30.1 Influenza, COVID-19 & Viral Respiratory Syndromes

ABFM Family Medicine Certification Study Guide Exam

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1 Chapter 1: Exam Structure, Blueprint & Clinical Decision-Making

1.1 Exam Blueprint, FMCLA vs. One-Day Exam & Board Scoring 1.2 Clinical Reasoning, Test-Taking Strategy & Ambiguity Management 1.3 Evidence-Based Medicine & Clinical Practice Guidelines in Primary Care

2 Chapter 2: Healthcare Systems, Practice Management & Quality

2.1 Practice Management, Billing, Coding & Documentation 2.2 Quality Improvement, Patient Safety & Diagnostic Stewardship 2.3 Interprofessional Team-Based Care & Care Coordination

3 Chapter 3: Ethics, Law, Biostatistics & Health Equity

3.1 Medical Ethics, Informed Consent & Surrogate Decision-Making 3.2 Biostatistics, Clinical Epidemiology & Diagnostic Test Interpretation 3.3 Social Determinants of Health, Health Equity & Cultural Humility 3.4 Physician Wellness, Burnout Prevention & Professionalism

4 Chapter 4: Lifestyle Counseling & Behavior Change

4.1 Alcohol Use Screening & Brief Intervention 4.2 Illicit Drug & Cannabis Counseling 4.3 Tobacco & Vaping Cessation Counseling

5 Chapter 5: Nutrition, Physical Activity & Chronic Disease Prevention

5.1 Dietary Counseling for Obesity & Chronic Disease Prevention 5.2 Exercise Prescription & Physical Activity Counseling 5.3 Genetic Risk Assessment & Family History Counseling

6 Chapter 6: Routine Adult Immunizations

6.1 ACIP Adult Respiratory Vaccines: Influenza, COVID-19, RSV & Pneumococcal 6.2 ACIP Adult Non-Respiratory Vaccines: Tdap, Shingrix, HPV & Hep B 6.3 Immunizations in Special Populations: Pregnancy, Asplenia & Immunocompromise

7 Chapter 7: Pediatric Health Maintenance & Surveillance

7.1 Newborn Examination & Early Postnatal Care 7.2 Well-Child Care, Developmental Surveillance & Milestones 7.3 Pediatric & Adolescent Screening: Vision, Hearing, Anemia & Lead 7.4 Pediatric Oral Health, Dental Screening & Fluoride

8 Chapter 8: Behavioral Health & Safety Screening

8.1 Primary Care Depression, Anxiety & Behavioral Health Screening 8.2 Domestic Violence, Intimate Partner Violence & Firearm Safety 8.3 Occupational Health & Recreational Safety Counseling

9 Chapter 9: Adult Preventive Screening: Cardiovascular & Metabolic

9.1 Cardiovascular Disease & Hypertension Screening 9.2 Type 2 Diabetes Screening & Prediabetes Risk Stratification 9.3 Abdominal Aortic Aneurysm & Osteoporosis Screening

10 Chapter 10: Adult Cancer Screening: Breast & Gynecologic

10.1 Breast Cancer Screening & Risk-Assessment 10.2 Cervical Cancer Screening & ASCCP Algorithms 10.3 Skin Cancer Clinical Assessment & Counseling

11 Chapter 11: Adult Cancer Screening: Colorectal, Lung & Prostate

11.1 Colorectal Cancer Screening Modalities & Intervals 11.2 Lung Cancer Screening with Low-Dose CT 11.3 Prostate Cancer Screening & Shared Decision-Making

12 Chapter 12: Sexual Health, Infectious Disease Screening & Contraception

12.1 Sexual Health Counseling & Risk-Reduction Strategies 12.2 STI Screening: Chlamydia, Gonorrhea, Syphilis & Trichomoniasis 12.3 Bloodborne Infectious Disease Screening: HIV & Hepatitis C

13 Chapter 13: Reproductive Life Planning, Lactation & Preoperative Care

13.1 Preconception Planning & Comprehensive Contraception Management 13.2 Lactation Support & Breastfeeding Counseling 13.3 Preoperative Medical Evaluation & Pre-Participation Sports Exam

14 Chapter 14: Emergent Cardiovascular Conditions

14.1 Acute Coronary Syndromes & Myocardial Infarction 14.2 Acute Chest Pain Risk Stratification & Triage 14.3 Acute or Critical Valvular Heart Emergencies

