The Link Between Flu Vaccines and Heart Diseases Explained
By: Shin Jie Yong
Tags: Technology, Vaccines, Health, Life, Science

The Link Between Flu Vaccines and Heart Diseases Explained

What the data from 1918 to 2020 tells us about influenza and heart diseases.

Image by Dimitris Christou from Pixabay

The data before 2000

A 1932 paper reported, for the first time, that influenza led to more excess deaths from heart diseases. Researchers noted a peak in cardiovascular deaths following every influenza outbreak in the U.S. in 1918, 1922, 1923, 1926, and 1928. An excess in deaths from kidney, lung, and cerebrovascular diseases and diabetes also happened, although to a much lesser extent than heart diseases.

The pattern repeats for subsequent influenza outbreaks.

“Peak months of mortality for ischemic heart disease, cerebrovascular disease, and diabetes mellitus coincided appropriately with peaks in pneumonia and influenza [during the winter of 1959 to 1999],” stated a study in the American Journal of Epidemiology. “Weather and other factors may determine the timing and modulate the magnitude of the winter-season increase in mortality, but the primary determinant appears to be the influenza virus.” Simply put, the flu best explains the excess deaths, especially from heart diseases.

“Recognition of influenza as a trigger for acute coronary events calls for more intensive efforts to increase the vaccination rate in subjects at risk of CHD [coronary heart disease].”

Another study in the European Heart Journal confirms these findings with eight years of autopsy data (total sample of 34,892) from 1993 to 2000. “Influenza epidemics are associated with a rise in autopsy-confirmed coronary deaths,” the study revealed, by 10–30%. “Recognition of influenza as a trigger for acute coronary events calls for more intensive efforts to increase the vaccination rate in subjects at risk of CHD [coronary heart disease].”

Using hospital data of over five million people from 1897 to 2001, a paper in the New England Journal of Medicine (NEJM) noted a 5- and 3-fold increase in incidents of heart attacks and stroke, respectively, during the first three days of a respiratory tract infection. But these heart and stroke risks disappeared in persons vaccinated for tetanus, pneumococcus, or influenza.

The data after 2000

Decades after the 1918 pandemic, the theory that influenza induces heart diseases still stands.

From 1999 to 2008, a study in the Journal of Infectious Diseases estimated that influenza accounted for 3.4% of cardiovascular deaths in the U.K. and 5.6% in Hong Kong — equating to tens of thousands of deaths. And the deaths often occurred within the first three days of influenza symptoms. A reason for the higher mortality in Hong Kong, the authors speculate, could be that influenza vaccines were only widely accessible in 2004.

“This study is key because it truly highlights how common it can be to experience serious heart complications after getting the flu — including in some people that did not have any prior chronic health conditions.”

Using patient data collected from 2004 to 2015 in New York, a paper issued this month in the Journal of American Heart Association found seasonality in heart diseases and strokes— just like the flu. “We found that if someone’s going to have a heart attack, it’s going to occur within seven days of the flu-like illness, during the acute phase,” said Amelia K. Boehme, professor of epidemiology at Columbia University who directed the study. “With stroke, we see an increased risk seven to 15 days after, similar to heart attacks.”

Lastly, a 2020 study in the Annals of Internal Medicine showed that 11.7% of 89,999 older adults hospitalized with influenza from 2010 to 2018 also had a subsequent cardiovascular event. “This study is key because it truly highlights how common it can be to experience serious heart complications after getting the flu — including in some people that did not have any prior chronic health conditions,” said Dr. Natasha Bhuyan, MD, family physician in Arizona.

Clearly, seasonal influenza viruses could trigger heart diseases. Why?

The burden on the heart

Reviewing the abovementioned studies, a paper published this month in the Journal of the American College of Cardiology (JACC) delved deeper into the literature to pinpoint the possible explanations for influenza-induced heart diseases:

Influenza raises the body's metabolic demands are raised to fight infection. The fight-or-flight system, immune system, and blood vessels carrying immune cells become highly activated, which could be overly taxing for persons at risk for heart diseases.

Influenza weakens the lung function, which increases the risk of other bacterial or viral respiratory infections that further burdens the lungs, heart, and blood vessels — the cardiovascular-pulmonary system.

Influenza initiates both systemic and localized inflammation, which could trigger the rupture of plaque in blood vessels.

Influenza has been shown to infect cells of the heart, lungs, and blood vessels in animals and cultured human cells. This infection further adds a burden on the organs already affected by inflammation.

Therefore, the rough idea is that influenza could be the final push for people at pre-heart disease stages.

Influenza vaccines and heart diseases

Since influenza could trigger — i.e., a risk factor — heart diseases, does that mean its vaccine is a protective factor? One observational study in the NEJM (discussed above) has found that risks of heart diseases and stroke disappeared in vaccinated (including influenza) persons.

However, observations show associations or possible causation. Randomized controlled trials (RCTs) are required to establish causation.

Most likely flu shots will not be the silver bullet. But it could make a huge difference even if it shows some efficacy given that heart diseases kill by the millions.

A 2015 Cochrane-standard meta-analysis (latest to date) synthesized eight RCTs with a pooled sample size of 12,029 persons with a cardiac history. They calculated that influenza vaccines halved the future death rates from heart diseases — from 5.1% (placebo group) to 2.1% (vaccine group).

“In patients with cardiovascular disease, influenza vaccination may reduce cardiovascular mortality and combined cardiovascular events,” the meta-analysis concluded. “However, studies had some risk of bias, and results were not always consistent, so additional higher-quality evidence is necessary to confirm these findings.”

Regardless, at least the flu shot does not increase the risk of heart diseases, so chances are that at-risk persons would benefit.

Source: Partial table from Behrouzi et al. (2020). Journal of the American College of Cardiology (JACC)

Other on-going big RCTs are investigating if influenza vaccines are a feasible strategy to prevent heart diseases (see figure). Most likely flu shots will not be the silver bullet. But it could make a huge difference even if it shows some efficacy given that heart diseases kill by the millions. It took 17.79 million lives in 2017 — nearly double that of the #2 leading cause of death, cancers. Not to mention that heart diseases are the most expensive disease to treat, and 80% of cases are preventable.

At the same time, efforts must also be made to improve flu shots’ efficacy. “It is also known that although the seasonal influenza vaccine is better than nothing, it is not nearly as effective as it can be, having ranged from 10% to 60% in estimated effectiveness in recent years,” the JACC review stated.

Short abstract

From 1918 to 2020, the flu has contributed to the countless lives lost to heart diseases. Evidence shows that influenza serves as a trigger of heart diseases, especially within a week of symptom onset and in those already at risk for heart diseases. And that influenza vaccines could mitigate this effect to some extent. Anyhow, some level of efficacy in reducing heart diseases — that kill by the millions and cost by the billions — would be precious. All the more reason to invest in improving the effectiveness of flu shots and the public’s perception of it.