Flu and heart attack risk

Catching the flu can double the risk of heart attack and stroke, reports The Daily Telegraph . People are “four times more likely to be affected within three days of falling ill with the flu and are at double the risk for up to a week”, the newspaper said. The Telegraph goes on to say that the risk did not depend on how old people were, or what sex they were  and the research renewed calls from experts and campaigners for all those with heart disease to have the flu jab to minimise risk.

The story is based on a large study of about 20,000 people who had had a heart attack or stroke. However, because of a study design, it is not possible to say that flu is a cause of heart attacks or stroke but the association shown by this study is strong enough to suggest a further reason for immunisation aginst flu in at-risk individuals.

Where did the story come from?

Dr Tim Clayton and colleagues from the London School of Hygiene and Tropical Medicine conducted the study in association with IMS, a company that collects data from a range of health care providers. There is no detail on how the study itself was funded, although the British Heart Foundation paid for the publication charges. The study was published in the open-access peer-reviewed European Heart Journal .

What kind of scientific study was this?

was a case-control study which used data collected during visits of about two million patients to about 500 GPs. The information on the database is anonymous and the participants in this study had to be registered with their GP for at least two years.

The researchers identified all people who were recorded as having a first-time diagnosis of heart attack (myocardial infarction – MI) or stroke (cases) from the database. They then used the same database to pick “controls” for comparison. These people were matched in order to be similar to the cases in age and sex, practice and date (calendar time) of presentation to the GP. Details were gathered about visits to the GP for respiratory infection over the year before the heart attack or stroke and whether people had other risk factors for heart attack or stroke.

What were the results of the study?

There were 11,155 cases of heart attack and 9,208 cases of stroke in the database. Of all the cases, 62% were men with an average age of 71 years. There were 326 respiratory infections in the month before the date that people had a heart attack and 260 respiratory infections in the month before the date that people had a stroke.

The researchers say there is strong evidence of an increased risk of both events in the seven days after infection, based on an adjusted odds ratio of 2.10 for cases of heart attack and an adjusted odds ratio of 1.92 for stroke. These increases in risk mean that the chance of developing a first-onset heart attack or stroke within seven days of a respiratory infection is about double the chance of developing these illnesses without a respiratory infection.

The researchers note that the strength of these associations fell with time . For the association between respiratory infection and heart attack, the increased risk occurred across all the patients, no matter what other risk factors for heart they had. The odds ratio adjustments took into account age, sex, month of presentation (and therefore seasonal changes) and practice.

What interpretations did the researchers draw from these results?

The researchers conclude that there are strong associations between recent respiratory infection and major cardiovascular events. For heart attack, this happens no matter what other risk factors for heart attack the patients had. They claim that the "benefits of reducing respiratory infection either through immunisation or treating or preventing infection may be substantial."

What does the NHS Knowledge Service make of this study?

• It is worth thinking about an individual’s chance of actually developing a heart attack or stroke following a respiratory infection when interpreting this study. The data from the study suggest that amongst about 11,000 cases of heart attack, 84 people had a respiratory infection the week before (0.8%) and in the controls, 34 of the 11,000 people also had a respiratory infection (0.3%). These are low rates and suggest that, in general, an individual is unlikely to suffer a heart attack following a respiratory infection.
• The number of respiratory infections in the control group used for the comparison (34 out of 11,155 controls) is an important number for this analysis, as the odds ratio that was calculated depends on the validity of this rate. The case-control design of this study makes it possible that there were other influences at work to produce these differences and a question hangs over whether the ‘controls’ were the right comparison group.
• Confounding factors need to be taken into account in this study. Possible sources of confounding include differences in socioeconomic status or the frequency of attendance at the doctor, etc. It is possible that those chosen as controls with respiratory infections were from a group of healthier, more frequent attenders to the doctor, or from a socioeconomic group with easier access to a GP and therefore less likely to develop heart disease.
• The study also relies on accurate recording and coding of all attendances and the researchers do not report any measures of how well this was done. 
• The authors also acknowledge that the early symptoms of unstable angina may be confused with respiratory symptoms.

This study proposes an interesting association, but before any causal relationship between respiratory infection and cardiovascular disease is proven, randomised controlled trials of interventions aimed at preventing respiratory infections and cardiovascular disease need to be carried out.

Sir Muir Gray adds...

Because all the blood passes through the lungs, any lung disease puts a strain on the vascular system, so to lower the risk of heart disease or stroke, prevention of flu seems sensible.