U.S. regulators approved the first flu vaccine that protects against four strains of the virus on Thursday. Food and Drug Administration officials approved the vaccine and expect the nasal spray to become available for the 2013-2014 flu season for people aged 2 to 49 years.

The new vaccine is a nasal spray that contains a weakened form of four flu strains. The weakened virus stimulates antibody production and immunizes against a virus strain without delivering the illness. Each year, flu experts choose which viruses to target with vaccines based on estimates of which strain is most likely to spread that year.

Conventional flu vaccines typically contained three strains - two vaccines against influenza A and one vaccine against influenza B. The new vaccine adds a second strain of influenza B.

Influenza A strains mutate quickly and are responsible for causing most flu outbreaks. Influenza A strains cause swine and bird flu, a strain blamed for the 1918 flu epidemic that killed an estimated 50 million people worldwide. Influenza B mutates more slowly than influenza A and often causes flu in children, according to the FDA. One strain of influenza C exists, but remains very rare.

Symptoms of the flu include fever, cough, runny or stuffy nose, body aches and fatigue.  

Flu infection rates fluctuate annually, but between 3,000 and 49,000 people died annually from the flu between 1976 and 2007, according to the Centers for Disease Control and Prevention. U.S. health authorities recommend getting the flu vaccine annually, and say it is the most important step in protecting against the virus.

CDC officials recommend those with the flu stay home and rest, drink water and other fluids to stay hydrated and treat fever and coughs with over-the-counter medicines.

Deaths, hospital days and outpatient visits from the flu cost the United Stated $10 billion annually, according to a 2007 study published in The Lancet. Flu-related costs could be reduced drastically if more people got the flu vaccine, according to the study.