Investigators Discover Link Between Influenza Epidemics and Acute Cardiovascular Events

Eric Chow, MD

While it is well known that influenza could contribute to acute cardiovascular events during annual influenza epidemics, the actual burden of this association is not entirely known.

A team, led by Eric J. Chow, MD, US Centers for Disease Control and Prevention, examined acute cardiovascular events and determined the different risk factors for acute heart failure (aHF) and acute ischemic heart disease (aIHD) in adults with a hospitalization linked with laboratory-confirmed influenza.

In the cross-sectional study, the investigators examined data from the US Influenza Hospitalization Surveillance Network during the 2010-2011 through the 2017-2018 influenza seasons. The study population included 89,999 adults hospitalized with laboratory-confirmed influenza, which were identified through influenza testing ordered by a practitioner.

The researchers ascertained acute cardiovascular events using discharge codes from the International Classification of Diseases (ICD), Ninth Revision, Clinical Modification, and ICD, 10th Revision.

The investigators included age, sex, race/ethnicity, tobacco use, chronic conditions, influenza vaccination, influenza antiviral medication, and influenza type or subtype as exposures in logistic regression models.

The team also estimated marginal adjusted risk ratios and 95% confidence intervals to describe the factors associated with aHF or aIHD.

Among the nearly 90,000 adult patients, 80,261 had complete medical record abstractions and available ICD codes. The median age of this group was 69 years old and 11.7% of the study had suffered from an acute cardiovascular event.

The most common of these events were aHF (6.2%) and aIHD (5.7%). The investigators also found older age, tobacco use, underlying cardiovascular disease, diabetes, and renal disease were significantly associated with higher risk for the 2 heart disorders in adults hospitalized with laboratory-confirmed influenza.

The investigators did admit to some study limitations, mainly the under detection of cases was likely due to influenza testing based only on practitioner orders. In addition, because acute cardiovascular events were identified by ICD discharge codes, they may be subject to misclassification bias.

Earlier this year, investigators found the influenza virus infections could be linked to respiratory and non-respiratory diagnoses, highlighting the broad scope of infection burden.

Chow also led this study, with the same patient population. However, in this study, the investigators abstracted demographic data; chronic medical conditions; clinical course and outcomes; and discharge summary data from each patient’s medical record.

Of the patients with an acute diagnosis, 94.9% had >1 acute respiratory diagnosis and 46.5% had >1 acute non-respiratory diagnosis—5.1% had only acute non-respiratory diagnoses.

For the nearly 95% of patients with >1 acute respiratory diagnosis, 43.3% had underlying respiratory comorbidities and 51.3% were current or former tobacco users.

Those with only acute non-respiratory diagnoses had a significantly higher frequency of underlying comorbidities than those with respiratory diagnoses. Comorbidities included neurologic (29.9% vs 24.6%; P <.001); cardiovascular (51.2% vs 40%; P <.001); metabolic (51.5% vs 42%; P <.001); renal (33.3% vs 19.4%; P <.001); hepatic (6.5% vs 3.9%; P <.001); immunosuppressive (19.4% vs 16.8%; P <.001); and hematologic (5.8% vs 4.2%; P <.001) diagnoses.

A key takeaway from both studies in contracting influenza increases the risk of negative outcomes.

“In this population-based study of adults hospitalized with influenza, almost 12% of patients had an acute cardiovascular event,” the authors wrote. “Clinicians should ensure high rates of influenza vaccination, especially in those with underlying chronic conditions, to protect against acute cardiovascular events associated with influenza.”

The study, “Acute Cardiovascular Events Associated With Influenza in Hospitalized Adults,” was published online in the Annals of Internal Medicine.