Influenza Flashcards

Influenza

Introduction to Influenza

• Influenza is an acute viral respiratory infection.
• It is predominantly caused by Influenza A and B viruses, with Influenza C being a rare cause.
• Influenza outbreaks occur annually on a global scale, predominantly during the winter season.

Epidemiology

• Influenza exhibits seasonal epidemics, particularly from October to March in the Northern Hemisphere.
• Influenza A is characterized by its rapid onset and a duration of 2-3 months.

Antigenic Subtypes of Influenza A Virus

The following table lists the antigenic subtypes of influenza A virus associated with pandemics or epidemics:

Note: Subtypes marked with an asterisk () were determined by retrospective serologic surveys.

• During the 2009-2010 H1N1 influenza A pandemic, mortality was relatively high among individuals <65 years and relatively low among those ≥65 years.

Transmission

• Influenza is transmitted through respiratory droplets, fomites, and aerosols.
• The incubation period is 1-4 days, with an average of 2 days.
• Peak viral shedding occurs 24-48 hours after symptom onset.

High-Risk Groups

• Children <5 years, especially <2 years
• Adults ≥65 years
• Immunosuppressed individuals
• Individuals with chronic medical conditions (COPD, heart disease)

Groups at Higher Risk for Influenza Complications

• Children <5 years, especially those <2 years (rates of hospitalization and mortality are greatest among those <6 months).
• Adults ≥65 years of age.
• Pregnant individuals or those up to 2 weeks postpartum.
• Residents of nursing homes and long-term care facilities.
• Non-Hispanic Black persons, Hispanic or Latino persons, and American Indian or Alaska Native persons.
• People with medical conditions, including:
  • Asthma.
  • Neurologic and neurodevelopmental conditions (e.g., cerebral palsy, epilepsy, stroke).
  • Chronic lung disease (e.g., COPD, cystic fibrosis).
  • Heart disease (e.g., congenital heart disease, congestive heart failure).
  • Blood disorders (e.g., sickle cell disease).
  • Endocrine disorders (e.g., diabetes mellitus).
  • Kidney diseases.
  • Liver disorders.
  • Metabolic disorders.
  • Weakened immune system (e.g., HIV/AIDS, cancer).
  • Children <19 years of age receiving long-term aspirin therapy.
  • People with Class III obesity (BMI ≥40 or ≥140% of the 95th percentile).
• Increased risk may be related to economic and social conditions (e.g., poverty, multigenerational households, limited access to vaccination).

Clinical Features – Adults

• Abrupt onset of fever, nonproductive cough, and myalgias.
• Malaise, sore throat, and headache.

Clinical Features – Children

• High variability from mild to severe symptoms.
• High fever, cough, and nasal congestion.
• Gastrointestinal symptoms are more frequent in children compared to adults.

Signs and Symptoms in Children with Influenza

The following table summarizes the signs, symptoms, and clinical diagnoses at the initial visit in different age groups of children with influenza:

*P-value is indicated. ND = not determined.

Complications

• Pneumonia (secondary bacterial)
• Exacerbation of chronic diseases (COPD, asthma)
• Neurological complications (rare)

Diagnosis of Influenza

• Clinical suspicion.
• Rapid antigen tests, PCR (gold standard).
• Normal chest radiography unless complications are present.

Influenza Diagnostic Tests

The following table summarizes influenza diagnostic tests for respiratory specimens:

Differential Diagnosis

• Bacterial respiratory infections
• RSV
• COVID-19
• Common cold (Rhinovirus)

Treatment in Adults

• Antivirals: Oseltamivir (preferred), Peramivir
• Prompt initiation within 48 hours of symptom onset
• Supportive treatment: hydration, analgesics

Hospitalized Patient Management

• Immediate antiviral therapy
• Oseltamivir preferred
• Empiric therapy despite negative initial tests if suspicion is high

Outpatient Antiviral Treatment in Adults

• For patients with symptomatic influenza who do not warrant hospitalization:
  • If progressive illness, risk for complications, or exposure to others at risk, consider prompt antiviral treatment if the interval since symptom onset is <48 hours.
  • Antiviral treatment may be associated with a modest reduction in illness duration (approximately 24 hours).
  • Antiviral treatment is generally not suggested if symptom onset was ≥48 hours prior.
  • Hospitalization is warranted for patients with dehydration, respiratory distress, hypoxemia, impaired cardiopulmonary function, or altered mental status.
  • Adults at risk for influenza complications include those ≥65 years old, pregnant or postpartum individuals, nursing home residents, immunocompromised patients, and those with comorbid conditions.

Pediatric Management

• Antiviral treatment in severe cases, hospitalized patients, and high-risk children.
• Oseltamivir dosing is weight/age-specific.

Antiviral Regimens for Treatment and/or Prophylaxis of Seasonal Influenza in Adults

Prevention – Vaccination Principles

• Annual vaccination is recommended.
• WHO provides annual vaccine strain selection.
• Shift from quadrivalent to trivalent vaccines.

Types of Vaccines (Adults)

• Inactivated vaccines (IIV): standard/high dose
• Recombinant Influenza Vaccine (RIV)
• Cell-culture based vaccines (egg-free)
• Live Attenuated Influenza Vaccine (LAIV)

Vaccination in Children

• Recommended from 6 months of age
• Special considerations: egg allergy, immunocompromised states

Vaccine Efficacy and Safety

• Reduces complications, hospitalization, and mortality.
• Common mild side effects.
• Rare severe reactions (e.g., Guillain-Barré syndrome).

Public Health Surveillance

• CDC FluView, WHO Flu Net
• Critical for managing outbreaks.

Summary

• Early diagnosis and treatment are important.
• Annual vaccination is key for prevention.
• Continuous surveillance is crucial.