Why Vaccinate? Because Flu Infection Increases Heart Attack Risk 6-Fold!

Vaccine opposition increased worldwide after the COVID-19 pandemic. Yet the discovery of vaccines, saving hundreds of millions of lives, has become one of modern medicine’s greatest achievements. The influenza vaccine is among these important preventive measures. Nevertheless, vaccines are blamed while the substantial increase in cardiovascular risk from respiratory infections themselves is forgotten. An important new study helps clarify this confusion.  

New research shows that flu infection increases heart attack risk sixfold. This once again highlights vaccination’s importance, particularly for people without a history of cardiovascular disease. Vaccination contributes to a healthier future by protecting both ourselves and our communities.

A Striking Study

A new study showed that influenza infection increases heart attack risk sixfold. It also found a particularly high risk in people not previously diagnosed with coronary artery disease. This was the higher-risk group identified.

Published on 25 June 2024 in NEJM Evidence, the study was led by Dr. Patricia Bruijning-Verhagen of the Netherlands, who said, “Our results confirm an increased heart attack risk during or immediately after acute severe influenza and suggest considering prophylactic anticoagulation [blood thinners] for these patients,” she said.

“Our results also suggest focusing on people without a history of cardiovascular disease and emphasize the importance of flu vaccination in this group,” she added.

In this observational case-series study, results from respiratory virus tests at 16 laboratories in the Netherlands, specifically PCR test results, were linked with national death, hospitalization, medication, and administrative records. Researchers compared acute heart attack incidence on days 1-7 after influenza (the risk period) with 1 year before infection and the 51 weeks afterward (the control period).

Study Details

The researchers identified 26,221 positive influenza PCR tests, representing 23,405 unique episodes. Examining acute heart attacks during the year before and after influenza, they found 25 events on days 1-7 after infection and 394 during the control period. Adjusted relative incidence during the risk period, days 1-7 after influenza, was 6.16 compared with control (95% CI, 4.11-9.24).

Relative acute heart attack incidence was 16.6 (95% CI, 10.45-26.37) in people not previously hospitalized for coronary artery disease, compared with 1.43 (95% CI, 0.53-3.84) in those previously hospitalized for it.

Several earlier studies reported a temporary rise in heart attack risk. A Canadian study by Kwong and colleagues in 2018 showed a sixfold increase after influenza, subsequently confirmed by studies from the United States, Denmark, and Scotland.

Bruijning-Verhagen and colleagues aimed to quantify the association between laboratory-confirmed influenza and acute heart attack further and examine subgroups that might guide more individualized prevention.

After laboratory-confirmed influenza, the first week brings a sixfold increase in acute heart attack risk.

A Surprising Result: People Without Cardiovascular Disease Are at Highest Risk

The researchers also examined risk differences between people with cardiovascular disease who did and did not have influenza.

Most earlier studies of flu and heart attacks did not distinguish between people with and without existing cardiovascular disease. Those that did could not demonstrate a difference confidently. There were suggestions of greater heart attack risk in acute influenza patients without known cardiovascular disease, but uncertainty remained.

The current study showed a substantial difference between these groups, with much higher influenza-associated heart attack risk in people without known cardiovascular disease.

One might expect patients with existing cardiovascular disease to be more susceptible to influenza-associated MI, making this a surprising finding.

Why Is the Difference So Large?

Influenza can cause conditions triggering heart attacks, including hypercoagulability (an increased tendency to clot), systemic inflammation, and vascular changes. This applies even to people previously thought not to be at risk. Acute infection also puts extra strain on the heart, increasing risk.

People already living with cardiovascular disease may be partially protected by their cardiovascular medications.

These results could justify short-term anticoagulation during the high-risk period in severe influenza. We give short-term prophylactic anticoagulation during surgery; this would be a similar approach. However, whether it works needs testing.

Clinical studies of such a strategy are currently underway.

Get a Flu Vaccine!

The results again emphasize vaccination for everyone eligible. They should add weight to the flu vaccination message. Even if you do not consider yourself at risk of cardiovascular disease, severe influenza may increase your heart attack risk.

Many countries recommend flu vaccination for everyone over 60 or 65 and younger people with cardiovascular disease. Vaccination data were unavailable in this study, but the average age was 74, meaning most patients were eligible.

In the Netherlands, where the study took place, vaccination is recommended for everyone over 60, with uptake around 60%.

Some cases will occur in younger people, but many more would need vaccination to demonstrate benefit, which may not be cost-effective.

Flu vaccination policies vary worldwide and consider many factors; some countries already recommend annual universal vaccination.

Vaccination is an easy, effective option for people who have not yet had their first heart attack. It is time to view flu vaccination as a routine preventive measure for people with acute cardiovascular risk factors, alongside statins (cholesterol-lowering drugs), blood pressure control, and smoking cessation.