Influenza vaccination: One of our most effective ways to prevent heart attacks and strokes

Influenza doesn’t just make you miserable, it also triggers heart attacks and stokes

Having influenza significantly increases the risk of heart attack and stroke. Epidemiologists first noticed in the 1930s that more people died of cardiovascular causes in severe flu seasons.¹ Modern studies back this up. Of note, a large Canadian study found that people who tested positive for influenza had a six‑fold increase in heart attack and stroke risk for at least a week.²  Hospital-based studies show that inpatients with confirmed influenza have a 12% chance of experiencing a major cardiovascular event during their hospitalisation.³

Seasonal influenza vaccination does far more than prevent influenza infection. A growing body of research, which includes dozens of studies and 28 systematic reviews, show the flu vaccination cuts heart attacks and strokes by around one third.^4-8

How does influenza cause heart attacks?

Influenza affects the heart and blood vessels in two major ways. First, there are the direct effects of the virus: Influenza can inflame or injure heart muscle directly leading to pericarditis and myocarditis.^9,10 More concerning is that it can also replicate within blood vessel walls. This process can destabilise plaques, triggering rupture and clot formation, leading to a heart attack or stroke.^9,10 

Second, there are the broader indirect effects of influenza illness: Fever, inflammation, and reduced oxygen levels all place stress on the heart. The inflammatory response also makes blood more prone to clotting and destabilises plaques in blood vessels. This cascade of direct and indirect effects from influenza infection greatly increases the likelihood of adverse cardiovascular events like heart attacks and strokes. No other common infection increases cardiovascular risk to this extent.^9,10 

The flu vaccine is as effective as commonly prescribed heart medications at preventing heart attacks and strokes

Research consistently shows that annual influenza vaccination reduces major cardiovascular events by about a third, especially in people already at moderate-to-high cardiovascular risk. This protection is comparable to the effect of cholesterol-lowering medications.^4-7,11,12 It is impressive and not well communicated. 

Not only does the annual flu vaccination prevent and reduce severe influenza, but it also provides important cardiovascular protection through “off-target” vaccine effects. Vaccines are increasingly recognised to have pleiotropic effects - boosting the immune system in ways that protect against diseases beyond the intended target.¹³ Table 1 (see Appendix) shows common vaccines and their recognised benefits in preventing non-communicable diseases, some of which are classified as “off-target”. Some vaccines can induce “trained immunity,” priming the body to respond more effectively to a wide range of infections not just the target disease.¹⁴ Influenza vaccination appears to fall into this category, offering a powerful layer of “off-target” cardiovascular protection.^4-6 

Summarising the different effects, several recent studies from Scandinavia demonstrate that influenza vaccine appears to prevents cardiovascular events in three different ways; first by preventing influenza, second by stabilising vulnerable plaques inside blood vessels by altering the response to inflammation, and finally by preventing other viral illnesses via trained immunity which can increase the risk of heart attacks and strokes albeit less so than influenza.^{13, 15,16}

Why this benefit matters for Aotearoa New Zealand

Heart disease and stroke remain among the leading causes of death in NZ, accounting for around a third of all deaths, approximately 12,000 each year.^17-20 Importantly from an equity perspective, Māori and Pacific peoples bear a disproportionate burden, developing cardiovascular disease earlier and dying from it at higher rates than NZ Europeans.¹⁹ Māori have a mortality rate from heart attacks twice that of NZ Europeans with 99.6 deaths per 100,000 people every year. Māori and Pacific people are on average 10 years younger when they are hospitalised with a heart attack or die from it compared to NZ Europeans.²¹

Yet flu vaccination coverage does not match the need. It tells a very different story.  Only around 60% of New Zealanders over 65 receive the flu vaccine annually with vaccination rates significantly lower among Māori, Pacific, and the most deprived communities - Precisely the groups with the highest cardiovascular risk. Last flu season, 52% of Māori and Pacific people who were eligible for a free influenza vaccine received one compared to 63% of NZ Europeans.²² Another way of looking at it is that there are approximately 372,800 New Zealanders over 65 who are missing out on the annual influenza vaccination, many of them Māori and Pacific people.²³

Improving flu vaccine uptake in these groups offers an important, currently underused opportunity to reduce health inequities and prevent thousands of major cardiovascular events. One practical measure would be to reinstate funding for free flu vaccine which had been extended in 2022 and 2023 to all Māori and Pacific people between 55 and 64 years of age to support their access to the vaccine.²⁴

A Clear Call to Action

Cardiovascular diseases are among the biggest health challenges facing NZ. The evidence is clear that flu vaccination can offer as much reduction to risk as the standard medications being prescribed commonly in general practice for prevention of heart attacks and strokes. Importantly, influenza vaccination has an additive protective effect and acts as a complementary intervention alongside blood pressure and cholesterol‑lowering medications.  There are major health gains possible for our communities. Increasing influenza vaccination rates is an effective, feasible, and immediate action with the potential for enormous impact.     

Author details

Dr Philip Shirley, Public health Registrar, Immunisation Advisory Centre

Dr Samantha Marsh, School of Population Health, University of Auckland | Waipapa Taumata Rau

Prof Nikki Turner, Department of General Practice and Primary Care, University of Auckland, and Principal Medical Advisor, Immunisation Advisory Centre

Prof Michael Baker, Department of Public Health, Ōtākou Whakaihu Waka, Pōneke | University of Otago, Wellington, and Director, Public Health Communication Centre

Appendix Table 1: Vaccines with known benefits for preventing or reducing non-communicable diseases (NCDs)

* Note – We have included influenza vaccine in three places to illustrate how it provides cardiovascular protection through three different mechanisms. Many other vaccines have multiple mechanisms of operation, which are not shown.