Pulmonary · PANCE / PANRE

Influenza

Acute viral respiratory illness from influenza A or B with seasonal epidemics and pandemic potential.

Also known as: influenza, flu, seasonal flu, influenza A, influenza B

Overview

Acute respiratory illness caused by influenza A or B viruses (single-stranded segmented RNA, family Orthomyxoviridae), characterized by abrupt onset of fever, myalgia, headache, and respiratory symptoms.

Epidemiology

Annual seasonal epidemics in temperate regions (Northern Hemisphere November-April). Causes 9-41 million illnesses, 100,000-700,000 hospitalizations, and 12,000-52,000 deaths annually in the US. Antigenic drift drives seasonal variation; antigenic shift in influenza A drives pandemics (1918, 1957, 1968, 2009).

🔒 Free preview limit reached

Keep reading — start your free trial

You've read your 2 free diagnosis previews. Create your free account to unlock the full Influenza outline — plus all 514 diagnoses, 3,500+ board-style questions, flashcards, and an AI tutor. Your 7-day free trial includes everything, and there's no credit card required.

Start your 7-day free trial → Sign in

Free to start · No credit card · Cancel anytime

Risk factors

Age <2 or ≥65 years
Pregnancy (especially 2nd and 3rd trimester) and up to 2 weeks postpartum
Chronic comorbidities: pulmonary (asthma, COPD), cardiovascular, renal, hepatic, neurologic, metabolic (diabetes), hematologic
Immunosuppression: HIV, cancer, transplant, immunosuppressive medications
Morbid obesity (BMI ≥40)
Residents of long-term care facilities
American Indian/Alaska Native populations (increased severe disease)

Pathophysiology

Influenza virus binds sialic acid receptors on respiratory epithelium via hemagglutinin (HA). Viral neuraminidase (NA) cleaves sialic acid to release new virions. Local respiratory epithelial injury, systemic inflammatory cytokine response, and impaired mucociliary clearance predispose to secondary bacterial pneumonia. Influenza A subtypes (H1N1, H3N2) and influenza B lineages (Victoria, Yamagata) co-circulate.

Clinical presentation

Symptoms

Abrupt onset (over hours) of fever, chills, myalgia, headache, fatigue
Respiratory: dry cough, sore throat, nasal congestion/rhinorrhea
GI symptoms (vomiting, diarrhea) more common in children
Severe prostration disproportionate to other URI causes
Usually self-limited 3-7 days, but cough and malaise may linger 1-2 weeks

Signs / physical exam

Fever, often 38.5-40°C
Conjunctival injection, mild pharyngeal erythema without exudate
Mild cervical lymphadenopathy
Respiratory exam usually unremarkable unless complicated (rales, wheeze suggest pneumonia)

Differential diagnosis

Common cold (rhinovirus, coronavirus) — Less systemic illness, prominent rhinorrhea/sore throat without high fever or severe myalgia
COVID-19 — Anosmia/ageusia prominent; positive SARS-CoV-2 testing; can have prolonged illness
RSV (older adults) — Wheeze, hypoxia; can be severe; multiplex PCR distinguishes
Acute bronchitis — Cough-predominant, mild systemic symptoms
Streptococcal pharyngitis — Sore throat with tonsillar exudate, anterior cervical adenopathy, no cough; positive strep test
Mononucleosis — Pharyngitis with posterior cervical adenopathy, splenomegaly, atypical lymphocytes; positive heterophile
Bacterial pneumonia (often post-influenza) — Persistent or recurrent fever after initial flu improvement, focal infiltrate, purulent sputum

Diagnostic workup

Labs

Often clinical diagnosis during peak season in otherwise well outpatients
Rapid influenza diagnostic tests (RIDT) — point-of-care, modest sensitivity (~50-70%), high specificity
Rapid molecular assays (NAAT/PCR) — higher sensitivity, preferred when treatment decision or hospitalization at stake
RT-PCR — gold standard; multiplex panels can simultaneously detect SARS-CoV-2, RSV, influenza
Severe disease: CBC (often lymphopenia), CMP, lactate, blood cultures if bacterial superinfection suspected

Imaging

Chest radiograph if hypoxia, focal findings, or worsening symptoms — to exclude primary viral pneumonia or secondary bacterial pneumonia
CT chest if diagnosis uncertain or complicated

