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Risk of death
Yes
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Time to symptoms
Countries affected
Active outbreaks
Influenza circulates year-round in tropical regions and seasonally elsewhere. Crowded travel settings increase risk. Annual vaccination recommended before travel.
Highly contagious acute respiratory illness caused by influenza viruses A and B, with annual seasonal epidemics and occasional pandemics.
Symptoms | Frequency | Severity | Onset |
|---|---|---|---|
| Fever | 95% | Moderate | Early |
| Myalgia | 85% | Moderate | Early |
| Chills | 85% | Mild | Early |
| Cough | 75% | Mild | Early |
| Fatigue | 80% | Moderate | Early |
| Headache | 90% | Moderate | Early |
| Nasal congestion | 50% | Mild | Early |
| Nausea | 30% | Mild | Early |
| Rhinorrhea | 40% | Mild | Early |
| Sore throat | 60% | Mild | Early |
| Shortness of breath | 15% | Moderate | Peak |
Influenza (commonly known as "the flu") is an acute respiratory illness caused by influenza viruses, belonging to the family Orthomyxoviridae. Two types are responsible for seasonal epidemics in humans: influenza A and influenza B. Influenza A viruses are further classified by surface glycoproteins hemagglutinin (H) and neuraminidase (N), with subtypes H1N1 and H3N2 currently circulating in the human population.
The disease is transmitted primarily through respiratory droplets and aerosols generated by coughing, sneezing, and talking. Transmission may also occur via contact with contaminated surfaces (fomites), though this route is considered secondary. The virus can survive on hard surfaces for up to 48 hours and on hands for approximately 5 minutes.
Influenza remains one of the most significant infectious disease threats globally, causing an estimated 1 billion infections, 3–5 million cases of severe illness, and 290,000–650,000 respiratory deaths annually according to WHO estimates. The disease exhibits clear seasonality in temperate regions (winter months) but circulates year-round in tropical and subtropical zones.
Periodic antigenic shift in influenza A viruses can produce novel subtypes to which the population has little or no immunity, resulting in pandemics. Notable pandemics include 1918 (H1N1), 1957 (H2N2), 1968 (H3N2), and 2009 (H1N1pdm09). Between pandemics, gradual antigenic drift necessitates annual reformulation of seasonal influenza vaccines.
Seasonal influenza is a vaccine-preventable respiratory infection that affects all age groups. The disease typically follows a self-limiting course of 3–7 days in otherwise healthy individuals, but can cause severe complications and death in high-risk populations including the elderly (≥65 years), children under 5 years, pregnant women, and individuals with chronic medical conditions such as cardiovascular disease, diabetes, chronic respiratory disease, and immunosuppression.
The overall case fatality rate (CFR) is below 0.1% in the general population, but rises to 2–3% among hospitalized patients and is significantly higher in those aged over 65 years. In the United States alone, influenza results in an estimated 140,000–710,000 hospitalizations and 12,000–52,000 deaths annually, depending on the circulating strains and vaccine effectiveness.
Annual vaccination is the cornerstone of influenza prevention and is recommended by the WHO for all individuals aged 6 months and older, with particular emphasis on high-risk groups and healthcare workers. Current vaccines include inactivated influenza vaccines (IIV), recombinant influenza vaccines (RIV), and live attenuated influenza vaccines (LAIV). Vaccine effectiveness varies from 10–60% depending on the match between vaccine and circulating strains.
Antiviral medications, principally neuraminidase inhibitors (oseltamivir, zanamivir) and the cap-dependent endonuclease inhibitor baloxavir marboxil, are available for treatment and post-exposure prophylaxis but must be initiated within 48 hours of symptom onset for maximum benefit.
