Many Are Unaware That Influenza Infections Also Increase the Risk of Stroke Just Like COVID-19!
Nikhil Prasad Fact checked by:Thailand Medical News Team Mar 08, 2025 1 month, 5 days, 3 hours, 10 minutes ago
Medical News: The Hidden Dangers of Influenza: More Than Just a Respiratory Illness
For many people, influenza, commonly known as the flu, is perceived as a seasonal nuisance that causes fever, cough, and body aches. However, what remains largely unknown is that influenza can also lead to severe complications, including an increased risk of stroke. Much like COVID-19, which has been widely reported to elevate the likelihood of cardiovascular events and stroke, many studies also confirm that influenza infections also contribute to a significant rise in stroke cases, particularly among older adults and those with preexisting health conditions.
Many Are Unaware That Influenza Infections Also Increase the Risk of Stroke Just Like COVID-19
For over 90 years, researchers have observed a link between influenza and cardiovascular events (CVEs), such as heart attacks and strokes. The earliest report on this connection dates back to 1932 when statistician Selwyn Collins noticed a surge in deaths unrelated to pneumonia or influenza during peak flu seasons. This
Medical News report highlights how modern research has further unraveled the underlying mechanisms responsible for this dangerous association. Beyond just triggering stress responses in people with heart disease, influenza appears to activate inflammatory pathways, disrupt normal blood flow, and increase the risk of clot formation, all of which heighten the likelihood of a stroke occurring shortly after infection.
Scientific Evidence Linking Influenza to Stroke Risk
Scientific studies over the years have strengthened the argument that influenza infections significantly contribute to an increased stroke risk. One of the most compelling pieces of evidence comes from a large-scale time-series analysis conducted in England, which examined hospital records between 2004 and 2015. Researchers found that laboratory-confirmed respiratory infections, including influenza, were strongly associated with ischemic strokes, especially in individuals over 75 years of age. This finding adds to a growing body of evidence that suggests respiratory infections can act as a trigger for acute cardiovascular events.
One of the major challenges in proving this connection lies in accounting for potential biases. Traditional ecological studies have been criticized for their inability to isolate influenza as the direct cause of stroke, as other factors could be at play. To overcome this issue, researchers have employed a more refined statistical approach known as the Self-Controlled Case Series (SCCS) methodology. This innovative method allows scientists to compare the risk of stroke within the same individual before and after an influenza infection, thereby eliminating the influence of external variables such as genetic differences, lifestyle choices, and preexisting conditions.
Key Study Findings: How Influenza Elevates Stroke Risk
A landmark study conducted by Smeeth et al. in 2004 provided the first clear evidence that influenza infections drastically increase stroke risk within the first few days of illness. Their findings showed
that the risk of stroke was highest during the first three days following an acute respiratory infection, with an Incident Rate Ratio (IRR) of 3.19, meaning patients were over three times more likely to suffer a stroke during this critical period. The risk gradually declined but remained elevated for up to 90 days after infection.
A more recent study by Boehme et al. echoed similar results, revealing that the odds ratio (OR) of ischemic stroke peaked between Days 1 - 15 following an influenza-like illness (ILI). Interestingly, their analysis uncovered a surprising age-related trend: younger individuals aged 18 - 45 years showed the highest stroke risk, with an OR of 9.28, meaning they were over nine times more likely to suffer a stroke after an influenza infection compared to those who were not infected.
Further supporting evidence comes from studies conducted in Scotland, Denmark, and the United States, all of which applied SCCS analysis to confirm that stroke risk spikes in the first 1 - 7 days post-infection, with some cases showing IRRs as high as 4.0–10.3. Alarmingly, this increased risk was observed for both ischemic and hemorrhagic strokes, emphasizing that the impact of influenza on the brain is not limited to a single type of stroke.
Understanding the Biological Mechanisms
Researchers believe that the underlying mechanisms linking influenza to stroke involve a combination of inflammatory responses, blood clot formation, and endothelial dysfunction. When the immune system fights off a flu infection, it releases inflammatory cytokines, which can make blood vessels more prone to clotting. In addition, the virus itself can cause direct damage to blood vessel linings, increasing the likelihood of blockages in arteries that supply the brain. This explains why the risk of stroke is highest in the immediate aftermath of an influenza infection and remains elevated for several weeks.
Another crucial factor is the severity of the flu illness. Patients who experience more intense symptoms, including high fever, prolonged bed rest, and dehydration, may be at even greater risk. This highlights the importance of flu prevention strategies, particularly vaccination, to lower the chances of severe complications.
Why This Matters: Prevention and Awareness
With influenza affecting millions of people worldwide each year, these findings carry significant public health implications. While much attention has been given to the cardiovascular risks of COVID-19, the dangers posed by influenza remain underappreciated. Many individuals, particularly those in younger age groups, may not consider themselves at risk for stroke and might dismiss flu symptoms as merely inconvenient rather than potentially life-threatening.
Healthcare professionals strongly recommend annual flu vaccinations, especially for high-risk groups such as older adults, individuals with heart disease, and those with compromised immune systems. Additionally, maintaining good hygiene, seeking prompt medical attention for flu-like symptoms, and managing underlying health conditions can all contribute to reducing the risk of flu-related strokes.
Studies Also Required to See if Previous Exposure to COVID-19 in a Person Contracting Flu, Further Compounds the Risk of Stroke.
As many studies have already validated that COVID-19 greatly increases the risk of stroke for weeks to months and even years after exposure, it is important to assess the effects of previous SARS-CoV-2 exposure in those also contracting influenza.
Conclusion
The connection between influenza and stroke risk is a crucial yet often overlooked health concern. Decades of research have established that influenza infections do more than just affect the respiratory system; they also increase the likelihood of cardiovascular events, including stroke, particularly in the days and weeks following infection. The self-controlled case series methodology has provided robust evidence that flu infections lead to a significant spike in stroke risk, particularly among younger individuals and older adults alike.
Given the potential severity of these outcomes, it is critical for both the general public and healthcare providers to recognize the cardiovascular risks associated with influenza. Preventative measures such as flu vaccination, timely treatment, and public awareness campaigns can go a long way in reducing stroke incidents linked to the flu.
References:
https://www.jstor.org/stable/4580606
https://evidence.nejm.org/doi/abs/10.1056/EVIDe2400178
https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(09)70233-6/abstract
https://academic.oup.com/jid/article/206/11/1652/897528
https://academic.oup.com/jid/article-abstract/203/12/1701/927766
https://academic.oup.com/cid/article/67/1/8/4791948
https://onlinelibrary.wiley.com/doi/full/10.1002/acn3.545
https://www.nejm.org/doi/full/10.1056/NEJMoa1702090
https://academic.oup.com/jid/article/230/3/e722/7603794
https://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2020.25.17.1900199
https://www.nejm.org/doi/full/10.1056/NEJMoa041747
https://onlinelibrary.wiley.com/doi/full/10.1111/irv.13304
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