Influenza Clinical Presentation and Complications

General Symptoms and Signs

• Influenza is characterized by the sudden onset of constitutional and respiratory symptoms, including fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis, typically with fever, in the general population 1
• The illness usually resolves within 7-10 days, though cough and malaise may persist for weeks 2, 3
• Children may present with higher fevers and more prominent gastrointestinal symptoms, including nausea and vomiting, compared to adults, while elderly individuals may present with less pronounced fever but more severe complications 1, 4

High-Risk Groups and Complications

• The World Health Organization identifies persons aged >65 years, very young children, pregnant women, and persons with underlying medical conditions, such as chronic respiratory disease, chronic cardiac disease, and immunosuppression, as being at high risk for complications 2, 3, 5
• Secondary bacterial pneumonia, primary viral pneumonia, encephalitis/encephalopathy, myositis, Reye's syndrome, myocarditis/pericarditis, and acute respiratory distress syndrome are potential complications of influenza 6, 5
• Cardiovascular, neurological, and muscular complications can occur, including alterations in ECG, myocarditis, pericarditis, encephalitis/encephalopathy, transverse myelitis, myositis, and myoglobinuria 5, 7, 8

Warning Signs for Severe Illness

• The American Academy of Pediatrics recommends monitoring for warning signs of severe illness, including difficulty breathing, persistent pain or pressure in the chest or abdomen, persistent dizziness, confusion, or inability to arouse, seizures, and severe muscle pain 6, 5
• Severe influenza is characterized by persistent high fever, severe cough, dyspnea, altered mental status, or dehydration, in patients with severe illness 9

Transmission and Prevention

• Transmission of influenza occurs primarily through large respiratory droplets from coughing and sneezing, within 1 meter of infected individuals, as reported by the European Centre for Disease Prevention and Control, similar to guidelines from the World Health Organization and the CDC 10, 11
• Frequent handwashing with soap and water or alcohol-based hand rubs is recommended for infection control 12
• Implement Standard and Droplet Precautions for hospitalized patients, including placing patients in private rooms with doors kept closed, and having healthcare personnel wear surgical masks, nonsterile gloves, and gowns, as recommended by the American Society of Transplantation, similar to guidelines from the CDC 13
• Annual influenza vaccination is the most effective preventive strategy, in the general population 1
• Chemoprophylaxis with oseltamivir or zanamivir can be considered for high-risk individuals exposed to influenza, in high-risk individuals 1

Diagnosis and Treatment

• Diagnosis is primarily clinical, based on symptoms and knowledge of local influenza prevalence, in outpatient cases, with laboratory confirmation not necessary for most outpatient cases but may be useful in hospitalized patients, patients where diagnosis will change treatment decisions, and during early influenza season to confirm circulation 1
• Neuraminidase inhibitors, such as oseltamivir and zanamivir, should be started within 48 hours of symptom onset, preferably within 24 hours, for maximum benefit, in high-risk patients and those with severe illness, as recommended by the American Academy of Pediatrics (AAP) and the Infectious Diseases Society of America (IDSA) 1, 2
• Oseltamivir is recommended at a dosage of 75 mg twice daily for 5 days, with dosage adjustment needed for renal impairment, in adults 1
• Symptomatic treatment for uncomplicated influenza includes antipyretics/analgesics, rest, adequate hydration, and avoiding smoking, in the general population 6
• Secondary bacterial pneumonia requires antibiotic therapy, with oral co-amoxiclav or tetracycline for non-severe cases and IV antibiotics for severe cases, in patients with secondary bacterial pneumonia 6

Stages of Illness and Return to Work/School

• The following stages of illness have been identified:

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• Individuals should not return to work or school until fever has resolved for at least 24 hours without fever-reducing medications, most acute symptoms have improved, and they are able to perform normal activities without excessive fatigue, as recommended by the CDC and the WHO 3, 13, 7