Scientists say that the flu vaccination may "halve the risk of heart attack in middle-aged people with narrowed arteries."
The researchers from the School of Public Health and Community Medicine in Australia publish their study in the journal Heart.
The study analyzed 559 patients over the age of 40. Of these patients, 275 had suffered a heart attack, while 284 had not. All patients were referred to a tertiary hospital over consecutive flu seasons between 2008 and 2010.
Flu is a cause of significant morbidity and mortality, particularly in the elderly, during seasonal epidemics. The researchers add:
"Studies show an increase in rates of acute myocardial infarction (AMI) and death during the annual inﬂuenza season. Acute infections in general have been shown to be associated with an increased risk of AMI in observational epidemiological studies."
Nose and throat swabs, and blood samples were taken from the patients at the beginning of the study, and again 4-6 weeks later. Around 50% of all patients had flu shots within the year.
The results of these tests showed that around 1 in 8 (12.4%) of the heart attack patients recently had flu, compared with 7% of patients who did not have heart attacks.
'Protective effect' against ischemic heart disease
Heart attack patients showed double the risk of experiencing respiratory problems. But the researchers add that when taking into account other influential factors - including age, smoking and high cholesterol, there was no evidence to suggest flu increases the risk of heart attack.
However, they say the flu vaccination appeared to be protective against heart attacks, reducing the risk by 45%.
The study authors say:
"The estimated vaccine effectiveness of inﬂuenza vaccine against acute myocardial infarction was 45%, suggesting potential population health beneﬁts of vaccination in adults at risk of ischemic heart disease."
"While other studies have looked at the association of inﬂuenza, inﬂuenza vaccination and acute myocardial infarction, none estimate inﬂuenza vaccine effectiveness against acute MI."
Call for more awareness of immunization benefits
In the UK and Australia, people aged between 50 and 64 are not included in the national flu vaccination programs.
The UK vaccination program aims to to encourage people who are more susceptible to flu to have the vaccination. Those aged over 65 are offered the shot annually, as are those under 65 who are vulnerable for "clinical risk" reasons such as chronic illness.
The researchers say the findings from this study suggest that further investigation into extending the schedule may be called for.
"There has been little international policy debate surrounding the use of inﬂuenza vaccination in the prevention of ischemic cardiac disease morbidity in people less than 65 years of age," say the researchers.
"The role of an expanded vaccination program for adults over 50 years of age, which would capture a signiﬁcant proportion of people at risk of acute MI, should be explored by further research."
In the US, flu information from the Centers for Disease Control and Prevention (CDC) suggests that everyone over the age of 6 months should get a flu shot.
Certain groups are highly recommended by the CDC to receive the shot. These include:
- People who have certain medical conditions, such as asthma, diabetes and chronic lung disease
- Pregnant women
- People aged 65 and over Anyone at risk of developing more serious conditions, such as pneumonia, as a result of flu
- Carers and others in close contact with patients at high risk of complications due to flu.
The researchers call for clinicians to be more aware of flu as an underlying and poorly diagnosed infection in hospitalized patients. They also call for appreciation of the potentially preventive benefits of the flu shot in patients at risk of heart attack.
The authors say that flu had not been diagnosed in 10% of people who were found in the study to have the infection, adding that flu infection may be overlooked when patients are in hospital with other illnesses.