15 Chapter 15: Cardiac Arrhythmias, Heart Failure & Hypertensive Crises

15.1 Acute Atrial & Ventricular Arrhythmias 15.2 Acute Decompensated Heart Failure & Cardiogenic Shock 15.3 Hypertensive Emergency & Urgency Management

16 Chapter 16: Vascular & Thromboembolic Emergencies

16.1 Pulmonary Embolism & Deep Vein Thrombosis 16.2 Acute Aortic Syndromes & Peripheral Vascular Emergencies 16.3 Critical Limb Ischemia & Acute Arterial Occlusion

17 Chapter 17: Neurologic Emergencies: Stroke & Altered Consciousness

17.1 Acute Ischemic Stroke & Intracranial Hemorrhage 17.2 Syncope Evaluation & Status Epilepticus 17.3 Acute Delirium, Encephalopathy & Severe Confusion

18 Chapter 18: Neurologic Trauma & Acute Cranial Emergencies

18.1 Concussion, Moderate-to-Severe TBI & Intracranial Injury 18.2 Thunderclap & Acute Severe Secondary Headaches 18.3 Acute Ophthalmologic Emergencies

19 Chapter 19: Acute Abdominal, Pelvic & Gastrointestinal Emergencies

19.1 Acute Abdominal & Pelvic Pain Triage 19.2 Upper & Lower Gastrointestinal Bleeding 19.3 Acute Hepatic Failure, Cholangitis & Severe Jaundice

20 Chapter 20: Acute Pulmonary & Airway Emergencies

20.1 Acute Hypoxic Respiratory Failure & ARDS 20.2 Severe Community-Acquired Pneumonia & Empyema 20.3 Acute Severe Epistaxis & Airway Obstruction

21 Chapter 21: Acute Traumatic Injuries & Wound Care

21.1 Major Trauma Resuscitation & Initial Assessment 21.2 Burn Assessment, Inhalation Injury & Resuscitation 21.3 Laceration Management, Complex Wounds & Animal Bites

22 Chapter 22: Acute Orthopedic & Musculoskeletal Emergencies

22.1 Acute Ambulatory Fractures & Initial Splinting 22.2 Compartment Syndrome & Major Orthopedic Trauma 22.3 Septic Arthritis & Acute Microcrystalline Crises

23 Chapter 23: Severe Infections, Sepsis & Systemic Emergencies

23.1 Sepsis, Septic Shock & Initial Fluid Resuscitation 23.2 Necrotizing Soft Tissue Infections & Severe CNS Infections 23.3 Anaphylaxis Diagnosis, Epinephrine Dosing & Airway Protection

24 Chapter 24: Metabolic, Endocrine & Electrolyte Emergencies

24.1 Diabetic Ketoacidosis & Hyperosmolar Hyperglycemic State 24.2 Critical Electrolyte Emergencies: Dyskalemias & Dysnatremias 24.3 Severe Calcium Abnormalities & Acute Adrenal Crisis

25 Chapter 25: Hematologic, Urologic & Dental Emergencies

25.1 Severe Symptomatic Anemias & Coagulopathic Emergencies 25.2 Testicular Torsion, Priapism & Acute Urinary Retention 25.3 Urgent Odontogenic Infections & Dental Trauma

26 Chapter 26: Psychiatric, Behavioral & Obstetric/Gynecologic Emergencies

26.1 Suicidal Ideation, Acute Agitation & Behavioral Emergencies 26.2 Acute Physical & Sexual Assault Care 26.3 Life-Threatening Vaginal Bleeding & Ectopic Pregnancy 26.4 Environmental Heat Stroke & Severe Hypothermia

27 Chapter 27: Acute Head, Neck & Neurologic Symptoms

27.1 Primary Acute Headaches: Migraine, Tension & Cluster 27.2 Concussion & Minor Closed Head Injury Evaluation 27.3 Transient Ischemic Attack Evaluation & ABCD2 Risk Scoring

28 Chapter 28: Acute Cranial Nerves, Hearing & Sensory Symptoms

28.1 Peripheral Neuropathies: Bell's Palsy, Carpal Tunnel & Radiculopathy 28.2 Acute Tinnitus, Sudden Sensorineural Hearing Loss & Cerumen Impaction 28.3 Acute Dizziness, Benign Paroxysmal Positional Vertigo & Vestibular Neuritis

