This work, led by Annemarijn de Boer, PhD, with the Julius Center for Life Sciences and Primary Care, UMC Utrecht in Utrecht, the Netherlands, comes 5 years after a 2018 study by Canadian researchers found a similar strong connection between flu and heart attack in people hospitalized for heart attacks.
The current findings will be presented by de Boer at the at this year's European Congress of Clinical Microbiology & Infectious Diseases in Copenhagen, Denmark, on April 18.
de Boer's team explains that the connection between influenza and heart attack lies in the influenza virus's ability to increase the stickiness or clotting of blood. That coagulation, along with inflammation from the body's immune response, can weaken fatty plaques in the arteries. If a plaque ruptures, they note, a blood clot can form and cause a heart attack.
"Respiratory infections in general can cause an obstruction like this," de Boer said in an interview.
The aim of this study was to replicate the study by Canadian researchers Kwong and colleagues but strengthen analyses by including mortality data across broader populations. The previous study did not include deaths from heart attack that occurred outside of the hospital.
de Boer used test results from 16 laboratories across the Netherlands (covering around 40% of the population), along with death and hospital records for broader analysis.
In the Netherlands, she said, flu testing and diagnosis is typically done in the hospital, so the patients who were studied were those with severe illness.
"Unfortunately, we were not able to look at flu in the general population," she said.
The laboratories confirmed 26,221 cases of influenza between 2008 and 2019. Among those with influenza, 401 people had at least one myocardial infarction within 1 year of flu diagnosis (a total of 419).
Of the 419, 25 cases were in the first 7 days after flu diagnosis, 217 within the year before diagnosis, and 177 occurred in the year after influenza diagnosis (excluding those in the first 7 days.)
About one third of the people (139 in 401) died of any cause within 1 year of flu diagnosis, the authors write.
The Dutch researchers calculated that study participants were 6.16 times more likely to have a heart attack in the week after an influenza diagnosis than they were in the year before or after. The Canadian researchers found that they were 6.05 times more likely to have a heart attack in that week.
But, when the Dutch researchers excluded the mortality data outside the hospital, just as the Canadian researchers did, they got a much lower but still significant risk number - 2.42 times the risk for heart attack in the first week instead of 6.16 times the risk.
The explanation for the different results when the same parameters for the data were used may come from the fact that testing for flu in outpatient settings is less common in the Netherlands than it is in Canada.
Researchers say the findings show how different the results can be with incomplete data.
de Boer said that it is important for patients and physicians to be aware of the connection and to watch for any heart attack symptoms.
Sherif Mossad, MD, with the Department of Infectious Disease at the Cleveland Clinic in Ohio, told Medscape Medical News, "It is not surprising that the Dutch study found only 2.4 times the increase in heart attacks within one week following flu diagnosis when excluding out-of-hospital deaths, since only a proportion of these deaths are expected to be due to heart attacks."
Mossad said that the findings add more robust evidence that the flu is not always a mere inconvenience with a short duration. They also support the necessity of vaccination, he said.
"[This study] raises more awareness about the seriousness and consequences of influenza," he said. "It could precipitate serious, life-threatening problems."
He pointed out that previous research has demonstrated that influenza infection can exacerbate underlying conditions such as chronic obstructive pulmonary disease, liver and kidney disease, and multiple sclerosis.
Mossad noted that the data were collected before the pandemic and said that the numbers of people with influenza having heart attacks in the pandemic might be even higher than what these numbers show.
During the height of the pandemic, he explained, people experiencing chest pain or other symptoms were less likely to come into the hospital for care, most likely due to fear of COVID-19 infection.
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