A new university college London study warns flu viruses can spark heart attacks.
Researchers looked at nearly 40 previously done studies on the subject.
They found heart disease caused up to half of all unexpected flu deaths analyzed.
That, they say, means there's a consistent link between the flu and heart trouble.
They say cardiac patients in particular should get a flu shot, because it could ultimately save their lives.
Researchers say only about a third of heart patients in the United States regularly get vaccinated.
The study appears in the journal The Lancet Infectious Diseases.
The CDC recommends people with underlying medical conditions like chronic heart disease get both a seasonal flu shot and an H1N1 swine flu shot when it becomes available next month.