29 Chapter 29: Acute Upper Respiratory Tract Infections

29.1 Acute Viral Rhinosinusitis vs. Bacterial Rhinosinusitis 29.2 Acute Pharyngitis: Group A Streptococcus & Centor Criteria 29.3 Acute Otitis Media, Otitis Externa & Serous Effusion

30 Chapter 30: Acute Lower Respiratory Infections & Cough

30.1 Influenza, COVID-19 & Viral Respiratory Syndromes 30.2 Acute Bronchitis & Lower Respiratory Tract Infections 30.3 Ambulatory Community-Acquired Pneumonia

31 Chapter 31: Allergic, Nasal & Non-Infectious Pulmonary Conditions

31.1 Allergic Rhinitis & Acute Non-Anaphylactic Allergic Reactions 31.2 Acute Dyspnea in Ambulatory Care 31.3 Interstitial Lung Diseases & Sarcoidosis Presentation

32 Chapter 32: Acute Chest, Cardiac & Vascular Symptoms

32.1 Palpitations Evaluation & Arrhythmia Triage in Primary Care 32.2 Deep Vein Thrombosis & Superficial Thrombophlebitis 32.3 Ambulatory Hypotension & Orthostasis Evaluation

33 Chapter 33: Acute Upper Gastrointestinal Conditions

33.1 Peptic Ulcer Disease & Gastritis 33.2 Dysphagia, Esophagitis & Acute Esophageal Disorders 33.3 Acute Nausea, Vomiting & Foodborne Illness

34 Chapter 34: Acute Lower Gastrointestinal Conditions

34.1 Acute Infectious Diarrhea & Gastroenteritis 34.2 Acute Constipation & Functional Bowel Changes 34.3 Ano-Rectal Disorders: Hemorrhoids, Fissures, Abscesses & Pruritus Ani

35 Chapter 35: Inflammatory, Functional & Malabsorptive GI Disorders

35.1 Acute Flares of Inflammatory Bowel Disease 35.2 Irritable Bowel Syndrome: Diagnosis, Subtyping & Management 35.3 Celiac Disease & Malabsorption Syndromes

36 Chapter 36: Acute Abdominal, Hepatic & Biliary Disorders

36.1 Ambulatory Abdominal & Pelvic Pain Triage 36.2 Acute Cholecystitis, Biliary Colic & Pancreatitis 36.3 Acute Liver Injury & Viral Hepatitis

37 Chapter 37: Acute Abdominal Wall, Hernias & Dental Conditions

37.1 Abdominal Wall, Inguinal & Ventral Hernias 37.2 Acute Dental Pain, Odontogenic Abscess & Aphthous Ulcers 37.3 Acute Edema: Unilateral vs. Bilateral Diagnostic Evaluation

38 Chapter 38: Acute Infectious Dermatoses

38.1 Bacterial Skin Infections: Impetigo, Cellulitis & Cutaneous Abscesses 38.2 Fungal Skin Infections: Tinea Corporis, Cruris, Pedis & Onychomycosis 38.3 Viral Cutaneous Infections: Herpes Simplex, Zoster & Molluscum Contagiosum

39 Chapter 39: Inflammatory, Allergic & Autoimmune Dermatoses

39.1 Atopic Dermatitis, Contact Dermatitis & Eczematous Flares 39.2 Non-Infectious Rashes: Urticaria, Drug Eruptions & Erythema Multiforme 39.3 Cutaneous Malignancies & Premalignant Lesions: AK, BCC, SCC & Melanoma

40 Chapter 40: Acute Shoulder, Elbow & Upper Extremity Injuries

40.1 Rotator Cuff Pathology, Tendinopathy & Shoulder Impingement 40.2 Lateral & Medial Epicondylitis, Olecranon Bursitis & Elbow Sprains 40.3 Wrist & Hand Soft-Tissue Injuries: De Quervain, TFCC & Finger Sprains

41 Chapter 41: Acute Knee, Ankle & Lower Extremity Injuries

41.1 Meniscal Tears, Ligamentous Knee Injuries & Patellofemoral Pain 41.2 Ankle Sprains, Ottawa Ankle Rules & Achilles Tendinopathy 41.3 Plantar Fasciitis, Shin Splints & Overuse Syndromes