Seek immediate medical attention if any of the following warning signs develop during influenza illness:
In adults:
Difficulty breathing or shortness of breath at rest
Persistent chest pain or pressure
Confusion, altered mental status, or inability to arouse
Severe or persistent vomiting with inability to maintain hydration
Cyanosis (bluish discoloration of lips or face)
Symptoms that improve then return with worsening fever and cough (suggesting secondary bacterial infection)
Oxygen saturation below 92% on pulse oximetry
In children:
Fast or labored breathing, nasal flaring, or intercostal retractions
Cyanosis or gray skin color
Dehydration (no tears when crying, decreased urination, sunken fontanelle in infants)
Inconsolable irritability or failure to interact when awake
Febrile seizures, especially if prolonged or recurrent
Inability to feed (in infants)
High-risk patients — including those aged ≥65, pregnant women, immunocompromised individuals, and those with chronic cardiopulmonary disease — should contact their healthcare provider early in the course of illness (within 48 hours) to discuss antiviral treatment, even if symptoms appear mild initially. Rapid deterioration can occur, particularly from days 4–7 of illness.
Most common signs and symptoms
Influenza characteristically presents with abrupt onset of systemic and respiratory symptoms, distinguishing it from the more gradual onset of the common cold. The hallmark symptom triad includes high fever (38–40°C), myalgia, and dry cough. Fever typically lasts 3–5 days and may be accompanied by rigors and diaphoresis.
Systemic symptoms predominate in the first 2–3 days and include severe headache, generalized myalgia (particularly in the back and limbs), arthralgia, profound malaise, and anorexia. Many patients describe overwhelming fatigue that may persist for 1–2 weeks after other symptoms resolve.
Respiratory symptoms include nonproductive cough (which may become the most prominent symptom as systemic symptoms wane), sore throat, nasal congestion, and rhinorrhea. The cough may persist for 2 weeks or longer. Ocular symptoms such as photophobia, tearing, and burning are common.
In children, influenza may present with higher fevers, febrile seizures, otitis media, nausea, vomiting, and abdominal pain more frequently than in adults. Elderly patients may present atypically with confusion, falls, or functional decline without classic febrile illness. In immunocompromised patients, symptoms may be prolonged and atypical, with lower respiratory tract involvement more common.
The disease can range from an asymptomatic infection (estimated in 16–33% of cases based on serological studies) to fulminant viral pneumonia with acute respiratory distress syndrome (ARDS).
Knowing the symptoms is the first step to a quick response.
The natural course of influenza follows a well-characterized temporal pattern. The incubation period ranges from 1–4 days, with a median of 2 days. Viral shedding begins approximately 1 day before symptom onset and continues for 5–7 days in adults and up to 10–14 days in young children and immunocompromised patients.
Day 1–2: Abrupt onset of systemic symptoms including high fever (38–40°C), severe myalgia, headache, malaise, and anorexia. Respiratory symptoms (dry cough, sore throat) begin concurrently but are initially overshadowed by systemic manifestations. This phase represents peak viremia and highest infectivity.
Day 3–4: Fever typically begins to subside. Systemic symptoms gradually improve while respiratory symptoms (particularly cough and nasal congestion) become more prominent. Some patients experience a biphasic fever pattern with brief defervescence followed by recurrence. Viral load begins to decline but the patient remains infectious.
Day 5–7: Resolution of fever and progressive improvement in most symptoms. Cough may worsen temporarily as airways recover. Infectivity diminishes in most adults. This is a critical period for development of secondary bacterial pneumonia, signaled by recrudescent fever, purulent sputum, and clinical deterioration.
Week 2–3: Cough gradually resolves but may persist for 2–3 weeks. Fatigue and reduced exercise tolerance are common during this recovery phase. Most patients return to baseline functional status by 2–3 weeks. In high-risk groups, complete recovery may take 4–6 weeks. Development of complications at any stage warrants reassessment.
How this disease is identified
Influenza diagnosis relies on a combination of clinical assessment and laboratory confirmation. Clinical diagnosis during known influenza activity periods has a sensitivity of approximately 60–70% in adults presenting with the classic triad of fever, cough, and acute onset. However, laboratory confirmation is essential for definitive diagnosis, especially outside of peak season.
Rapid influenza diagnostic tests (RIDTs) detect viral nucleoprotein antigens in respiratory specimens and provide results within 15–30 minutes. Their main advantage is point-of-care availability, but sensitivity is only 50–70% (with higher specificity of 90–95%), meaning a negative result does not exclude influenza. Newer digital immunoassay rapid tests offer improved sensitivity of 80–90%.
