Despite repeated E. coli outbreaks linked to undercooked ground beef, millions of Americans continue to favor their burgers slightly pink inside.

That’s the gist of a survey by researchers at the Centers for Disease Control and Prevention, whose results were published in the February edition of the Journal of Food Protection.

Thousands of Americans have been sickened, and hundreds have died in recent years due to E. coli O157:H7 or other bacteria in undercooked ground beef traced to a variety of sources – from grocery stores to fast-food restaurants. Beef can become contaminated by cow feces at slaughterhouses or at other processing stages, and pathogenic bacteria can become interspersed throughout the meat when it is ground.

Consumers have been advised repeatedly that cooking hamburger thoroughly will kill harmful bacteria.

The CDC survey suggests that many Americans continue to ignore those warnings.

According to the CDC researchers, 8,543 consumers in 10 American regions were interviewed about their attitudes toward ground beef.  Of those, 75 percent reported eating ground beef at home, and 18 percent of them said the hamburger was pink when they ate it.

Extrapolated to the American public at large, that suggests that some 40 million Americans have consumed undercooked hamburger in their homes.

Researchers said men and people of medium incomes ($40,000 to $75,000)  were more likely to eat pink meat, as were people with graduate degrees. People over age 65 were less likely to eat hamburgers served rare.

Overall consumption of ground beef has not changed in recently surveys, the researchers report.

The authors note that their study had several limitations, including self-reported questionnaire responses, and no questions on consumption of pink ground beef prepared outside the home. Also, color is not a completely reliable indicator of ground beef doneness. Ground beef patties not only can turn brown well before they reach 160 °F, but some lean ground beef will remain pink well above the 160 °F final cooking temperature.

The safest way to test for doneness is with a tip sensitive digital thermometer. The questionnaire did not ask about thermometer use.