A new study reveals that persons who have been diagnosed with influenza confirmed with laboratory diagnostic tests, are around six times more likely to be admitted for acute myocardial infarction (AMI) or heart attack within a week compared to their risk before they had had the flu.
The study titled, “Acute Myocardial Infarction after Laboratory-Confirmed Influenza Infection,” is published in the latest issue of the peer reviewed journal New England Journal of Medicine.
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The team of researchers looked at a large population or cohort to arrive at this conclusion. They noted that older individuals are more likely to have a greater risk of heart attack than younger persons after a bout of flu. This result of independent of the factor if the individual was vaccinated against flu or had a previous history of being hospitalized for a heart attack, they write.
The authors of the study also noted that flu as well as other respiratory infections can raise the risk of heart attacks. Dr Jeffrey C Kwong from the University of Toronto, ON, explained that this association between respiratory infections and AMI has been studied before and speculated upon. However, this was the first study that showed a definitive connection between lab-confirmed influenza and MI within 7 days.
Kwong and his team looked at the Ontario health insurance records and selected all the people who had tested for respiratory viruses from May 2009 to May 2014. They then noted the connection the infection had with MI. Kwong added that the fact that despite being vaccinated, this risk was unaltered, should not deter the people from getting vaccinated against flu. He explained that this study was not designed to see if the vaccine was protective against consequences of flu. He said, “if vaccinated patients have influenza of sufficient severity to warrant testing, their risk of acute myocardial infarction is increased to a level that is similar to that among unvaccinated patients.” This study adds to the public health message to get vaccinated against flu he said by showing what harm a bout of flu can bring about.
The study analyzed 364 hospitalizations for acute MI in 332 patients. These were all assessed within 1 year before and 1 year after a laboratory report had confirmed the presence of influenza. They noted that there were 48 percent women and the rest men. Of the population 24 percent had been previously hospitalized for MI. Among the 364 hospitalizations, 20 were seen to be during the first 7 days after a laboratory had confirmed flu from a respiratory specimen they noted. This 7 day gap is called the “risk interval”. The other 344 hospitalizations that occurred within the two year period were termed the “control interval”. Results from the analysis showed that the risk of MI related hospitalization rose by six fold during the risk interval when compared to the control interval. According to Kwong, the risk was highest during that 7 day period before falling to normal levels. For example the incidence ratio was 6.05 during the first week after the bout of flu and fell to 0.75 by 15th to 28th day after the flu. Further infection with influenza B carried a greater risk than influenza A they found.
According to Kwong, the reasons for this risk could be the rise in the inflammatory chemicals in the body during the flu. There may be rise of proinflammatory cytokines he explained which could lead to dysfunction of the walls of the arteries or endothelium. It could also rupture and dislodge the plaques that could be responsible for the MI. Detailed studies on this are necessary he added. As of now the mechanism is still a speculation but the risk is real.
Reference: http://www.nejm.org/doi/full/10.1056/NEJMoa1702090
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