Federal health officials have approved the first vaccine that protects against four strains of the common flu, offering one additional layer of protection against the influenza virus that affects millions each year.

The FluMist Quadrivalent vaccine from AstraZeneca's MedImmune unit protects against two strains of influenza A and two strains of influenza B. The Food and Drug Administration approved the spray-based vaccine for people ages 2 to 49.

Previously all vaccines contained two strains of influenza A and one strain of influenza B, chosen annually by medical experts based on their potential to spread the virus. Having an extra strain of influenza B increases the likelihood that the vaccine will protect against illness, the FDA said in a statement.

Similar to the previously approved FluMist, the new vaccine is a nasal spray that delivers weakened strains of the virus.

"Illness caused by Influenza B virus affects children, particularly young and school-aged, more than any other population," said Dr. Karen Midthun, M.D., director of the FDA's biologics center.

The severity of illness and death due to flu varies widely from season to season. Between 1976 and 2007, flu-related deaths have ranged from an estimated low of about 3,000 to a high of about 49,000 people, according to the FDA.