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Skip Navigation LinksPennsylvania Department of Health > My Health > Emergency Preparedness > Pandemic > How Flu Becomes a Pandemic



In nature, different strains of the flu virus occur all the time among animals. Most of these flu viruses do not produce disease in people. But sometimes human flu viruses and those found in animals, like pigs and birds, mix together to produce new strains. Since the resulting virus is new to the human immune system, people have little defense against it. Some of these viruses, like the H5N1 bird flu, do not currently spread well in people. Others do. If a new flu virus spreads easily, it can produce a flu pandemic. The pandemic will likely spread quickly around the globe.

In the last 100 years, flu pandemics have occurred four times and spread around the world within months, even when most people traveled between countries by ship. In today's world, given the speed and amount of country-to-country air travel, a flu pandemic could spread much more quickly. The 2009 flu pandemic reached all continents in only 3 months.

What about Vaccines?

Right now, more than 100 national flu centers worldwide study disease trends and monitor for the flu year-round. Each year, before the flu season arrives, scientists choose the flu viruses they will include in the upcoming season's flu vaccines. They base their selections on which strains are circulating, how they are spreading, and how well the vaccines would help protect against the expected strains of seasonal flu.

  • Seasonal flu vaccines are updated yearly. They protect against three types of circulating flu viruses. Since flu pandemics are caused by a new strain of the virus, seasonal flu vaccines do not protect against them.
  • Pandemic vaccines can only be produced after a flu pandemic virus is identified. With current methods, it takes a number of months to produce vaccines for a newly recognized pandemic strain.