Does the flu shot lower heart-attack risk? Trials point that way
In heart patients, randomised trials link influenza vaccination to fewer cardiovascular events and deaths. The effect is clearest in the highest-risk patients, and the largest trial stopped early.
| Group | Value (%) |
|---|---|
| Placebo | 7.2 |
| Influenza vaccine | 5.3 |
In people who already have heart disease, getting a flu shot is associated with fewer cardiovascular events and deaths, and the strongest evidence now comes from a randomised trial rather than just observation. In that trial, patients vaccinated shortly after a heart attack had a primary composite event rate of 5.3% at one year against 7.2% on placebo — a hazard ratio of 0.72 (95% CI 0.52 to 0.99) [s1]. This is a claim about secondary prevention in cardiac patients, not proof that a flu shot protects an otherwise healthy person's heart.
The IAMI trial
The landmark test is IAMI, an investigator-initiated, randomised, double-blind, placebo-controlled trial across 30 centres in eight countries [s1]. It enrolled 2,571 patients — 1,290 assigned to inactivated influenza vaccine and 1,281 to a saline placebo — given shortly after a myocardial infarction (99.7% of patients) or, in a small fraction, with high-risk stable coronary disease [s1]. The primary endpoint was a composite of all-cause death, myocardial infarction or stent thrombosis at 12 months [s1].
Vaccination came out ahead on the main outcome and on mortality. The composite occurred in 67 vaccinated patients (5.3%) versus 91 on placebo (7.2%), a hazard ratio of 0.72 (95% CI 0.52 to 0.99; P=0.040) [s1]. All-cause death was 2.9% versus 4.9% (HR 0.59, 95% CI 0.39 to 0.89; P=0.010) and cardiovascular death 2.7% versus 4.5% (HR 0.59, 95% CI 0.39 to 0.90; P=0.014) [s1]. Recurrent myocardial infarction alone did not differ significantly, 2.0% versus 2.4% (HR 0.86, 95% CI 0.50 to 1.46; P=0.57) — so the benefit was carried more by fewer deaths than by fewer new infarctions [s1].
Two limits sit inside those numbers. The confidence interval on the primary result reaches almost to 1.0, meaning the benefit, while statistically significant, could be modest [s1]. And the trial was halted before reaching its planned sample size when the COVID-19 pandemic disrupted enrolment, which reduces its statistical power and means it was smaller than intended [s1].
What the earlier meta-analysis found
IAMI landed on top of a body of randomised evidence pointing the same way. A 2013 meta-analysis in JAMA pooled five published and one unpublished randomised trials — 6,735 patients, mean age 67, 51.3% women, with a mean follow-up of 7.9 months [s2]. Influenza vaccine was associated with a lower risk of major adverse cardiovascular events, 2.9% versus 4.7%, a relative risk of 0.64 (95% CI 0.48 to 0.86; P=.003) [s2].
That analysis also located where the effect concentrates. Among patients who had had a recent acute coronary syndrome within the prior year, the risk reduction was large (RR 0.45, 95% CI 0.32 to 0.63), whereas in patients without recent acute coronary syndrome it was not significant (RR 0.94, 95% CI 0.55 to 1.61), a statistically significant interaction (P=.02) [s2]. In other words, the sicker and more recently unstable the heart, the bigger the apparent benefit — the same signal IAMI later confirmed in post-infarction patients [s1][s2].
How to read this
The mechanism is biologically plausible: influenza infection triggers inflammation and clotting that can tip a vulnerable coronary artery into an event, so preventing the infection may prevent some of those events [s2]. But the evidence base is specific in a way the headline can lose. It is about people with established or recent coronary disease, the largest trial stopped early, and the meta-analysis found the benefit essentially absent in lower-risk patients [s1][s2]. It does not establish that a flu shot lowers a healthy young adult's heart-attack risk; that population was not where the signal appeared [s2].
For heart patients, though, the direction is consistent across a randomised trial and a meta-analysis of randomised trials, which is a stronger footing than the observational data these questions usually rest on [s1][s2]. It sits alongside coverage of what flu-vaccine "effectiveness" actually measures and the separate finding on the recombinant shingles vaccine and cardiovascular events, part of a wider pattern of vaccines being studied for effects beyond the infection they target; uptake itself is examined in adult vaccination rates.
This article describes what the studies found and is not medical advice.
Sources
Sources
- Influenza Vaccination After Myocardial Infarction: A Randomized, Double-Blind, Placebo-Controlled, Multicenter Trial — Circulation , August 30, 2021
- Association between influenza vaccination and cardiovascular outcomes in high-risk patients: a meta-analysis — JAMA , October 23, 2013
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