How the flu increases the risk of heart attack and stroke

“The flu” is a respiratory illness caused by an influenza virus. Common symptoms include fever, chills, fatigue, body aches, headaches, coughing, sore throat, and congestion. In some cases, influenza only causes mild illness. In others, the flu can lead to serious complications, hospitalization, and death.

People with cardiovascular disease are considered at high risk for severe outcomes from the flu. Also referred to as “heart disease” and “heart and blood vessel disease,” cardiovascular disease is a group of many different conditions that affect blood flow throughout the body.

Studies have found that influenza infections significantly increase the risk of heart attack and stroke in people who have cardiovascular disease.

Influenza and arterial plaque

Most cardiovascular diseases involve atherosclerosis, or plaque buildup along the inner walls of the arteries. Plaque is made of cholesterol, fatty substances, calcium, cellular waste, and clotting substances. Plaque buildup causes the blood vessels to narrow, reducing blood flow and making the heart work harder to pump blood through the body.

When a person is infected with the influenza virus, the immune system responds by releasing a variety of inflammatory chemicals. This release of inflammatory chemicals prevents the spread of infection and clears away infected cells. However, it can also promote the formation of plaques and cause existing plaques to rupture and break loose. A plaque that breaks loose can block off a blood vessel, preventing blood and oxygen from reaching certain organs or areas of the body.

Influenza and blood clots

Influenza and other respiratory infections are also associated with an increased risk of blood clots. Clotting or coagulation is another one of the body’s defense mechanisms. It’s what stops blood vessels from leaking when there is an injury. For example, it’s what causes blood to go from liquid to gel-like to scab when there is a cut or abrasion. This type of clot will dissolve on its own as the injury heals.

Sometimes blood clots can form on the inner lining of blood vessels. These types of blood clots do not dissolve naturally and can interfere with normal blood flow. Like arterial plaques, clots can also break loose and cause blockages in blood vessels.

The immune response that occurs with influenza is believed to trigger an increase in substances that cause blood to clot, which means more clotting substances will be circulating in the blood.

A blood clot can also occur when arterial plaque ruptures. The rupture can cause injury to the blood vessels the plaque was attached to, and the clot forms as the body works to heal that injury.

Heart attack, stroke, and other complications

If a blood clot or a plaque breaks loose, it can partially or completely block blood flow to certain parts of the body and cause a number of serious and potentially life-threatening complications.

Blockage that affects blood flow to the heart causes heart attack, and blockage that affects blood flow to the brain causes stroke. Blockages can also occur in the blood vessels that supply the lungs (an event called pulmonary embolism) and other parts of the body, such as the major blood vessels of the legs.

Preventing complications from the flu

If you are living with any form of cardiovascular disease, it is important to take precautions to protect your health during flu season (fall and winter). Precautions include steps like frequent handwashing and avoiding crowded places. But the most important thing a person with cardiovascular disease can do to protect their health from the flu is to get an annual flu shot.

Flu shots prevent transmission of the flu, prevent severe illness if a person does become infected with the flu, and reduce the risk of heart attack and stroke among people who have cardiovascular disease.

Different types of flu shots may be recommended for different people, depending on a person’s age and health conditions. Inactivated injectable vaccines are the preferred choice for people with cardiovascular disease (nasal spray vaccines that contain a weakened-but-live virus are not preferred).

Your healthcare provider or pharmacist can advise you on what flu shot is recommended.