42 Chapter 42: Acute Ambulatory Fractures & Dislocations

42.1 Distal Radius, Scaphoid & Common Upper Extremity Fractures 42.2 Metatarsal, Clavicle & Common Lower Extremity Fractures 42.3 Joint Dislocation Management: Nursemaid's Elbow & Shoulder Subluxation

43 Chapter 43: Acute Axial Musculoskeletal Pain & Falls

43.1 Acute Low Back Pain: Red Flags, Strain & Disc Herniation 43.2 Acute Cervical Strain, Whiplash & Axial Neck Pain 43.3 Evaluation of Falls in Ambulatory & Older Patients

44 Chapter 44: Acute Inflammatory Arthropathies & Generalized Pain

44.1 Acute Inflammatory Arthritis & Spondyloarthropathies 44.2 Myalgias, Rhabdomyolysis & Post-Viral Pain Syndromes 44.3 Acute Non-Headache Pain Management Strategies

45 Chapter 45: Acute Renal, Urinary & Genitourinary Conditions

45.1 Uncomplicated UTI, Pyelonephritis & Acute Urethritis 45.2 Hematuria Workup & Glomerulonephritis 45.3 Nephrolithiasis & Acute Urinary Tract Obstruction

46 Chapter 46: Acute Male Genitourinary & Prostatic Conditions

46.1 Acute Prostatitis, Epididymitis & Orchitis 46.2 Acute Scrotal Pain: Torsion vs Appendage, BPH & Male Genital Emergencies 46.3 Acute Urinary Retention & Incontinence Evaluation

47 Chapter 47: Acute Women's Health & Early Pregnancy Care

47.1 Early Pregnancy Diagnosis, Dating & Initial Complications 47.2 Abnormal Uterine Bleeding & Acute Menstrual Disorders 47.3 Premenstrual Syndrome & Premenstrual Dysphoric Disorder

48 Chapter 48: Breast Disorders, STIs & Pediatric Elimination

48.1 Palpable Breast Mass, Mastitis & Nipple Discharge 48.2 Sexually Transmitted Infections: Diagnosis, Staging & CDC Guidelines 48.3 Pediatric Enuresis & Encopresis

49 Chapter 49: Systemic Signs, Metabolic Dysregulation & Miscellaneous Infections

49.1 Acute Fever, Sepsis Screening & FUO in Primary Care 49.2 Acute Hyperglycemia & Symptomatic Hypoglycemia 49.3 Electrolyte Abnormalities: Mild-to-Moderate Disturbances 49.4 Vector-Borne, Tick-Borne & Rickettsial Illnesses

50 Chapter 50: Chronic Cardiovascular Diseases: Hypertension & Ischemia

50.1 Essential Hypertension Management: 2025 AHA/ACC Guideline & JNC Strategies 50.2 Chronic Coronary Artery Disease & Post-MI Secondary Prevention 50.3 Chronic Heart Failure: HFrEF, HFpEF & Quadruple GDMT

51 Chapter 51: Chronic Vascular, Valvular & Dyslipidemic Diseases

51.1 Peripheral Arterial Disease & Claudication 51.2 Chronic Valvular Heart Disease & Murmur Evaluation 51.3 Lipid Disorders & ASCVD Risk-Based Statin Therapy

52 Chapter 52: Chronic Respiratory Diseases: Asthma & COPD

52.1 Chronic Asthma Management: GINA Stepwise SMART Therapy 52.2 Chronic Obstructive Pulmonary Disease: GOLD Recommendations 52.3 Solitary Pulmonary Nodule Evaluation & Fleischner Society Criteria

53 Chapter 53: Chronic Metabolic Diseases: Diabetes Mellitus

53.1 Type 2 Diabetes Management: ADA Pharmacotherapy Algorithms 53.2 Type 1 Diabetes: Basal-Bolus Insulin, CGMs & Diabetic Complications 53.3 Diabetic Microvascular & Macrovascular Disease Surveillance

54 Chapter 54: Obesity, Nutrition & Metabolic Bone Diseases

54.1 Medical & Behavioral Management of Obesity & Incretin Mimetics 54.2 Osteoporosis & Metabolic Bone Disease: DEXA, FRAX & Antiresorptives 54.3 Nutritional Deficiencies: Vitamin D, B12, Iron & Malnutrition

