Walgreen Co., the largest drugstore chain in the U.S., said Monday it is now offering seasonal flu vaccinations at all of its stores during pharmacy and clinic hours.

The shots will cost $31.99 in cash, or $29.99 for members of Walgreen's Prescription Savings Club. Medicare Part B covers the flu shots, and the company said health insurers including UnitedHealth Group Inc. and Cigna Corp. are also covering the vaccinations.

All Walgreen stores will also offer Fluzone HD, a high-dose vaccine intended for people over 65.

The vaccination protects against swine flu, influenza B, and the H3N2 strain. National drugstore chains like Walgreen and CVS Caremark Corp. have started giving flu shots earlier in the past couple of years following the 2009 swine flu outbreak. Flu season typically peaks in January and February.

The company said that for the first time, shoppers can use its website to schedule an appointment, although an appointment is not required.

Walgreen has said it is the largest provider of flu shots other than the federal government. It runs 7,742 stores across all 50 states, and about 350 of its locations have Take Care retail clinics. The clinics can give shots to patients age 2 and older.

The U.S. Centers for Disease Control and Prevention recommends that everyone six months and older get a seasonal flu shot every year unless they have severe allergic reactions to past flu shots or to eggs, which are used in growing the vaccine, or Guillain-Barre Syndrome, a rare condition that is sometimes linked to vaccines.

Walgreen administered about 6.4 million flu shots during the 2010-11 flu season. In the previous flu season, Walgreen gave about 7 million vaccinations.