Walgreen provided enough flu shots last season to protect a population roughly twice the size of Los Angeles.
CVS doled out more than 5 million, or double its total from a few years ago. And Wal-Mart, the world's largest retailer, offers flu shots at more than 4,200 of its U.S. stores that have pharmacies.
The nation's biggest drugstores and other retailers are grabbing larger chunks of the immunization market, giving customers more convenient options outside the doctor's office to protect themselves against the flu, pneumonia and other illnesses. In fact, nearly half of all flu vaccines provided to adults are now administered in non-medical settings like drugstores or worksite clinics.
But this push by retailers muscles into an area of health care that was once largely the domain of the family doctor. And that stirs some concern from doctors who want to stay tuned into the health of their patients and keep track of who has received a vaccine.
Doctors often use flu shot appointments to ask patients about chronic conditions such as diabetes they may be treating, said Dr. Robert Wergin, a family physician in Milford, Nebraska, and president-elect of the American Academy of Family Physicians.
"That relationship, knowing the patient, that's important," he said, adding that many doctor offices have worked to become more convenient by offering same-day appointments for flu shots.
Retailers say their pharmacists can notify a customer's primary care doctor that the patient had a flu shot, if the patient gives them the contact information, and their expansion into vaccinations isn't meant to replace a primary care doctor visit.
"It's all about providing our customers with access to affordable health care," said Danit Marquardt, a Wal-Mart spokeswoman.
Retailers, some of which also offer vaccinations against diseases such as yellow fever and chickenpox, don't disclose how much they make off vaccines. They likely represent a small slice of overall profit, but their value extends beyond what they contribute to the bottom line.
"The vaccine is, I think ... a good marketing tool to bring people in," said Eric Keuffel, a Temple University health economist.
The first flu shot Lynn Bruggemann got was an impulse purchase she made in 2011 while visiting a CVS store to buy toothpaste and shampoo. It took about 15 minutes and was covered by her insurance. That convenience has brought the 50-year-old Wyckoff, New Jersey, resident back to drugstores every year since.
"If you try to make an appointment at a doctor's office, sometimes it's forever to get an appointment," she said.
In 1999, only 22 states allowed pharmacists to administer flu shots. A decade later, all 50 permitted the practice, after the outbreak of swine flu, or the H1N1 virus.
Drugstores, which had spent years opening locations nationwide, offered plenty of places for people to quickly get flu shots to guard against that global pandemic. Their stores are open on weekends and for longer hours during the day than a typical doctor's office. Plus, they can give shots without appointments.
"It really helped increase access to the pandemic vaccine," said Dr. Carolyn Bridges, associate director for adult immunizations for the U.S. Centers for Disease Control and Prevention. "That was, I would say, kind of the turning point."
The number of flu vaccines distributed in the United States has nearly doubled since 2000 to 134.5 million in the 2013 to 2014 season. Bridges said she isn't sure how much of this growth can be attributed to retail locations, but research shows that easy access is a big factor in adults getting immunizations.
Here are some common questions and answers from the CDC to consider as flu season approaches:
— Who should get a flu shot?
The government recommends annual vaccinations for all people ages six months or older.
— When should I get vaccinated?
The CDC prefers that you do this by October because it takes about two weeks for the protection to kick in. Cases of the flu usually peak between December and February, but the season can stretch as late as May and start as early as October.
— How long will the protection last?
Most healthy people produce enough antibodies to protect them through the season. But older people or those with weakened immune systems may have a shorter window for protection.
— What type of vaccinations are available?
People can get shots or nasal sprays. Some vaccines also are designed to protect against four flu viruses instead of the usual three. People ages 65 and older can get a high-dose version.
For more information, visit http://www.cdc.gov/flu/about/qa/flushot.htm.