55 Chapter 55: Chronic Endocrine Disorders

55.1 Primary & Subclinical Hypothyroidism, Hyperthyroidism & Nodules 55.2 Adrenal Disorders: Addison's Disease & Cushing's Syndrome 55.3 Hypercalcemia, Hyperparathyroidism & Pituitary Disorders

56 Chapter 56: Chronic Kidney Disease & Venous Disorders

56.1 Chronic Kidney Disease: KDIGO Staging & Renoprotective Therapy 56.2 Chronic Venous Insufficiency, Stasis Dermatitis & Lymphedema 56.3 Chronic Electrolyte & Acid-Base Management in CKD

57 Chapter 57: Chronic Gastrointestinal & Hepatic Diseases

57.1 Gastroesophageal Reflux Disease, Barrett's Esophagus & Dyspepsia 57.2 Chronic Liver Disease: MASLD/NAFLD, Cirrhosis & Portal Hypertension 57.3 Viral Hepatitis B & C: Long-Term Monitoring & DAA Therapy

58 Chapter 58: Colorectal Surveillance & Chronic Pancreaticobiliary Care

58.1 Post-Polypectomy Colorectal Polyp Surveillance Guidelines 58.2 Chronic Pancreatitis & Exocrine Pancreatic Insufficiency 58.3 Chronic Nausea, Gastroparesis & Functional GI Disorders

59 Chapter 59: Chronic Musculoskeletal & Rheumatologic Disorders

59.1 Osteoarthritis: Hip, Knee & Hand Management 59.2 Spinal Conditions: Spinal Stenosis, Spondylolisthesis & Compression Fractures 59.3 Chronic Myopathies & Fibromyalgia Syndrome

60 Chapter 60: Chronic Neurologic & Neurodegenerative Disorders

60.1 Dementia: Alzheimer's, Vascular & Lewy Body Disease 60.2 Parkinson's Disease & Essential Tremor 60.3 Epilepsy: Chronic Antiseizure Medications & Post-Stroke Care

61 Chapter 61: Chronic Peripheral Nerves, Pain & Sensory Disorders

61.1 Diabetic & Chronic Peripheral Neuropathies 61.2 Chronic Pain Syndromes & Chronic Daily Headaches 61.3 Chronic Tinnitus, Presbycusis & Persistent Dizziness

62 Chapter 62: Chronic Psychiatric Disorders

62.1 Major Depressive Disorder & Generalized Anxiety Disorder 62.2 Bipolar Disorder: Maintenance Pharmacotherapy & Lithium Monitoring 62.3 Adult & Pediatric Attention-Deficit/Hyperactivity Disorder

63 Chapter 63: Chronic Behavioral Health & Neurodevelopmental Conditions

63.1 Chronic Behavioral Conditions: PTSD, OCD & Somatization 63.2 Autism Spectrum Disorder & Intellectual Disabilities 63.3 Eating Disorders: Anorexia Nervosa, Bulimia & Binge Eating

64 Chapter 64: Chronic Substance Use Disorders & Tobacco Cessation

64.1 Alcohol Use Disorder: Pharmacotherapy & Relapse Prevention 64.2 Opioid & Stimulant Use Disorders: MOUD & Harm Reduction 64.3 Longitudinal Tobacco Cessation Pharmacotherapy

65 Chapter 65: Chronic Dermatologic Conditions

65.1 Acne Vulgaris, Rosacea & Seborrheic Dermatitis 65.2 Chronic Plaque Psoriasis & Biologic Therapies 65.3 Chronic Eczema, Hidradenitis Suppurativa & Cutaneous Lupus

66 Chapter 66: Chronic Reproductive, Infectious & Geriatric Special Care

66.1 Chronic Menstrual Disorders, Infertility & Sexual Dysfunction 66.2 Menopause Management & Hormone Replacement Therapy 66.3 Chronic HIV Infection Management & Primary Care Integration 66.4 Gender-Affirming Primary Care & Hormone Therapy 66.5 Advance Care Planning, Palliative Care & End-of-Life Symptom Control

67 Chapter 67: Continuity Care: Sleep, Fatigue, Cancer, Pregnancy & the Adult Preventive Visit

67.1 Chronic Insomnia & Obstructive Sleep Apnea 67.2 Chronic Malaise, Fatigue & ME/CFS 67.3 Primary Care During Cancer Treatment & Survivorship 67.4 Routine Prenatal Care & Longitudinal Pregnancy Management 67.5 The Periodic Adult Health Examination & Medicare Annual Wellness Visit

30.1 Influenza, COVID-19 & Viral Respiratory Syndromes

Key Takeaways Explain This Topic AI

Clinical diagnosis of influenza is defined by the abrupt onset of high fever, rigors, severe myalgias, headache, and non-productive cough; rapid molecular NAAT/RT-PCR (sensitivity 90-95%) is the preferred diagnostic method over rapid antigen tests (sensitivity 50-70%).

