It’s been suspected as a trigger for heart attack for many years, but it has taken until now for researchers to confirm that influenza infection is linked to acute myocardial infraction .

In a case control study involving 364 cases of acute myocardial infarction in Ontario, researchers found a six-fold higher risk of the coronary event in the seven days after laboaratory-confirmed diagnosis of influenza.

The study, published in the NEJM, also found smaller increased risks of acute MI following other acute respiratory tract infections such as respiratory syncytial virus  (RSV).

In a review of almost 148,000 influenza tests in people over 35, of which almost 20,000 were positive, they found that the average rate of hospitalisation for acute MI over a 12 month period was three per week. However in the seven days following  influenza infection the rate of acute MI was 20 per week.

The incidence ratios for acute MI within seven days were 10 for influenza B,  5 for influenza A, 3.5 for RSV, and 2.8 for other viruses. The incidence of acute MI after influenza also appeared to be higher for people over the age of 65, though the result did not reach statistical significance.

The researchers said the findings supported the hypothesis that acute respiratory infection may cause anacute coronary syndrome through acute inflammation, biomechanical stress, and vasoconstriction.

“Infections create a thrombogenic environment through platelet activation and endothelial dysfunction. Furthermore, infections increase metabolic demand and may induce hypoxemia, hypotension, or other stress on the vascular system that can lead to the development of an occlusive thrombus and subsequently an acute coronary syndrome,” they said.

And while earlier studies had shown associations between acute MI and doctor visits for influenza-like illness, “it is important to confirm the association between influenza and acute myocardial infarction because cardiovascular events triggered by influenza are potentially preventable by vaccination,” they added

“Our findings, combined with previous evidence that influenza vaccination reduces cardiovascular events and mortality, support international guidelines that advocate for influenza immunisation in persons older than 65 years of age to protect against ischaemic coronary event,” they concluded.