Individuals who are diagnosed with flu are six times more likely to have a heart attack in the week after they test positive for the virus than they are in the year before or afterwards, a Dutch study being presented at this year’s European Congress of Clinical Microbiology & Infectious Diseases (ECCMID 2023, Copenhagen, 15-18 April) has found.
The link between flu and heart attacks has been made before, in a 2018 Canadian study of individuals hospitalised for heart attacks. The Canadian study, however, did not include information from death records which meant out-of-hospital deaths from heart attacks were not included.
More complete picture
Dr Annemarijn de Boer, of the Julius Center for Life Sciences and Primary Care, UMC Utrecht, Utrecht, The Netherlands, used the results of tests from 16 laboratories across The Netherlands (covering around 40% of the population), along with death and hospital records to produce a more complete picture.
26,221 cases of influenza were confirmed by the laboratories between 2008 and 2019. 401 individuals had at least one heart attack (myocardial infarction) within one year of their flu diagnosis (419 heart attacks in total).
Of the 419 heart attacks, 25 were in the first seven days after flu diagnosis, 217 within the year before diagnosis and 177 in the year after flu diagnosis (not including the first seven days.)
Around a third of the individuals (139/401) died, of any cause, within a year of being diagnosed with flu.
The researchers calculated that the individuals studied were 6.16 times more likely to have a heart attack in the seven days following a flu diagnosis than in the year before or after. This compares to a figure of 6.05 times in the Canadian study.
However, excluding data from death records, as in the Canadian study, reduced the increase in heart attack risk in the first week to 2.42 times. The researchers say this illustrates the impact that incomplete data can have on study findings.
They add that differences in testing practices in the two countries may help explain why they found the association to be weaker than in the Canadian study. Testing for flu in out-of-hospital settings is less common in the Netherlands than in Canada. However, the association is still significant and, by using similar methodology to the Canadian team, they’ve been able to confirm that the increase in risk applies across different populations.
The influenza virus is known to increase the coagulation (the “stickiness” or clotting of blood). It is thought that this, along with the inflammation that is part of the body’s immune response against the virus, can weaken fatty plaques that have built up in the arteries. If a plaque ruptures, a blood clot can form, blocking the blood supply to the heart and causing a heart attack.
Dr de Boer concludes: “With the potential public health implications of an association between influenza virus infection and acute heart attacks, showing robustness of results in a different study population is important.
“Our results endorse strategies to prevent influenza infection, including vaccination. They also advocate for a raised awareness among physicians and hospitalised flu-patients for symptoms of heart attacks.
“While it isn’t clear from our results if those with less severe flu are also at risk, it is prudent for them to be aware of the link.”