It starts with a runny nose. Maybe it’s just allergies, you think.

Then fatigue. It has been a long week at work or school. A little rest will take care of that.

But once the body ache starts, there is no more denying it: you have the flu.

It’s going to be a rough couple of days, but you’ll get through it. A few days on the couch, watching TV and drinking tea. Maybe a quick trip to the coronary care unit at your local hospital to treat your heart attack.

Wait….what? Heart attack? From the flu? Seriously?

Seriously. That little virus is a big problem, especially for your heart. How does that happen and what can you do to prevent it?

This digitally-colorized, negative-stained transmission electron microscopic (TEM) image depicts the ultrastructural details of an influenza virus particle, or “virion”. A member of the taxonomic family Orthomyxoviridae, the influenza virus is a single-stranded RNA organism. Credit: Frederick Murphy, CDC.
This digitally-colorized, negative-stained transmission electron microscopic (TEM) image depicts the ultrastructural details of an influenza virus particle, or “virion”. A member of the taxonomic family Orthomyxoviridae, the influenza virus is a single-stranded RNA organism. Credit: Frederick Murphy, CDC.

How does influenza work?

The influenza virus enters the body through the respiratory system when airborne virus particles are inhaled. The body has a number of protective mechanisms that reduce the ability of the virus to infect cells. When the amount of virus overwhelms the body’s defenses, the infection begins.

Influenza viruses have two key proteins for infection: hemagglutinin and neuraminidase. Hemagglutinin binds to cells in the body and helps the virus get inside. Influenza typically first infects a type of cell in the respiratory system called epithelial cells. Once inside, the virus hijacks the machinery of the cell and uses it to reproduce.

A protein produced by the virus called neuraminidase helps it escape from inside the cells it infects so that it can move on and infect more cells. There are many types of hemagglutinin and neuraminidase, and influenza viruses are named based on the types of proteins they have. For example, the H1N1 influenza virus has type 1 hemagglutinin and type 1 neuraminidase.

How does the vaccine work?

For people with a healthy immune system, influenza is not a big threat. Immune cells in the body recognize proteins on the surface of the virus as foreign. These proteins that start the immune response are called ‘antigens’. Immune cells including macrophages, B-lymphocytes and T-lymphocytes work together to kill off infected cells. Without the ability to use cells to reproduce, the virus dies out.

Antibodies mark pathogens for destruction by phagocytic cells, they coat key sites on pathogens necessary for infection, and they induce the complement cascade to react against antibody-bound pathogens. Credit: selvanegra.
Antibodies mark pathogens for destruction by phagocytic cells, they coat key sites on pathogens necessary for infection, and they induce the complement cascade to react against antibody-bound pathogens. Credit: Selvanegra.

Vaccines work by tricking the immune system into thinking a virus has infected the body. There are many types of vaccines, but they all contain antigens for the virus they are trying to stop. The antigens are injected into the body and are recognized as foreign by immune cells called B-lymphocytes. The lymphocytes produce antibodies or ‘immunoglobulins’ that bind to the antigen and mark it for destruction by the T-lymphocyte. It may take several weeks, but the body builds up a supply of T-lymphocytes that ‘remember’ the antigen. Later, when a virus invades the body, it is quickly recognized and the immune system that was primed by the vaccine quickly disposes of it before it can cause a serious infection.

Are Vaccines Safe?

For most people, the reaction to a vaccine is highly controlled and is a minor stress. Symptoms of infection like fever are small or even unnoticeable. Some influenza vaccines are produced in chicken eggs and traces of egg protein may be found in the vaccine. However, allergic reactions to egg proteins in vaccines are very rare. The U.S. Centers for Disease Control and Prevention recommends that people with egg allergies receive the influenza vaccination from a healthcare provider who can recognize and treat any reactions.

Despite overwhelming evidence that vaccines are safe, a number of myths about the dangers of vaccines persist. One of the more common myths is the danger of formaldehyde poisoning from vaccination. Formaldehyde is sometimes used in the early stages of vaccine production. Although it is largely removed from the end product, some vaccines do contain small amounts of formaldehyde.

Not all influenza vaccines contain formaldehyde, but those that do have between 5 and 100 micrograms1. To put this in perspective: adults have about 12.5 grams of formaldehyde circulating in their blood naturally. Formaldehyde is actually a necessary chemical for the body to make proteins and nucleic acids. In fact, every day the body produces 50 grams of formaldehyde as part of its normal metabolism.

