Indianapolis-based WellPoint Inc., the biggest U.S. insurer by enrollment, agreed to acquire CareMore Health Group to expand Medicare coverage in California, Arizona and Nevada.

CareMore, owned by CCMP Capital Advisors LLC, serves 54,000 members and operates 26 primary-care clinics, WellPoint said Wednesday in a statement.

The insurer paid almost $800 million, according to people familiar with the deal who declined to be identified because they weren’t authorized to release the information. WellPoint declined to comment on the sale price or provide other financial details, said Kristin Binns, a spokeswoman.

Medicare, the U.S. government health program for the elderly and disabled, is significant to WellPoint’s growth strategy, CEO Angela Braly said in the statement. The acquisition won’t help earnings until 2013, the insurer said.

WellPoint is the latest managed-care company to expand into providing medical care directly, said Thomas A. Carroll, an analyst at Stifel Nicolaus & Co. in Baltimore.

“Medicare Advantage is a nice long-term growth story from here on out,” Carroll said. “We’ve got a 15-year hill ahead of us where more and more people are going to become eligible.”

WellPoint stock was down fell 17 cents, to $75.76 per share, in afternoon trading.

Insurers offering plans and clinics “may end up being the new joint model moving forward,” said Jason Gurda, an analyst at Leerink Swann & Co. in New York. Humana Inc. acquired Concentra Inc., operator of more than 300 medical centers in 40 states, in December 2010 for $790 million.

On first impression, WellPoint appears to have paid “a substantial premium for the asset,” Ana Gupte, an analyst at Sanford C. Bernstein & Co., wrote in an e-mail, saying she needed time to evaluate what possible contributions other than membership CareMore represents for WellPoint. CareMore is based in Cerritos, Calif.

WellPoint is the third-largest provider of Medicare Advantage plans after UnitedHealth Group Inc. in Minnetonka, Minn, and Louisville-based Humana, according to Bloomberg data. Medicare Advantage is the name for the commercial managed-care plans marketed to Medicare beneficiaries.

Morgan Stanley served as the investment bank for the deal, according to WellPoint.