Neuraminidase inhibitors (oral oseltamivir 75 mg BID for 5 days) and the endonuclease inhibitor baloxavir marboxil provide maximal clinical benefit when initiated within 48 hours of symptom onset; however, antiviral therapy is indicated regardless of symptom duration in hospitalized patients, severe/progressive illness, and high-risk populations (age <2 or ≥65, pregnancy/postpartum, BMI ≥40, chronic cardiopulmonary, renal, hepatic, or neurologic disease, immunosuppression, and nursing home residents).

Secondary bacterial pneumonia following influenza presents as a biphasic 'double sickening' (initial clinical improvement followed by recrudescent fever, purulent sputum, and new dyspnea), most commonly caused by Streptococcus pneumoniae, Staphylococcus aureus (including MRSA with necrotizing cavitation), and Group A Streptococcus.

First-line outpatient pharmacotherapy for mild-to-moderate COVID-19 in high-risk patients is oral nirmatrelvir-ritonavir (Paxlovid 300/100 mg BID for 5 days, initiated within 5 days of onset; dose-reduced to 150/100 mg BID for eGFR 30-59 mL/min, contraindicated if eGFR <30 mL/min), which requires vigilant review for severe CYP3A4 drug-drug interactions (statins, DOACs, antiarrhythmics, anticonvulsants); IV remdesivir (3-day course initiated within 7 days) is the preferred alternative.

Systemic corticosteroids (e.g., dexamethasone) are strictly contraindicated in outpatient mild-to-moderate COVID-19 without hypoxemia (SpO2 ≥94% on room air), as they provide no benefit, delay viral clearance, and increase mortality.

Last updated: September 2026

Influenza: Virology, Clinical Features & Diagnostic Assessment

Influenza is an acute respiratory infection caused by negative-sense, single-stranded RNA viruses of the family Orthomyxoviridae . Annual epidemics cause substantial morbidity and mortality, particularly among extremes of age and individuals with chronic medical comorbidities.

Virology & Pathophysiology

Influenza A : Infects humans and animal reservoirs (aquatic birds, swine). Characterized by two major surface glycoproteins: Hemagglutinin (HA) , which binds sialic acid receptors on respiratory epithelial cells to facilitate viral entry, and Neuraminidase (NA) , which enzymatically cleaves terminal sialic acid to release newly formed virions from infected host cells. Influenza A undergoes both antigenic drift (minor point mutations in HA and NA leading to seasonal epidemics) and antigenic shift (genetic reassortment between animal and human strains, creating novel surface antigens and pandemic potential).

Influenza B : Infects humans almost exclusively and is categorized into two lineages (B/Yamagata and B/Victoria). Influenza B undergoes antigenic drift only, causing seasonal outbreaks but never pandemics.

Incubation & Transmission : The incubation period averages 2 days (range 1 to 4 days). Transmission occurs via respiratory droplet aerosolization (coughing, sneezing) and fomites. Viral shedding commences 24 to 48 hours prior to symptom onset, peaks during the initial 48 to 72 hours of illness, and abates by 5 to 7 days in immunocompetent adults. Prolonged viral shedding (>10 to 14 days) occurs in young children, elderly adults, and immunocompromised hosts.

Clinical Presentation

Classic Adult Presentation : Abrupt, explosive onset of systemic and respiratory symptoms ("patients can pinpoint the exact hour they became ill"). Hallmark manifestations include:

High fever (typically 38.5°C to 40.0°C / 101°F to 104°F) accompanied by shaking chills and rigors;

Severe, debilitating diffuse myalgias and arthralgias, prominent in the paraspinal and lower extremity musculature;

Retro-orbital or frontal headache, photophobia, profound prostration, and anorexia;

Non-productive dry cough, substernal burning or chest soreness, sore throat, and clear rhinorrhea.

