Ask almost any paramedic, ER doctor, or ICU nurse and they will tell you that heart attacks and strokes are more common in the winter. So much so, that these events are commonly called `Christmas Coronaries’ or `Holiday Heart Attacks’.
In 1998, a study looked at the rate of heart attacks in the United States (see Seasonal distribution of acute myocardial infarction in the second National Registry of Myocardial Infarction), and found that Acute Myocardial Infarctions (AMIs) ran as much 53% higher during the winter months than during the summer.
While cold weather combined with strenuous physical activity (like clearing snow from sidewalks) has often been blamed for this spike, even in balmy Southern California, studies have shown a 33% increase in heart attacks over the holidays (see When Throughout the Year Is Coronary Death Most Likely to Occur?).
Admittedly, non-climate related factors – like overindulgence in food and drink - combined with holiday stressors like shopping, running up debt, traveling, meal preparation, and the angst that comes from dysfunctional family gatherings are all plausible contributors to this yearly spike.
But over the past 15 years studies have increasingly linked this seasonal increase in heart attacks and strokes to acute influenza and respiratory infections. A few (of many) studies include:
PloS One: Early Risk of Acute Myocardial Infarction Following Hospitalization for Severe InfluenzaAt the same time, we've seen growing evidence that the uptake of the seasonal flu vaccine may reduce the incidence of heart attack and stroke, even if the vaccine doesn't prevent influenza infection.
JAHA: Another Study Linking ILI To Increased Risk Of Heart Attack & Stroke
PLoS One: Transient Depression of Myocardial Function After Influenza Virus Infection
NEJM: Acute Myocardial Infarction After Laboratory-Confirmed Influenza Infection
Eur. Resp.J.: Influenza & Pneumonia Infections Increase Risk Of Heart Attack and Stroke
All of which brings us to a new study, published yesterday in Eurosurveillance, which seeks to confirm and expand upon these earlier findings using Danish national health records from across the 2014-2025 influenza seasons.
They found that people (age 40+) with a confirmed flu infection were more likely to be hospitalized for a heart attack or stroke in the first 7 days after testing positive. The risk was highest in the first 3 days, and returned to normal after 2 weeks.
Intriguingly, they found the short-term risk was reduced by about 50% among those with a positive influenza test, but vaccinated.
Today's study is quite detailed, and many will want to read it in its entirety. I've only reproduced the link, and summary, followed by a link to an accompanying editorial.
I'll return with a brief postscript about an unexpected finding after the break.
Influenza vaccination attenuates acute myocardial infarction and stroke risk following influenza infection: a register-based, self-controlled case series study, Denmark, 2014 to 2025
Roberto Croci1,2,* , Johanna J Young2,* , Hanne-Dorthe Emborg2 , Palle Valentiner-Branth2 , Steen Ethelberg2,3 , Christian Holm Hansen2
What did you want to address in this study and why?
Catching influenza increases the short-term risk of heart attack and stroke. Influenza vaccination has been shown to reduce this risk by preventing infection, but it is unclear whether it also offers protection among people who become infected despite vaccination. We wanted to quantify how much more at risk of heart attack and stroke adults in Denmark are shortly after catching influenza, and to assess if prior vaccination can attenuate this risk.
What have we learnt from this study?
In adults 40 years or older in Denmark, hospital admissions for heart attack and stroke were more frequent in the first week after testing positive for influenza than during any other period in the year before and after their test, almost threefold for stroke and fivefold for heart attack. This increased risk was about half as high among people who tested positive for influenza but had received the influenza vaccine that season.
What are the implications of your findings for public health?
Influenza vaccination may offer cardiovascular protection even in instances when it does not prevent infection. If confirmed by additional studies in other settings, this would strengthen the case for prioritising influenza vaccination among people at risk of heart disease or stroke and would support refining recommendations across Europe.
(SNIP)
Conclusions
Our findings add to the evidence that influenza vaccination confers cardiovascular protection. In this study, prior vaccination halved the excess risk of acute myocardial infarction or stroke following breakthrough influenza infection. These results strengthen the case for prioritising influenza vaccination in high-risk groups. Highlighting the dual protection offered by vaccination, against both infection and its cardiovascular complications, could have a substantial public health impact. Factoring this into economic and burden analysis might improve the cost-effectiveness profile of vaccination programmes.
Does influenza vaccination protect people with breakthrough infections from acute cardiovascular events?
Jeffrey C Kwong
Unlike with influenza, previous flu vaccination showed no attenuating effect.
They wrote:
Unexpectedly, infection with Campylobacter spp., a food-borne, non-respiratory bacterial pathogen, was associated with a threefold increase in acute cardiovascular events.
One explanation may be that any infection severe enough to warrant microbiological testing can result in proinflammatory events. Alternatively, unmeasured respiratory co-infections might have determined the risk increase, at least in part.
Meta-analyses have shown that respiratory pathogens other than influenza can trigger acute cardiovascular events [24]. Research should clarify whether this is true for non-respiratory pathogens as well. Importantly, the absence of effect modification by influenza vaccination in our Campylobacter-negative exposure analysis reinforces that the observed risk attenuation after influenza infection is exposure-specific.
While cardiac complications have been reported following Campylobacter infection (see Review of Campylobacter Species Related Cardiac Disease), today's findings suggest this may be more common than previously believed.
Although the current flu vaccine is nowhere near as effective as we'd like it to be, and the promise of a universal flu vaccine remains elusive (see J.I.D.: NIAID's Strategic Plan To Develop A Universal Flu Vaccine), we continue to see evidence that the benefits of vaccination may extend beyond simply reducing the risk of influenza infection.
Which is why I'll be rolling up my sleeve this fall for both the Flu and COVID vaccines. Because at my age - and in my physical condition - I can use all the advantages I can get.