Reverse transcription polymerase chain reaction (RT-PCR) is the gold standard for influenza diagnosis, with sensitivity and specificity both exceeding 95%. RT-PCR can differentiate between influenza A and B, identify subtypes (H1N1, H3N2), and detect co-infections. Results are typically available within 1–6 hours. Multiplex PCR panels can simultaneously test for influenza and other respiratory pathogens (RSV, SARS-CoV-2, adenovirus, etc.).
Preferred specimens include nasopharyngeal swabs or aspirates, which yield higher viral loads than oropharyngeal or nasal swabs. Specimen collection within 48–72 hours of symptom onset optimizes detection sensitivity, as viral shedding peaks at 24–72 hours. Other methods include viral culture (3–7 days, now primarily used for surveillance and vaccine strain selection) and immunofluorescence assays (moderate sensitivity, rapid turnaround).
Available treatment methods
Treatment of influenza combines supportive care with antiviral therapy in appropriate patients. Supportive measures include adequate hydration, antipyretics (paracetamol or ibuprofen; avoid aspirin in children due to the risk of Reye syndrome), rest, and cough management.
Antiviral therapy is recommended for all hospitalized patients and outpatients at high risk for complications, regardless of illness duration at presentation. In otherwise healthy outpatients, antivirals provide maximum benefit when initiated within 48 hours of symptom onset. Currently recommended agents include:
Oseltamivir (Tamiflu): 75 mg twice daily for 5 days (oral); the most widely used antiviral with extensive safety data including in pregnancy. Reduces symptom duration by approximately 1 day and may reduce complications.
Zanamivir (Relenza): 10 mg (2 inhalations) twice daily for 5 days (inhaled); contraindicated in patients with underlying airway disease due to risk of bronchospasm.
Baloxavir marboxil (Xofluza): single dose, weight-based (40–80 mg); a cap-dependent endonuclease inhibitor that achieves rapid viral load reduction. Approved for patients ≥5 years.
Peramivir (Rapivab): 600 mg single IV dose; an option for patients unable to take oral or inhaled medications.
Secondary bacterial pneumonia, the most common serious complication, requires empiric antibiotic therapy targeting Staphylococcus aureus (including MRSA), Streptococcus pneumoniae, and Streptococcus pyogenes. Patients with severe disease may require intensive care, mechanical ventilation, and in refractory cases, extracorporeal membrane oxygenation (ECMO).
Most cases are effectively treated with early diagnosis.
How to protect yourself
Annual influenza vaccination is the most effective measure for preventing influenza and its complications. The WHO recommends vaccination for all individuals aged ≥6 months, with highest priority for healthcare workers, pregnant women, elderly persons (≥65), children aged 6 months to 5 years, and individuals with chronic medical conditions. Current vaccine options include:
Inactivated influenza vaccine (IIV4): quadrivalent, intramuscular; standard-dose and high-dose formulations available. The high-dose and adjuvanted formulations are preferred for adults ≥65 years.
Recombinant influenza vaccine (RIV4): egg-free production, suitable for individuals with severe egg allergy.
Live attenuated influenza vaccine (LAIV4): intranasal, approved for ages 2–49 years; contraindicated in pregnancy and severe immunosuppression.
Vaccine effectiveness varies annually (10–60%) depending on the match between vaccine strains and circulating viruses, but even in mismatched seasons, vaccination reduces the severity of illness, hospitalization rates by 40–60%, and mortality. Vaccination should occur before the start of influenza season (typically October–November in the Northern Hemisphere).
Non-pharmaceutical interventions play a crucial complementary role: frequent handwashing with soap and water (or alcohol-based hand sanitizer), respiratory hygiene (covering coughs and sneezes), avoidance of touching the face, maintaining distance from ill individuals, and staying home when symptomatic for at least 24 hours after fever resolution. In healthcare settings, droplet and contact precautions are standard, with airborne precautions for aerosol-generating procedures.
