vaccine, it does not match the strain of swine flu that has been making people sick, and so the vaccine will likely not provide full protection against the flu.
"It's unclear at this time whether previous flu shots or having had the flu in the past will protect you from swine flu," said Dr. Christopher Ohl, an associate professor of infectious diseases at Wake Forest University Baptist Medical Center.
"Certainly, to have full protection is not going to be possible," he said. "However, it may be that some partial protection may be provided by earlier shots or having had the flu."
Seasonal flu vaccine development is well under way, but the first viruses from which manufacturers will make swine flu vaccines are only being delivered this week.
The CDC expects those to be made and tested at the end of June and then made so that they can be available in the fall, when flu season begins.
At that time, CDC officials will be tracking the swine flu to see if they need to deploy the swine flu vaccine.
So, to be fully protected against flu strains likely to be around next flu season, you will likely need more than one vaccination.
As the WHO notes, Level 5 -- the current level for swine flu -- indicates that the disease has spread from person to person and a pandemic is considered "imminent."
At this point, the WHO advises countries to be prepared for a possible pandemic.
A pandemic is not considered to be under way unless the pandemic alert level is raised to Level 6.
As the WHO notes on its Web site, "While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication and implementation of the planned mitigation measures is short."
Do you want to know more about swine flu? Visit the ABCNews.com OnCall+ Swine Flu Center to get all your questions answered.