Dear Doctor:

Last year, my best friend, who was home in bed with a bad case of the flu, died of a heart attack. He was fit, strong and relatively young — only 42 years old — and his death was a shock. But now research suggests having the flu actually increases the risk of a heart attack. Can we protect ourselves?

Dear Reader: We’re very sorry for your loss and understand why it would be shocking. When we think of the complications that arise from the flu, it’s usually something like an ear or sinus infection. When things get more serious, flu can lead to pneumonia.

However, inflammation caused by the influenza virus can affect the body in even more severe and unexpected ways. This includes developing encephalitis or myocarditis, which are inflammation of the brain and the heart respectively, or sepsis, a full-body inflammatory response that can lead to multiple-organ failure and death. Individuals with the flu can also sometimes experience a worsening of chronic medical conditions, such as asthma or heart disease.

Now, as you mention, researchers have uncovered what they believe is a surprisingly strong connection between influenza infection and heart attack. In a study published last year in the New England Journal of Medicine, scientists at the Institute for Clinical Evaluative Sciences in Toronto reported that in the seven days following a lab-confirmed diagnosis of influenza, heart attacks were six more times as likely as they were either a year prior to or following the diagnosis. The researchers arrived at this conclusion using details from hospital admissions in Ontario between 2008 and 2015. They analyzed data from individuals with confirmed cases of influenza, as well as 364 heart attacks.

According to the data, the hospital saw 3.3 heart attack admissions per week in the year before and after a flu diagnosis. But during the week of a flu diagnosis, the heart attack rate rose to 20 admissions per week. Of the 332 people in the study who had a heart attack during the seven-day window following a flu diagnosis, 69 percent had not received a flu shot that year. The data also suggested that individuals older than 65 were at a slightly higher risk than younger people of suffering a post-flu heart attack. Other acute respiratory infections can also increase heart attack risk, although not as sharply as influenza, according to the study.

Previous studies have tied influenza infection to a three-fold increase in the risk of stroke. This range of extreme complications is believed to arise in response to the sudden and systemic inflammation that accompanies an acute respiratory infection.

The best way to protect yourself against the flu is by getting your annual flu shot. Research has shown that hospitalizations and deaths are markedly lower among people who get a flu vaccine when compared with those who do not. People who get the flu despite being vaccinated tend to have milder and shorter illnesses. Anyone who experiences heart attack symptoms during or shortly after an acute respiratory illness, including chest pain, arrhythmia, shortness of breath, exhaustion or edema, should seek immediate medical care.