Does the Flu Increase Your Heart Rate?

Influenza reliably raises your resting heart rate, often by a noticeable margin. Fever alone accounts for a large share of that increase, but your immune system’s inflammatory response and shifts in your nervous system’s control of heart rhythm also play a role. A study of urgent-care patients found that each degree Celsius of fever was associated with a heart rate roughly 8 to 14 beats per minute higher than normal, depending on the person’s age. What makes the heart rate story during the flu more interesting than a simple “yes” is that the increase can begin before you feel sick, can persist after the fever breaks, and in rare cases can signal a genuine cardiac complication.

How Fever Pushes Heart Rate Up

The most straightforward reason the flu raises your heart rate is fever. When your body temperature climbs, your heart has to work harder to move blood toward the skin for cooling and to keep organs supplied with oxygen. A large study of children attending urgent and emergency care found that, on average, heart rate rose by about 12 beats per minute for every 1°C increase in body temperature, after accounting for age and oxygen levels. The effect was strongest in the youngest patients, where a single degree of fever corresponded to roughly a 14 bpm increase, and somewhat smaller in older children at around 9 bpm per degree.¹ Adults follow a similar pattern, though the per-degree bump tends to be a bit smaller than in young children.

A typical flu fever runs between 38°C and 40°C (about 100.4°F to 104°F). If your normal resting heart rate sits around 70 bpm, even a modest fever of 38.5°C could push you into the mid-80s or higher. During the peaks of a high fever, resting rates well above 100 bpm are not unusual, especially if you are dehydrated from sweating, vomiting, or not drinking enough. Dehydration reduces blood volume, so the heart compensates by beating faster to maintain the same output. The combination of fever and fluid loss is one reason people with the flu sometimes feel their heart pounding even while lying still.

The Immune System’s Effect on Heart Rhythm

Fever is only part of the picture. Your immune system’s inflammatory response to the flu virus independently shifts the balance of your autonomic nervous system, the branch of the nervous system that controls heart rate without you thinking about it. Under normal conditions, sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) signals keep your heart rate in a steady range. During a flu infection, inflammatory signaling molecules tilt that balance toward sympathetic dominance, which speeds the heart up.

A study examining the effects of an influenza A vaccine, which triggers a mild inflammatory reaction without a full-blown infection, found that even that limited immune activation was enough to shift autonomic balance toward sympathetic predominance. The researchers observed significant correlations between levels of C-reactive protein, a marker of inflammation, and changes in heart rate variability, suggesting a direct link between immune-driven inflammation and cardiac autonomic control.² In a real flu infection, the inflammatory response is far larger, and so is its effect on the heart.

Cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α), which surge during influenza, have been shown to increase sympathetic nervous system tone directly. That accelerates the heart rate and raises the heart’s demand for oxygen.³ This cytokine-driven mechanism explains why your heart rate can stay elevated even after you take a fever reducer and your temperature temporarily normalizes. The underlying inflammation hasn’t gone away just because the thermometer reads lower.

Heart Rate Rises Before You Feel Sick

One of the more striking findings from wearable-device research is that resting heart rate begins climbing before flu symptoms appear. Since the influenza virus has an incubation period of one to four days, the immune system is already mounting a response, and the heart is already reacting, before you notice a sore throat or body aches. A study using Fitbit data from thousands of users found elevated resting heart rates in the two days surrounding the onset of influenza-like illness symptoms, with the uptick starting before the person reported feeling unwell.⁴

A controlled inoculation study went further, challenging volunteers with influenza virus and monitoring biometric data continuously. Researchers found they could identify an infected participant roughly 24 to 36 hours before symptom onset by looking at physiological signals including heart rate changes.⁵ That presymptomatic window is short but significant. It means the heart is already under mild stress from the immune response during a phase when many people are going about their daily lives unaware they are infected.

Research comparing heart rate patterns in both COVID-19 and flu-like illness found that about a quarter of COVID-19 patients had highly elevated resting heart rates in the two days around symptom onset, compared to about 13 percent in the period well before symptoms.⁶ Influenza-like illness showed a similar pattern of presymptomatic heart rate elevation, reinforcing that this is not unique to one virus but a general feature of how the body responds to respiratory infection.

When the Heart Rate Increase Signals Something More Serious

For most people, the faster heart rate during the flu is uncomfortable but not dangerous. It is a normal physiological response and resolves as the infection clears. But influenza can occasionally cause genuine cardiac problems, and an elevated heart rate is sometimes an early sign.

A systematic review of influenza-associated cardiac arrhythmias catalogued several mechanisms by which the virus can disrupt heart rhythm beyond the normal fever response. These include direct inflammatory damage to heart tissue (focal myocarditis), the same IL-6 and TNF-α mediated inflammatory cascade described above, sympathetic overactivation, and effects on a protein called angiotensin-converting enzyme 2 that helps protect the heart.⁷ These mechanisms can trigger arrhythmias ranging from benign extra beats to more dangerous rhythm disturbances, particularly in people with preexisting heart conditions.

