Overview

Influenza, more commonly known as the flu, is a contagious respiratory disease known for its seasonal outbreaks. It primarily affects the respiratory system; however, research has found that contracting influenza can increase the risk of complications with the heart and the cardiovascular system. Influenza can also worsen existing heart conditions which can lead to life-threatening heart attacks. Both influenza and heart disease are prevalent and impactful conditions worldwide, and understanding the connection between these illnesses can improve healthcare for patients.

What is influenza?

Influenza is a respiratory infection that mainly affects the nose, throat, and lungs. It is a common illness found all over the world, with approximately 1 billion cases of the flu every year. The virus is most commonly spread through the inhalation of water droplets found in the air from someone sneezing or coughing and can be spread when touching dirty surfaces. Influenza is caused by the influenza virus, which has 4 reported variants; A, B, C and D. There are slight differences between each type:

• Type A: The most common variant that is known to cause seasonal flu outbreaks, accounting for roughly 75% of flu cases. Type A influenza can affect both humans and animals and is the only variant known to cause pandemics. 
• Type B: Less common than Type A influenza but still highly contagious and can cause dangerous side effects to health. Is only transmitted between humans.
• Type C: Generally causes mild symptoms similar to a cold.
• Type D: Mainly affects cattle and other animals, but not humans.¹ 

Symptoms of influenza

Generally, the symptoms of influenza develop quickly and may come without notice. For most people, symptoms should clear within about a week. Symptoms can include:

• Fever (38 °C or more)
• General aches and pains
• Fatigue
• Dry cough
• Headaches
• Runny nose
• Sore throat
• Sneezing
• Body chills²

What is heart disease?

Heart disease, or cardiovascular disease, covers a wide range of diseases that revolve around the heart and the blood vessels. The most common heart disease includes:

• Coronary heart disease – A condition where the blood supply to the heart is disrupted due to the buildup of fatty deposits in blood vessels. These fat deposits called plaques can cause blood clots in severe cases. This reduces the amount of oxygen-rich blood reaching the heart, causing pain and even heart attacks.
• Angina – Chest pain caused by reduced blood flow to the heart. Angina can either be stable with a known trigger or unstable which is more predictable.
• Heart attacks – Also known as myocardial infarction, heart attacks are serious emergencies where the blood supply to the heart is blocked, causing damage to and the eventual death of heart cells.

Causes of heart disease

Each disease is different and will have a specific cause(s) attached to it.

Several heart diseases are attributed to the narrowing of blood vessels. One common cause of this narrowing is the buildup of plaques caused by a process called atherosclerosis, where fats, cholesterol, and other substances in the blood collect in and around vessel walls.³ This forms a plaque that narrows the opening of a blood vessel and reduces blood flow through that vessel, disrupting the functioning of the heart.

Over time, the plaque may rupture and detach from the blood vessel wall, and form a blood clot. If this blood clot completely blocks a blood vessel, this causes a heart attack.

What is the link between influenza and heart disease?

Heart disease is an established risk factor for serious flu complications, implying there is an inherent link between the two conditions. Some research suggests that contracting influenza can worsen cardiovascular issues by triggering a cardiovascular event or worsening an existing issue. 

If an elderly person develops even the mildest case of influenza, the likelihood of a cardiovascular event, such as a heart attack or stroke, can increase 2-fold in the first 2 weeks of infection. In vulnerable patients with severe cases, the likelihood increases to more than 4-fold in the same period.⁶

One study in 2018 found a significant link between respiratory diseases and heart attacks. After analysing cases of laboratory-confirmed influenza and hospitalisation records, they concluded that patients were 6 times more likely to be hospitalised for a heart attack in the first 7 days of being diagnosed with the flu.⁴

The CDC also examined cardiovascular events in adults who were hospitalised from influenza from 8 different flu seasons. This study found that roughly 12% (about 1 in 8) of patients experienced a serious issue with their heart or cardiovascular system, such as acute heart failure or ischemic heart disease.⁵ 

The mechanisms between influenza and heart disease

Research suggests that the influenza virus either directly affects the cardiovascular system or causes inflammation, causing cells to die.  

Effects on the cardiovascular system

The influenza virus can enter the cells in the artery walls and endothelial cells. Molecules on the artery wall interact with the virus, causing white blood cells called leukocytes to migrate to the area. Also, the influenza virus can cause smooth muscle cells in the heart to release cytokines. Cytokines are chemical messenger molecules that cause specific interactions and effects between cells. These cytokines can be pro-inflammatory and cause inflammation, which damages cells and can contribute to acute cardiovascular events. 

Direct effects on heart cells

The entry of the influenza virus into cells is dependent on trypsin, an enzyme that speeds up digestion. Trypsin can be found at elevated levels during an influenza infection and can induce myocarditis and inflammation of the heart muscles. Influenza cells can cause fibrosis in heart cells and cell death, which can lead to a reduction in cardiac function and an increased chance of heart failure. 

Influenza and plaques

The presence of the influenza virus triggers the migration of macrophages to the affected area. Macrophages can interact with cholesterol in the arteries and create foam cells, these cells form the core of plaques. Alongside this, smooth muscle cells of the arterial wall produce a cap made of fibres that engulf the plaque and increase the chance of a plaque rupture.⁷ 

How can you manage influenza and heart disease? 

One simple and easy action that will reduce the chance of contracting influenza is to get the flu vaccine. The vaccine should be available at most pharmacies and GPs and greatly reduces the risk of serious complications or hospitalisations associated with influenza. The vaccine is particularly recommended for anyone over the age of 65, those who are pregnant, and those with chronic heart conditions. 

If someone you know has the flu, avoid direct contact with them and keep your distance from crowded places during flu season. This greatly reduces the risk of getting influenza from coughing or sneezing. Also, practising proper handwashing and sanitising will help prevent infection. 

If you have any concerns about your health, talk to your GP or any health practitioner, and they should be able to offer you guidance on how to better protect yourself. They may prescribe medication or suggest lifestyle changes for you. 

Conclusion

Heart disease and influenza are both very common diseases found throughout the world and have a huge impact on public health. Despite being classed as different conditions, they seem to be more closely related than previously thought. Influenza has been found to increase the risk of heart attacks and strokes, and research has suggested mechanisms that link them. Understanding how to manage influenza with a heart condition is crucial in protecting a person’s health and well-being.