Physical Examination : Flushed facies, warm diaphoresis, hyperemic pharynx without purulent exudates, and mild cervical lymphadenopathy. Pulmonary auscultation is usually normal or reveals scattered dry rhonchi and mild wheezes. Focal crackles or signs of consolidation suggest pneumonia.

Atypical Presentation in Older Adults : Adults aged ≥65 years frequently present without classic high fevers or prominent respiratory signs. Instead, influenza often manifests as acute delirium, confusion, lethargy, anorexia, unexplained falls, or acute decompensation of underlying congestive heart failure or COPD.

Diagnostic Modalities: Molecular NAAT vs. Rapid Antigen Tests

Diagnostic Test Sensitivity Specificity Turnaround Time Clinical Utility & Pearls

Rapid Molecular NAAT / RT-PCR 90% - 99% >98% 15 - 30 minutes Gold standard / Preferred test in all outpatient and inpatient settings. Differentiates Influenza A and B and identifies subtyping; high negative predictive value rules out disease.

Rapid Influenza Diagnostic Tests (RIDTs) 50% - 70% 90% - 95% 10 - 15 minutes Immunoassay detecting viral nucleoprotein. High false-negative rate (30-50%). A negative RIDT does NOT exclude influenza during peak community prevalence; empiric antiviral therapy should never be withheld based on a negative rapid antigen test.

Viral Culture >95% 100% 3 - 7 days Not clinically useful for acute bedside decision-making; reserved for public health surveillance and antigenic drift surveillance.

Important

Clinical Decision Rule During Peak Epidemics :

When influenza is actively circulating in the community, the presence of abrupt fever and cough in an adult has a positive predictive value exceeding 80%. Empiric antiviral therapy should be initiated immediately in patients with high-risk conditions without delaying for laboratory confirmation, and a negative rapid antigen test should not deter clinical treatment.

Antiviral Pharmacotherapy: Formulations, Timing & High-Risk Indications

Antiviral agents reduce the duration of fever and systemic symptoms by 24 to 36 hours, accelerate functional recovery, and decrease the incidence of secondary complications (acute otitis media, lower respiratory tract infection, hospitalization, and antibiotic exposure). INFLUENZA ANTIVIRAL THERAPY

Confirmed or High-Suspicion Influenza
│
┌──────────────────────────┴──────────────────────────┐
▼ ▼
Onset <=48 Hours Onset >48 Hours
│ │

│ Assess Risk Criteria:

│ • Hospitalized / Severe illness
│ • Age <2 or >=65 years
│ • Chronic pulmonary / cardiac disease
│ • Diabetes / CKD / Cirrhosis
│ • Immunocompromised / Cancer
│ • Pregnancy / <=2 wk postpartum
│ • BMI >=40 kg/m²
│ • Nursing home resident
│ │
├─────────────────────────────────────────────────────┤
│ │
▼ ▼
ANTIVIRAL INDICATED ANTIVIRAL INDICATED
Oseltamivir 75 mg PO BID x 5d (preferred) (If ANY high-risk criterion met)
Baloxavir single weight-based dose ─────────────────────────────────

Zanamivir 10 mg INH BID (no asthma/COPD) If Healthy Host with Onset >48h:

Peramivir 600 mg IV single dose • Antiviral NOT indicated
Symptomatic care only
Antiviral Drug Classes & Dosing Regimens
1. Neuraminidase Inhibitors (NAIs)

Oral Oseltamivir (Tamiflu) :

Adult Dosing : 75 mg PO twice daily for 5 days.

Renal Impairment Adjustments :

eGFR >60 mL/min: 75 mg PO BID;
eGFR 30 to 60 mL/min: 30 mg PO BID;
eGFR 10 to 30 mL/min: 30 mg PO once daily;

End-stage renal disease on hemodialysis: 30 mg PO immediately after each dialysis session.

Pregnancy & Lactation : Oseltamivir is the first-line drug of choice across all trimesters of pregnancy and during the first 2 weeks postpartum. Maternal physiologic benefits significantly outweigh potential drug risks.

Adverse Effects : Transient nausea (10%) and vomiting (8%), which are markedly alleviated by taking medication with meals. Rare neuropsychiatric events (confusion, hallucinations, delirium) have been reported, primarily in pediatric populations in East Asia.

