Influenza and Acute CV Events Can Go Hand in Hand

One in eight US adults hospitalized with influenza experiences an acute CV event, most commonly heart failure and ischemic heart disease, according to an 80,000-patient cross-sectional study. This knowledge, researchers say, should encourage wider vaccination against the flu and raise awareness among clinicians to screen for influenza in at-risk patients.

The study, published online recently in the Annals of Internal Medicine, was designed and conducted by the US Centers for Disease Control and Prevention (CDC).

“What most surprised us was the frequency of sudden, serious heart complications,” senior author Shikha Garg, MD, of the agency’s influenza division, told TCTMD in an email. These events had repercussions, she pointed out. “Among hospitalized patients with flu who had an acute cardiac event, almost one-third were admitted to the ICU and 7% ultimately died while hospitalized, underscoring the severity of cardiac complications that can be associated with flu infection.”

If anything, the retrospective analysis likely underestimates the connection between influenza and acute CV events, she said, since it captured only patients who were actually diagnosed with the viral infection. Many cases, and their fallout, may have gone undetected.

When encouraging patients to get their annual flu shot, ideally as early as September or October, it’s important to mention that influenza and other respiratory viral infections can lead to complications, Garg advised. “During flu season, clinicians should have a low threshold to suspect flu in patients who present with cardiovascular symptoms, regardless of the presence of respiratory symptoms.”

Mohammad Madjid, MD (University of Texas Health Science Center, Houston), said the results confirm earlier reports and add more details on the variety of cardiovascular events seen with influenza. This information can be used to counsel patients—and household members with whom they regularly interact—on their need to get a flu shot.

“The number one reason for people with heart disease not taking the vaccine . . . is that they do not know they are at high risk for flu complications,” he commented. “As doctors, we have to sit down and talk with them and tell them that this is because of their [existing disease].”

These discussions may have greater traction, both Garg and Madjid said, in the COVID-19 era.

Flu Shot Lowered Risk

The CDC investigators, led by Eric J. Chow, MD, examined data on 80,261 adults from the US Influenza Hospitalization Surveillance Network (FluSurv-NET) for flu seasons between 2010 and 2018, using discharge codes to identify acute CV events. Most common were heart failure (6.2%) and ischemic heart disease (5.7%), with 11.7% of patients experiencing at least one acute CV event; less common types were hypertensive crisis (1.0%), cardiogenic shock (0.3%), acute myocarditis (0.1%), acute pericarditis (0.05%), and cardiac tamponade (0.02%).

Events occurred in 20.6% of hospitalized patients with chronic cardiovascular disease, 19.3% of those with chronic renal disease, and 14.8% of those with diabetes.

Predictors of acute heart failure were older age, extreme obesity, current tobacco use, A-fib, chronic heart failure or cardiomyopathy, CAD, diabetes, and chronic renal disease. For ischemic heart disease, the predictors were largely the same, except that obesity and female sex were associated with lower risk and there was no link to A-fib.

Less than half (47.2%) had been vaccinated against influenza in the current season, while 39.2% had not (vaccination status was unknown for 13.6%). Compared with those who didn’t get a vaccine, people vaccinated at least 2 weeks before hospitalization had a lower risk of heart failure (adjusted RR 0.86; 95% CI 0.80-0.92) and ischemic heart disease (adjusted RR 0.80; 95% CI 0.74-0.87).

Patients who received late versus early antiviral treatment were at increased risk of both types of acute events, though the researchers did not have access to information on whether the antivirals were given before, after, or during the CV events.

The means by which influenza could be linked to CVD are many, Chandini Raina MacIntyre, MBBS, PhD (University of New South Wales, Sydney, Australia), writes in an accompanying editorial. “Influenza may unmask undiagnosed cardiovascular disease and may exacerbate known disease. It may trigger acute cardiovascular events (such as stroke or myocardial infarction), acting through direct viral effects on the myocardium, vasculature, and receptors and through indirect effects, including cytokine production, plaque disruption, acute thrombosis, vasoconstriction, tachycardia, and hypoxia.”

As reported by TCTMD, many of these same mechanisms have been cited in COVID-19.

In terms of future directions for research, Garg said it would be valuable to know what happens to people after they leave the hospital, such as how many die or are readmitted. Also, “prospective studies with systematic flu testing among persons hospitalized with acute cardiac events during flu season may help to define the underrecognized burden of flu in this population,” she suggested, adding that it’s also important to look into vaccine effectiveness in patients at risk for acute heart disease.

The COVID Connection

COVID-19’s seismic effects present an opportunity to raise awareness about influenza vaccination, said Madjid. Only 50% to 60% of people get a flu shot, he noted, a proportion that hasn’t budged over the decades.

Garg agreed: “Getting a flu vaccine is more important than ever this year—not only to reduce the individual patient’s risk from flu, but also to help lessen the burden on the nation’s healthcare system during the unprecedented COVID-19 pandemic.”

While the influenza vaccine of course cannot prevent COVID-19, it does decrease the risk of flu-related hospitalization and death, she pointed out. “A flu vaccine is the most important thing people can do to protect themselves, their loved ones, and their community from flu and its potentially serious complications. And now, more than ever, everyone needs to do their part to prevent the spread of respiratory illnesses like flu and COVID-19.”

What with social distancing, masks, and other precautions, this year’s flu season may well be moderate, Madjid said. Groups like the CDC, as well as the American College of Cardiology and American Heart Association, should seize the moment, though, and “join forces to see how we can start educational programs,” he stressed. “I think honestly the COVID-19 situation can bring more attention to this and improve vaccination [rates].”