Heart problems may account for a huge share of deaths from influenza, according to a study published on Tuesday that recommends cardiac patients be vaccinated against flu.
The paper, published in the journal The Lancet Infectious Diseases, reviews mortality figures for acute myocardial infarction -- a sudden heart attack -- and cardiovascular disease during outbreaks of flu between 1932 and 2008.
The current pandemic H1N1 virus was not included in the snapshot.
Between 35 and 50 percent of the increase in deaths recorded during influenza outbreaks could be attributed to cardiovascular problems, it says.
The authors, led by infectious disease epidemiologists Charlotte Warren-Gash and Andrew Hayward at University College London, say the flu virus causes inflammation and acts on the molecular pathways that control blood coagulation.
These effects could destabilise fatty deposits that line the arterial wall and cause clots that block coronary arteries, they say.
Only a few investigations have been carried out into whether flu vaccination helps protect cardiac patients, but the little evidence available suggests it does, the paper says.
"We believe influenza vaccination should be encouraged wherever indicated, especially in those people with existing cardiovascular disease," it says.
At present, vaccines for "seasonal" flu are recommended in many countries for individuals with chronic medical conditions. They generally include cardiovascular disease but not other cardiac problems such as hypertension.
Relatively few people at risk take up the vaccine, though. In Britain, only 47.2 percent of people with chronic conditions received the jab for seasonal flu, according to figures quoted in the study.