Antiviral chemoprophylaxis with oseltamivir (75 mg once daily) may be considered for high-risk exposed individuals who cannot be vaccinated, for a duration of 7–10 days after last known exposure.
Preparation is the best protection.
Influenza poses a year-round risk for international travelers, as the timing of influenza seasons differs by hemisphere and the virus circulates continuously in tropical regions. In the Northern Hemisphere, peak activity occurs December–March; in the Southern Hemisphere, June–September. In the tropics, influenza can occur at any time, often with multiple peaks.
Travelers should ensure they have received the current season's influenza vaccine before departure. Those traveling between hemispheres during the opposite season may have reduced protection if circulating strains differ from their vaccine formulation. The CDC recommends that travelers to the Southern Hemisphere during June–September consider revaccination with the Southern Hemisphere formulation if available.
High-risk travel scenarios for influenza include: cruise ship voyages (outbreaks are common in closed settings), the Hajj pilgrimage (mass gathering with documented annual influenza outbreaks), large international sporting events, and group tours. In these settings, attack rates may reach 20–40% among unvaccinated individuals.
Travelers should carry a supply of antipyretics and consider carrying a prescription for oseltamivir if they are at high risk for complications and traveling to areas with limited healthcare access. Standard respiratory hygiene measures — hand hygiene, avoiding crowded enclosed spaces when possible, and use of face masks during outbreaks — are advisable.
Travelers who develop influenza-like illness during or after travel should seek medical evaluation, especially if returning from areas with reported avian influenza (H5N1, H7N9) or novel influenza A virus activity.
Statistics and geographic data
Influenza is a globally distributed disease with a well-documented epidemiological pattern. The WHO Global Influenza Surveillance and Response System (GISRS), comprising 143 institutions across 113 countries, monitors circulating strains and informs annual vaccine composition.
Seasonal epidemiology: In temperate regions, influenza exhibits distinct winter seasonality with epidemics lasting 6–8 weeks. In the Northern Hemisphere, peak activity occurs between December and March; in the Southern Hemisphere, between June and September. Tropical regions experience year-round circulation with variable peaks, often associated with rainy seasons. Annual attack rates are estimated at 5–10% in adults and 20–30% in children.
Global burden: The WHO estimates that seasonal influenza causes approximately 1 billion infections, 3–5 million cases of severe illness, and 290,000–650,000 respiratory deaths annually. The economic burden is substantial: in the United States alone, annual direct medical costs exceed $10 billion, with total economic impact (including lost productivity) estimated at $87 billion.
Circulating strains: Since 2009, seasonal epidemics have been caused by influenza A(H1N1)pdm09, A(H3N2), and influenza B (Victoria and Yamagata lineages). Notably, the B/Yamagata lineage has not been detected since March 2020 and may be functionally extinct. A(H3N2) seasons typically produce higher morbidity and mortality than A(H1N1) seasons, particularly in the elderly.
Pandemic potential: Influenza A viruses circulate in animal reservoirs (avian, swine) and can undergo reassortment to produce novel pandemic strains. Avian influenza viruses (H5N1, H7N9) cause sporadic zoonotic infections with high CFRs (30–60%) but limited human-to-human transmission to date. Global pandemic preparedness remains a WHO priority.
Who is most at risk
Risk factors for influenza infection and severe outcomes can be categorized into factors affecting susceptibility to infection and those predisposing to severe disease and complications.