A meta-analysis looking at cardiovascular complications in patients with lab-confirmed influenza found that heart failure occurred in roughly 17 percent of cases, arrhythmia of any kind in about 6 percent, myocarditis in about 3 percent, and heart attacks in about 2 percent.⁸ Those numbers reflect hospitalized patients, not everyone who catches the flu, so they overrepresent severe cases. Still, they underscore that influenza is not purely a lung disease. The cardiovascular system takes a hit, and people with existing heart disease, older adults, and those with other chronic conditions are at higher risk of those complications.⁹

Warning signs that a fast heart rate during the flu may be more than a normal response include chest pain or pressure, feeling your heart racing or skipping beats while at rest after the fever has been controlled, sudden breathlessness that is out of proportion to any respiratory symptoms, and dizziness or fainting. Any of these warrants medical evaluation. For most flu patients, though, the elevated heart rate is the body’s expected response to fighting off infection and will come down on its own.

How Long the Heart Rate Stays Elevated

Most people notice their resting heart rate returns to normal within a few days of the fever breaking, but the timeline varies. Autonomic recovery, the process by which your nervous system restores its normal sympathetic-parasympathetic balance, can lag behind the disappearance of other symptoms. A study of children infected with the 2009 H1N1 pandemic influenza virus found that markers of autonomic function, specifically heart rate variability, were significantly lower in infected children compared to healthy controls.¹⁰ Lower heart rate variability reflects a nervous system that is still tilted toward sympathetic dominance, and it can take days to weeks to fully normalize.

If you wear a fitness tracker, you may notice your resting heart rate remains a few beats per minute above your usual baseline for a week or more after you feel better. This is common and generally not a cause for concern. It reflects residual inflammation and the nervous system slowly returning to its resting state. Persistent elevation beyond two to three weeks, especially if accompanied by fatigue or exercise intolerance, is worth mentioning to a doctor, as it could point to lingering inflammation or a complication like myocarditis.

The Flu Vaccine Comparison

A reasonable question is whether the flu vaccine itself raises your heart rate, since it also triggers an immune response. A study that compared physiological reactions to COVID-19 and influenza vaccination using wearable sensors found a clear difference between the two. COVID-19 vaccination produced a statistically significant increase in heart rate that peaked about 22 hours after the shot, averaging roughly 4.5 beats per minute above baseline and returning to normal by day six. Influenza vaccination, by contrast, produced no statistically significant change in heart rate or stress measures.¹¹

That finding aligns with the vaccine study mentioned earlier, which found that even though the influenza vaccine triggered mild inflammation and a detectable shift in autonomic balance, the effect was subtle enough that it did not translate into a heart rate bump most people would notice.² The immune response from a flu shot is orders of magnitude smaller than from an actual flu infection. If you have noticed your heart rate climbing after a flu shot, it is more likely related to anxiety about the injection, mild dehydration, or coincidental factors than to the vaccine itself.

Practical Tips for Managing an Elevated Heart Rate During the Flu

Understanding why your heart rate rises during the flu points toward some practical steps. None of these will prevent the increase entirely, since it is part of how your body fights the infection, but they can keep it from getting worse than it needs to be.

Stay hydrated: Dehydration from fever, sweating, and reduced fluid intake compounds the heart rate increase. Sipping water, broth, or electrolyte drinks steadily throughout the day helps maintain blood volume so your heart does not have to work as hard.
Manage fever sensibly: Over-the-counter fever reducers like acetaminophen or ibuprofen can bring your temperature down, which will lower your heart rate somewhat. They do not address the inflammatory nervous-system effects, so do not expect your heart rate to drop all the way to normal.
Rest genuinely: Physical activity during an acute flu infection places additional demand on a cardiovascular system that is already under stress. Resting is not laziness during the flu; it is protecting your heart from having to work even harder on top of the immune-driven increase.
Skip the caffeine: Stimulants that you tolerate fine when healthy can push an already-elevated heart rate into uncomfortable territory during illness.
Monitor if you have heart disease: If you have a preexisting cardiac condition, pay closer attention to how your heart feels during the flu than to your other symptoms. The combination of inflammation, sympathetic overdrive, and dehydration creates a higher-risk window for complications.

Why Your Smartwatch Freaks Out When You Get Sick

If you use a smartwatch or fitness band, you have probably noticed it flagging abnormal readings during a cold or flu. Some devices will notify you that your resting heart rate is unusually high or that your heart rate variability has dropped. This is not a glitch. It is the device accurately reflecting the physiological changes described above. Researchers have taken advantage of this phenomenon to build flu surveillance tools. By aggregating anonymized wearable data from large populations, public health teams can detect spikes in resting heart rates across a region that correlate with rising flu activity, sometimes before hospital data catches up.⁴

For individual users, the practical takeaway is that a sudden, unexplained rise in resting heart rate, especially during flu season, can be an early signal that your body is fighting something off. Several studies have shown the ability to detect infection from wearable data a full day or more before traditional symptoms appear.⁵ If your watch says your heart rate is unusually high and you cannot explain it with exercise, stress, or caffeine, it might be worth taking your temperature and watching for early flu symptoms. You are not diagnosing yourself with a wristband, but you are getting a genuinely useful heads-up from your own physiology.

One limitation is that smartwatches measure resting heart rate somewhat inconsistently depending on how still you are, when you last ate, and how tightly the band sits. A single high reading means little. A pattern of elevated readings over 12 to 24 hours is more meaningful, and that is what the research actually uses to make predictions. If your device offers a multi-day resting heart rate trend, that is more informative than any single snapshot.