Myocardial infarction admissions were six times more likely to occur in the week after a positive test for influenza than in the year before or the 51 weeks after the infection, according to analysis of a Canadian cohort that links laboratories with administrative databases.
The investigators used these cohort data to define definitions of “risk interval” – the first seven days after flu detection – and a combined
“control interval” – 52 weeks before the flu detection and 51 weeks after the end of the risk interval.
Among the total of 364 hospital admission for MI in patients with confirmed influenza, 20 occurred during the defined one-week risk interval (20 admissions/week) and 344 occurred during the control interval (3.3 admissions/week), giving an incidence ration (IR) of 6.05.
There was little difference between days one and three after flu confirmation (IR, 6.3) and days four to seven (IR, 5.8), risk dropped off quickly after that, with IRs of 0.6 at days eight to 14 and 0.75 at days 15-28. Risk was increased for older adults, those with influenza B infection, and those who had their first MI, the investigators said.
MI incidence also was elevated after infection with noninfluenza respiratory viruses, although to a lesser extent than with influenza, which suggests that “influenza is illustrative of the role that acute respiratory infections have in precipitating acute myocardial infarction.”