Visits to retail clinics -- the medical offices in places like CVS and Wal-Mart -- have quadrupled between 2007 and 2009, according to health economists Ateev Mehrotra and Judith Lave. Here's a chart from their new Health Affairs paper:

Those big spikes you see, every fall, mostly represent seasonal flu shots. The green line through the middle shows the steady rise of non-vaccine visits. With 5.97 million visits in 2009 -- each generating, on average, $78 in spending -- retail clinics now are a $460 million industry.

Americans seem to be using retail clinics for pretty routine care. The top reason for visits tend to be vaccines and respiratory infections. A sizable number of visits -- 44 percent -- happen during hours when doctors' offices tend to be closed, in the evening and over the weekend.

Mehrota and Lave see retail clinics as filling a much needed gap in care left by a primary care doctor shortage. "The clinics continue to provide mostly simple acute and preventive care, and they continue to serve a population of patients who do not report having a primary care physician," they write.

At the same time, there are also some concerns about what it means for coordinated care to have visits take place at a retail clinic. If a visit happens at a CVS clinic, for example, will a primary care doctor even know about it?

Increasingly though, the answer turns out to be yes: Retail clinics have begun partnering with nearby health-care systems to increase communication between care delivered in the doctor's office and at the store. CVS announced last year a new partnership with Minnesota hospitals to share data with its 24 clinics in the area. That could mean lower-cost and better-coordinated care -- two things most health-care experts would love to see more of.