Some formaldehyde is taken in through foods that contain the chemical naturally. A single pear contains 120X the amount of formaldehyde that is in any vaccine. The bottom line is that the maximum amount of formaldehyde in any vaccine is several times lower than levels that occur naturally in the body.

How effective is the vaccine?

Influenza vaccines reduce hospitalizations and save lives. In 2016-17, just under half of the US population received the influenza vaccine. Even with relatively low coverage of the population, vaccination prevented 5.3 million cases of influenza and 85,000 hospitalizationsin this timeframe.

There is a catch. Despite their potential health benefits, influenza vaccinations are only effective 50-70% of the time. Why are some influenza vaccines less effective? Every year the types of influenza that appear change. Scientists and medical professionals who study viruses use a large amount of data to try and predict which influenza virus will be the most common any given year. Sometimes their predictions are accurate, sometimes they aren’t.

Even when scientists accurately predict which type of influenza virus will be the biggest risk for the year, the rapid evolution of viruses can cause significant problems. Viruses copy their genetic information and pass it to their offspring. In humans, the process of replicating genetic information is very tightly controlled, but viruses lack a proofreading mechanism. This allows for mutations to build up frequently. If enough mutations appear in the viral genes that produce the target antigen, our immune system may not recognize the new mutated antigen. Vaccines designed to recognize viral antigens that have subsequently evolved into new forms via mutations are ineffective.

However, some studies show that even when the match between vaccine and viral antigen is not perfect, the severity of the illness is still reduced with vaccination. Especially for at-risk individuals, this makes getting the flu vaccine, despite the fact that it isn’t 100% effective, a good idea.

The Heart of the Matter

For many people, catching the flu is a minor inconvenience, other than fatigue, body aches and the always fun vomiting. It might require a few days of taking off from work or missing school. But for others, the flu can be a serious threat. In 2018, up to 646,000 people died from influenza infections worldwide3. People most at risk are over 75 years of age or under 5 years of age, and from economically poor countries.

The flu isn’t just about body aches. It can actually cause heart ache. Technically, those ‘heart aches’ are heart attacks. In a study of 20,000 Canadians, researchers from the University of Toronto found that within 7 days of being infected with influenza, the risk for a heart attack increased 600%!4 The increase in heart attacks occurred with other respiratory infections as well, but the risk was highest with influenza. Although other studies have reported a link between influenza infection and heart attacks, this study was strengthened by the fact that all cases were confirmed with laboratory testing, and didn’t rely on patients self-diagnosing.

How does influenza infection cause heart attacks? The short answer is that we don’t know. But there are some ideas. Infections can stress the body including blood vessels. This can cause damage and blood clots may form or plaques in the arteries may break away. Clots and plaques get stuck in small blood vessels and when those vessels are in the heart…well, that is a heart attack.

Action Plan

What can people do to decrease their risk? It’s quite simple, really: get the flu vaccine.

Many studies show that influenza vaccination decreases the risk of cardiovascular diseases like heart attacks. In 2017, a study of over 80,000 elderly people investigated the occurrence of heart attacks over a 13 year period5. They found that influenza vaccination decreased the risk for heart attacks by one-forth. Remember the part above about how influenza vaccinations are not always effective at preventing infection? This study also found that even in people whose vaccination did not prevent them from getting infected, the vaccine still offered some protection against heart attacks.

In another study by Mohseni and colleagues, heart failure patients who received the annual influenza vaccination had a decreased rate of hospitalization6. These studies show that even in people who are at high risk for death or hospitalization, something as simple as a flu shot protects their health.

So roll up your sleeve and take a shot for your heart.


  1. Children’s Hospital of Philadelphia. Retrieved September 21, 2018.
  2. U.S. Centers for Disease Control and Prevention. Retrieved September 21, 2018.
  3. AD Iuliano et al. Estimates of global seasonal influenza-associated respiratory mortality: a modelling study 2018. Lancet. 391(10127): 1285-1300. 2018.
  4. JC Kwong et al. Acute Myocardial Infarction after Laboratory-Confirmed Influenza Infection. N Engl J Med. 378(4): 345-353. 2018.
  5. MH Chiang et al. Association between influenza vaccination and reduced risks of major adverse cardiovascular events in elderly patients. Am Heart J. 193: 1-7. 2017.
  6. H Mohseni et al. Influenza vaccination and risk of hospitalization in patients with heart failure: a self-controlled case series study. Eur Heart J. 38(5): 326-333. 2017.