Diagnostic algorithm

Antiviral	Route	Dose	Notes
Oseltamivir	Oral	75 mg BID × 5 days	First-line; renal dose adjust; nausea common
Zanamivir	Inhaled	10 mg (2 puffs) BID × 5 days	Avoid in asthma/COPD (bronchospasm)
Peramivir	IV	600 mg single dose	For patients unable to tolerate oral/inhaled
Baloxavir	Oral	40-80 mg single dose	Endonuclease inhibitor; single-dose convenience; resistance emerging

FDA-approved antivirals for influenza in adults (IDSA 2018).

Treatment

First-line

Antivirals — start ASAP for symptomatic patients (ideally within 48 h of onset):
Oseltamivir 75 mg BID × 5 days (oral; renal dose adjust) — most widely used
Zanamivir 10 mg (2 inhalations) BID × 5 days — avoid in asthma/COPD (bronchospasm risk)
Peramivir 600 mg IV × 1 dose — for those unable to tolerate PO
Baloxavir marboxil 40-80 mg PO × 1 dose — endonuclease inhibitor; single-dose; emerging resistance
Hospitalized patients: treat regardless of symptom duration
Outpatients: treat if high-risk (age, pregnancy, chronic conditions, immunocompromise, severe illness) or within 48 h of onset in healthy patients
Supportive care: hydration, rest, acetaminophen/NSAIDs for fever and myalgia (avoid aspirin in children — Reye syndrome)

Second-line / adjunct

Treat secondary bacterial pneumonia (S. pneumoniae, S. aureus including MRSA, H. influenzae): empiric beta-lactam ± vancomycin/linezolid based on severity
Post-exposure chemoprophylaxis with oseltamivir or zanamivir for high-risk unvaccinated close contacts (within 48 h of exposure) — 7 days
Prevention is paramount: ANNUAL influenza vaccination for everyone ≥6 months
Vaccine options: standard inactivated (IIV3/IIV4), high-dose (adults ≥65), adjuvanted (adults ≥65), recombinant (RIV4), live-attenuated nasal (LAIV4, ages 2-49 non-pregnant non-immunocompromised)
Infection control: droplet precautions in hospitals; isolation of infected patients

Complications

Primary influenza viral pneumonia (especially in pregnancy, immunocompromised)
Secondary bacterial pneumonia (S. pneumoniae, S. aureus including MRSA, H. influenzae) — often biphasic with initial improvement then deterioration
ARDS, multi-organ failure
Exacerbation of chronic conditions: COPD, asthma, heart failure
Myocarditis, pericarditis
Encephalitis, transverse myelitis, Guillain-Barré syndrome
Myositis, rhabdomyolysis (especially influenza B in children)
Reye syndrome (children given aspirin)

PANCE pearls

Abrupt-onset fever, myalgia, and dry cough in a previously well adult during flu season is influenza until proven otherwise.
Antiviral benefit is greatest within 48 h of symptom onset — do not delay for testing in high-risk patients.
Biphasic illness (initial improvement, then recurrent fever and worsening) suggests secondary bacterial pneumonia, often S. aureus or S. pneumoniae.
AVOID aspirin in children with influenza or varicella — Reye syndrome risk.
Annual vaccination is the most important preventive intervention; everyone ≥6 months should receive it absent specific contraindication.

References

IDSA 2018 — Clinical Practice Guidelines by IDSA: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenza (Uyeki et al., Clin Infect Dis 2019)
CDC 2024 — Prevention and Control of Seasonal Influenza with Vaccines: ACIP Recommendations 2024-25 (MMWR Recomm Rep)
Baloxavir Trials — Baloxavir Marboxil for Uncomplicated Influenza in Adults and Adolescents (Hayden et al., NEJM 2018)
Cochrane Oseltamivir — Neuraminidase Inhibitors for Preventing and Treating Influenza in Healthy Adults and Children (Jefferson et al., Cochrane Database Syst Rev 2014)

Practice Pulmonary questions on FirstPassPA

Turn this outline into retention. 3,500+ board-style questions with an AI tutor that explains every answer — free to start, no card required.

Start studying free → Browse all 514 diagnoses

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants.