The number of seasonal influenza cases has increased sharply in the United States this year, with 42 states reporting high flu activity, according to the Centers for Disease Control and Prevention. For the week ending January 27, the proportion of people seeing their healthcare provider for influenza-like illness (ILI) was 7.1 percent, which is above the national baseline of 2.2 percent and is approaching the 7.7 percent peak of the 2009 pandemic.

The CDC also reported an additional 17 flu-related pediatric deaths, bringing the total number of flu-related pediatric deaths reported this season to 53 so far. This elevated flu activity is likely to continue for several more weeks.

Since Oct. 1, 14,676 hospitalizations have been reported through the Influenza Hospitalization Network (FluSurv-NET), a population-based surveillance network for laboratory-confirmed influenza-associated hospitalizations. This translates to a cumulative overall rate of 31.5 hospitalizations per 100,000 people in the United States.

As if there isn't enough to worry about this season, a new study in The New England Journal of Medicine shows the flu may increase the risk of heart attack. According to Dr. Jeff Kwong, an epidemiologist and family physician with the Institute for Clinical Evaluative Sciences and Public Health Ontario in Canada, chances of a heart attack increase sixfold during the first seven days after detection of laboratory-confirmed influenza infection.

The risk may be higher for older adults, patients with influenza B infections and patients experiencing their first heart attack. The researchers also found elevated risk with infection from other respiratory viruses, although not as high as for influenza.

The researchers looked at nearly 20,000 Ontario adult cases of laboratory-confirmed influenza infection from 2009 to 2014 and identified 332 patients who were hospitalized for a heart attack within one year of a laboratory-confirmed influenza diagnosis.

The study used the self-controlled case-series design to evaluate the association between laboratory-confirmed influenza infection and hospitalization for acute myocardial infarction. They used various high-specificity laboratory methods to confirm influenza infection in respiratory specimens, and they ascertained hospitalization for acute myocardial infarction from administrative data.

Kwong and his team identified 364 hospitalizations for acute myocardial infarction that occurred within one year before and one year after a positive test result for influenza. Of these, 20 occurred during the risk interval, and 344 occurred during the control interval.

Study results revealed that the incidence ratios for acute myocardial infarction within seven days after detection of influenza B, influenza A, respiratory syncytial virus and other viruses were 10.11, 5.17, 3.51 and 2.77, respectively. A prominent connection was noted between respiratory infections, particularly influenza, and acute myocardial infarction.

According to Kwong, the hypothesis that influenza may trigger acute cardiovascular events and death has been around since the 1930s. However, the studies that evaluated this association were neither sensitive nor specific for influenza, and the few studies that did use laboratory-confirmed influenza as a measure were underpowered and had inconsistent findings.

This new study, on the other hand, evaluated the association between laboratory-confirmed influenza infection and acute myocardial infarction using the self-controlled case series study design.