In September 2021, the United States surpassed 700,000 deaths due to COVID 19. This is certainly an enormous and tragic loss of life, but it is significant for another reason. The US now has more deaths from COVID 19 than it did during the 1918 influenza pandemic (675,000). While COVID 19 has caused more morbidity and mortality, these sobering numbers remind us not to forget about the flu. How does influenza affect the heart and how much protection does the flu vaccine offer?
Influenza (“the flu”) is a very common, very contagious respiratory illness. It is caused by an RNA virus. Influenza has three subtypes, A, B and C with A being the most common in humans (influenza can affect animals as well, causing, for example, swine flu or bird flu). The virus consists of an RNA core surrounded by proteins. The most significant surface proteins are hemagglutinin (H) and neuraminidase (N). When someone is infected with the flu, these surface proteins are targeted by the immune system. In addition, these surface proteins help identify the type of influenza virus. For example, in 2009 there was a major pandemic caused by the H1N1 strain of influenza. There are 17 H and 9 N types, but the most common human pathogens are H1, H2 or H3 and N1 or N2. H3N2 and H1N1 are the most common types of influenza A that infect humans.
Influenza inflicts a huge burden on the healthcare system each year. It is responsible for almost a million hospitalizations and about 30,000 deaths per year in the United States. Most cases of the flu resolve on their own, but certain populations are at risk for complications from influenza. This includes people over 65 years old, those with weakened immune systems, obese individuals, diabetics, and patients with underlying heart or lung disease.
How does influenza affect the heart? During the acute phase of the illness, influenza stresses the body due to the symptoms and high fever. This causes an elevated heart rate and either a significant increase in blood pressure or severe lowering of the blood pressure. These effects put an increased demand on the heart and can precipitate a heart attack or congestive heart failure (CHF). The virus may also infect the heart muscle directly and cause myocarditis (infection of the heart). In addition, influenza causes an intense inflammatory response. Inflammation plays a critical role in the progression of blockage in the heart arteries. Influenza infection accelerates the atherosclerotic process resulting in heart attacks. The inflammatory response also depresses heart function, leading to CHF. Over the years, many studies have shown that influenza infection increases the risk for heart attack, cardiovascular death and overall deaths. The flu vaccine helps to prevent these cardiac complications.
The influenza vaccine is designed to produce an immune response to the virus. Once the immune response occurs (usually within two weeks after vaccination) and someone becomes infected with the flu, the body is able to fight it off. The key is to include the influenza viruses most likely to cause infection in a given year. The flu viruses are constantly changing and can change from one year to the next or even within a single flu season. Each year an FDA committee meets and picks the strains of influenza that it deems likely to be prevalent that year. Then manufacturers produce the vaccine. Trivalent vaccines include strains of influenza A in the H1N1 and H3N2 families plus a strain of an influenza B virus. Quadrivalent vaccines include the H1N1, H3N2 strains plus two types of influenza B strains. The ideal time for vaccination is before the flu season begins, allowing the body to develop the immune response. Typically this between September and November, but the vaccine may still be effective even in the middle of flu season. Due to the wide variation in the strains of the influenza virus, the vaccine is only between 40 and 60% effective at preventing the flu (the Pfizer and Moderna COVID vaccines have an efficacy of about 95%). Now Pfizer and Moderna are working on flu vaccines based on the RNA technology used for COVID with trials starting soon. The RNA technology allows for more flexibility and more rapid manufacturing, potentially allowing for better strain matching.
How effective are influenza vaccines at preventing heart complications? In patients who have been hospitalized with the flu, there is a lower risk of heart attack and CHF in those vaccinated versus those who are unvaccinated. There is an 18% reduction in both heart deaths and overall deaths. In patients hospitalized with an acute heart attack, vaccinated patients had a 28% reduction in death or repeat heart attack over the following year compared to unvaccinated heart attack patients. In people with CHF, getting the vaccine resulted in a 50% drop in death during the flu season and a 20% risk of death during the rest of the year. This is why every hospitalized patient is asked whether they want the vaccine during their hospital stay. This is why every doctor touts the vaccine for his or her patients. Yet despite these impressive benefits and the public campaign for vaccination, only about 50% of heart patients over the age of 50 receive the flu vaccine in any given year.
So remain vigilant regarding COVID 19 but don’t forget about the flu. Go out today and get your vaccine, especially if you have underlying heart disease.
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