A pandemic is an epidemic that spreads rapidly around the world with high rates of illness and death. Although people are exposed to different strains of the flu virus every year, history has shown that several times each century, entirely new flu strains develop.
Because no one has had a chance to develop immunity to the new flu strain, it can spread rapidly and widely. If the changed virus causes serious illness and easily spreads from person to person, a pandemic can occur.
Pandemics are different from seasonal flu outbreaks. Seasonal flu outbreaks are caused by small changes in influenza viruses that people have already been exposed to. A new flu vaccine is developed each year to protect people against the expected changes in existing viruses. That's why annual flu shots are needed and are effective.
But since an influenza pandemic is caused by an entirely new strain of flu virus, preparing a vaccine in advance is not as simple as it is for seasonal flu.