Risk factors for infection:
Close contact with infected individuals (household, schools, dormitories, military barracks)
Crowded living or working conditions
Healthcare setting exposure (nosocomial transmission)
Lack of vaccination or waning immunity from prior season
Travel during influenza season in destination region
Mass gatherings (pilgrimages, festivals, sporting events)
Risk factors for severe disease and complications:
Age extremes: children <2 years and adults ≥65 years bear the highest burden; children <6 months cannot be vaccinated
Pregnancy and early postpartum (up to 2 weeks post-delivery): 3–4-fold increased risk of hospitalization
Chronic pulmonary disease: asthma, COPD, cystic fibrosis, bronchiectasis
Cardiovascular disease: heart failure, coronary artery disease (excluding isolated hypertension)
Metabolic conditions: diabetes mellitus (both type 1 and 2), morbid obesity (BMI ≥40)
Immunosuppression: HIV/AIDS, organ transplant recipients, cancer chemotherapy, chronic corticosteroid use, primary immunodeficiencies
Chronic renal or hepatic disease
Neurological/neuromuscular conditions: particularly those affecting respiratory function or secretion clearance
Residents of long-term care facilities: attack rates of 20–50% during institutional outbreaks
Individuals with multiple comorbidities face compounding risk; a patient with diabetes, obesity, and chronic kidney disease may have a 10-fold or greater increase in the odds of ICU admission compared to a healthy adult of the same age.
Potential complications
Influenza complications range from mild to life-threatening and are responsible for the substantial morbidity and mortality associated with seasonal epidemics. Complications are most frequent in high-risk groups but can occur in previously healthy individuals.
Pulmonary complications are the most common and clinically significant. Primary viral pneumonia presents with progressive dyspnea, hypoxemia, and bilateral infiltrates, often progressing to ARDS; it carries a mortality rate of 10–30% even with intensive care. Secondary bacterial pneumonia typically manifests 4–7 days into illness with recrudescent fever and purulent sputum; causative organisms include Staphylococcus aureus (including MRSA), Streptococcus pneumoniae, and Haemophilus influenzae. Exacerbation of underlying chronic lung disease (COPD, asthma) accounts for a significant proportion of influenza-related hospitalizations.
Cardiovascular complications are increasingly recognized as a major contributor to influenza-associated mortality. The risk of acute myocardial infarction increases 6–10-fold in the week following laboratory-confirmed influenza. Myocarditis occurs in an estimated 0.4–13% of hospitalized cases (subclinical involvement detected by troponin elevation may be higher), and can lead to fulminant heart failure. Acute stroke risk is similarly elevated in the post-influenza period.
Neurological complications include febrile seizures (primarily in children aged 6 months to 5 years), encephalitis/encephalopathy (rare but potentially devastating, more common with influenza A), Guillain-Barré syndrome, and transverse myelitis.
Other complications include rhabdomyolysis (with risk of acute kidney injury), Reye syndrome (associated with aspirin use in children), otitis media (in 10–50% of children), parotitis, and toxic shock syndrome associated with secondary staphylococcal infection.
Expected outcomes and recovery
The prognosis of seasonal influenza is excellent for the majority of patients. In otherwise healthy individuals, symptoms resolve within 3–7 days, although cough and malaise may persist for 2 weeks or more. The overall case fatality rate is below 0.1% in the general population.
However, prognosis varies significantly with patient demographics and comorbidities. The hospitalization rate is estimated at 1–2% of symptomatic cases, with the highest rates in children under 2 years and adults over 65 years. Among hospitalized patients, the CFR rises to approximately 2–3%, and intensive care admission rates of 10–25% have been reported during severe seasons.
Age-specific mortality is markedly skewed toward the elderly: approximately 70–85% of seasonal influenza deaths occur in individuals aged ≥65 years. In this age group, influenza may trigger fatal cardiovascular events (acute myocardial infarction, stroke) with incidence peaking in the week following infection. Among children under 5 years, the global mortality burden is estimated at 9,200–105,600 deaths annually.
Patients with chronic comorbidities face significantly worse outcomes. Chronic obstructive pulmonary disease, congestive heart failure, diabetes mellitus, chronic kidney disease, and immunosuppression each independently increase the risk of hospitalization and death by 2–5-fold. Pregnant women, particularly in the third trimester, have hospitalization rates comparable to elderly populations.
Post-influenza recovery is generally complete, but a proportion of patients experience post-infectious fatigue lasting weeks to months. Rare long-term sequelae include Guillain-Barré syndrome, transverse myelitis, and exacerbation of underlying autoimmune conditions.
This disease is vaccine-preventable. Effective protection is available through vaccination.
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