Can the flu cause heart damage?

A flu infection is commonly associated with familiar symptoms such as fever, runny nose, cough, body aches, and headaches. But the infection can also damage the heart and cause a heart attack.

Flu infection has a connection with serious cardiac events

Physicians have observed for a long time that in patients who have heart disease, getting the flu increases the chances of a heart attack. As early as the 1930s, they noted that influenza epidemics correlated with deaths from myocardial infractions (heart attacks).

The phenomenon was carefully studied by Dr. Jeffery Kwong and his colleagues in 2018. They identified 364 patients who had been hospitalized for heart attacks who tested positive for the flu a year before their heart attack or a year after it. They found that in this group, the incidence of hospital admission due to a heart attack was 6 times higher during the first 7 days after their flu diagnosis than at any other time the year before or the year after.

In another study, it was observed that out of 336,000 people who were hospitalized for the flu, 11.5% experienced a serious cardiac event such as a heart attack or cardiac arrest.

How flu infection can damage the heart

Reduced oxygen supply

According to Dr. Anthony L. Komaroff, MD, editor-in-chief of the Harvard Health Letter, the adverse effects of the flu on the heart may be attributed to a drastically reduced oxygen supply. When someone has the flu, their lungs can get infected which can result in lower blood oxygen levels which can in turn reduce the heart’s oxygen supply. This can be of critical consequence to people who already have health conditions that reduce the amount of oxygen that gets to their heart, such as those with atherosclerosis. Atherosclerosis is the buildup of plaque inside the arteries, a common condition for those over 50. The compounded lack of oxygen can lead to serious cardiac events.

Dr. Michael Sillis, MD, a board-certified cardiovascular specialist at Baylor Scott and White, also noted that the flu can strain the heart of individuals whose arteries are already blocked significantly. Flu-caused pneumonia is a lung infection that can similarly cause lower amounts of oxygen reaching the heart and can strain the heart due to a “mismatch” between the demand for oxygen by the heart and the supply from the lungs.

Flu virus can directly infect heart cells

In their seminal research published in 2022, Dr. Jacob Yount and colleagues showed that the flu virus could directly infect cardiac muscle cells. For their study, they used the influenza A (flu A) virus. Normally, the virus is capable of replicating both in the lungs and in the cardiac muscle cells. The researchers generated a version of the flu A virus that was unable to replicate in cardiac muscle cells, though it replicated normally in lung cells.

When mice were infected with this version, the respiratory symptoms in the mice were the same as a normal flu infection. However, they had reduced levels of electrical malfunctions or scarring of the heart compared to the mice that were infected with the wild-type (unchanged) flu A virus. The researchers also noticed that the infection by the wild-type flu A virus had to be quite acute and the viral load had to be over a certain threshold to produce symptoms in the heart.

These results indicate that the virus may somehow reach the heart and cause infection in cardiac muscle cells. It is not known if the virus infects other types of heart cells, but from these results, it appears that either it does not infect them, or the damage is quickly cleared by the mouse’s immune system.

Future studies will be needed to reveal if the two flu viruses (flu A and B) have similar effects on human cardiac muscle cells.

Who is at risk of flu-related heart damage?

Anyone who gets a bad enough infection from the flu virus can get heart damage. However, some groups are more susceptible. 

• Patients who have already been diagnosed with heart disease could have more severe cardiac symptoms from flu infection.
• Patients who have reduced immunity due to disease (e.g. HIV-positive individuals), and those who take immunity-suppressing drugs have a reduced natural capability to clear out the flu virus after infection. Therefore, the viral load may become high enough that it affects the heart.
• Individuals with diabetes that is not well-controlled have increased sugar levels in their blood. This weakens the immune system because the increased sugar forces the white blood cells of the immune system to work harder and be less effective.
• Most people after 50 have some amount of atherosclerosis (plaque starting to accumulate in their arteries), though many are not aware of it. Atherosclerosis can increase their susceptibility to heart damage due to the reduced oxygen supply to the heart.