CHICAGO (Reuters) - Patients with heart failure are especially vulnerable to influenza and most doctors recommend they get flu shots, but a study suggests these annual jabs may not offer them full protection, U.S. researchers said on Saturday.

Flu vaccine drips out of a syringe as a nurse prepares for a patient at a clinic in central London November 22, 2005. REUTERS/Dylan Martinez

They found heart failure patients in a study had lower immune responses to the vaccine compared with healthy people of similar ages, leaving them more vulnerable to infection.

“What we theorize is that heart failure as a condition leads to impaired immune function, which renders these patients less able to respond to the vaccine,” said Orly Vardeny of the University of Wisconsin, who presented the study at the American College of Cardiology meeting in Chicago.

Vardeny and her team studied 29 patients with heart failure, a condition in which a heart damaged by heart attack or other causes gradually loses its pumping power. More than 5 million Americans have heart failure.

The researchers compared the heart failure group to 17 healthy people. Both groups had blood tests done before and after their flu shots to assess their immune responses.

The flu vaccine each year contains three strains of flu virus and is reformulated each year to protect patients from the circulating flu strains. The vaccine works by causing people to develop antibodies to the strains in the vaccine, which will protect them when they are exposed to the flu.

In the study, healthy patients had stronger antibody responses than the heart failure patients. Those with heart failure were less able to develop an immune response to the newest strain of flu in the vaccine.

“Patients with heart failure had a reduced response to the newest virus introduced to the vaccine, meaning they had an impaired immune response compared with healthy individuals,” Vardeny told reporters at a press briefing.

She said it isn’t clear what mechanism in heart failure is impairing this immune response.

“We’re not saying that patients with heart failure should not get immunized,” Vardeny said. “But maybe more preventive measures should be taken,” like booster shots and frequent hand washing to decrease the transmission of germs.

Influenza causes 36,000 deaths and more than 200,000 hospitalizations in the United States each year, according to the U.S. Centers for Disease Control and Prevention.