Yet these high-risk adults are less likely to get the vaccine than others, the researchers told an American Heart Association meeting.
Adults over 50 who got flu vaccines during a hospitalization had a 28% lower risk of a heart attack the following year. They also had a 47% lower risk of a mini stroke or transient ischemic attack (TIA), 85% lower risk of cardiac arrest and 73% lower risk of overall death.
The researchers looked at the rate at which the flu vaccine was administered to hospital patients who are at high risk for flu and its complications. This includes people over the age of 50, HIV/AIDS patients, people in nursing homes and those who are obese.
Only 168,325 patients were vaccinated during hospitalization out of more than 7 million high-risk patients hospitalized, based on the 2014 National Inpatient Sample, the largest database of US hospitals.
About 1.8% of hospitalized adults over the age of 50 were likely to be vaccinated during hospitalization compared to 15% of the general population, according to researchers.
Vaccination rates were also lower for the other high-risk groups.
"The results we found are staggering. It's hard to ignore the positive effect the flu vaccine can have on serious cardiac complications," said Roshni Mandania, a medical student at Texas Tech University who led the study team.
"These groups should have the highest vaccination rates because they are the most at risk," Mandania added in a statement. "However, our findings show the opposite: flu vaccinations are under-utilized."
These preliminary findings only looked at immunization in the hospital setting, and it's possible some patients received the flu vaccine in outpatient care, according to the researchers.
The flu shot does more good
It turns out that the flu and pneumonia shots may also reduce a person's risk of developing Alzheimer's disease in the future, according to two separate abstracts presented at the Alzheimer's Association International Conference on Monday
There's no cure for Alzheimer's yet but but getting proper sleep, nutrition and exercise may influence a person's individual risk, research has shown. It's possible that getting vaccinated could fall into that category.
If getting flu or pneumonia vaccinations can reduce the risk of Alzheimer's, that's important to get out to the public, Alzheimer's Association Chief Science Officer Maria Carrillo told CNN.
"We do need more research to understand what that connection is," said Carrillo, who supervises research initiatives for the association.
"Is it direct, the vaccine to disease? Or is it protective, as a part of the risk reduction strategies that we have, like lower your BMI (body mass index), watch your sugar intake, keep an eye on your cholesterol and high blood pressure, exercise, get vaccinated," she added. "It's one of those sort of health tips that we need to make sure that our public knows about."