Inhaled Zanamivir (Relenza) :

Adult Dosing : 10 mg (two 5-mg inhalations via Diskhaler) twice daily for 5 days. Approved for age ≥7 years.

CRITICAL CONTRAINDICATION : Absolutely contraindicated in individuals with underlying reactive airway disease, including asthma, COPD, or bronchiectasis , due to the life-threatening risk of precipitating acute bronchospasm and severe bronchoconstriction.

Intravenous Peramivir (Rapivab) :

Adult Dosing : Single IV infusion of 600 mg over 15 to 30 minutes. Approved for age ≥6 months.

Indication : Acute uncomplicated influenza in patients who cannot tolerate oral or inhaled routes (e.g., severe refractory emesis, malabsorption syndromes, or altered gastrointestinal motility).

2. Cap-Dependent Endonuclease Inhibitor

Oral Baloxavir Marboxil (Xofluza) :

Mechanism of Action : Selectively inhibits the cap-dependent endonuclease within the viral RNA polymerase complex, arresting viral mRNA synthesis. Demonstrates significantly faster reduction in viral load and viral shedding compared to oseltamivir.

Adult Dosing : Single oral dose administered once:

Weight 40 kg to <80 kg: Single 40-mg dose.

Weight ≥80 kg: Single 80-mg dose.

Drug Absorption Warning : Chelation occurs when co-administered with polyvalent cations; do not administer simultaneously with dairy products, calcium-fortified beverages, antacids (aluminum, magnesium), iron supplements, or sucralfate.

Contraindications & Limitations : Not recommended in pregnancy or lactation (inadequate safety profile); avoid in severely immunocompromised patients due to rapid selection for treatment-emergent resistance mutations (I38T/M/F).

The 48-Hour Clinical Rule & High-Risk Exceptions

The Golden Window : Antiviral initiation within 48 hours of symptom onset provides maximal reduction in fever duration, symptom burden, hospitalizations, and secondary bacterial complications.

Mandatory Indications for Antiviral Therapy REGARDLESS of Symptom Duration (>48 Hours) :

Hospitalized Patients : Any patient with suspected or confirmed influenza requiring acute hospitalization.

Severe or Progressive Illness : Any outpatient demonstrating clinical deterioration, severe respiratory distress, hypoxemia (SpO2 <94%), or systemic toxicity.

High-Risk Populations (CDC & IDSA Guidelines):

Children aged <2 years (highest hospitalization rates in infants <6 months);
Adults aged ≥65 years;

Individuals with chronic pulmonary disorders (asthma, COPD, bronchiectasis, cystic fibrosis, interstitial lung disease);

Individuals with chronic cardiovascular disorders (congestive heart failure, coronary artery disease, cardiomyopathy; excluding isolated hypertension);

Chronic renal disease (CKD stage 3-5, ESRD on dialysis);
Chronic hepatic disease (cirrhosis, chronic viral hepatitis);
Metabolic disorders (diabetes mellitus types 1 and 2, thyroid disorders);

Neurologic and neurodevelopmental conditions (epilepsy, stroke, cerebral palsy, muscular dystrophy, spinal cord injury, cognitive impairment causing dysphagia or aspiration risk);

Immunocompromise (HIV/AIDS, hematologic malignancies, active chemotherapy, solid organ transplant recipients, daily chronic systemic corticosteroids [≥20 mg prednisone equivalent]);

Pregnancy and women within 2 weeks postpartum;
Severe obesity (Body Mass Index BMI ≥40 kg/m²);
Residents of nursing homes, assisted living facilities, and long-term care institutions;

American Indians and Alaska Natives.

Complications & Secondary Bacterial Pneumonia: The "Double Sickening" Phenomenon

While primary influenza viral pneumonia causes fulminant bilateral alveolar damage, the most common life-threatening pulmonary complication is secondary bacterial pneumonia .

The "Double Sickening" Trajectory

Clinical Presentation : The patient develops typical influenza, experiences noticeable clinical improvement over days 2 to 4 (defervescence of fever, easing of myalgias), and then suddenly experiences an abrupt recrudescence on days 5 to 10 with new high-grade spiking fevers, violent shaking chills, worsening dyspnea, pleuritic chest pain, and productive cough with purulent, blood-tinged or rust-colored sputum.

Pathophysiology : Influenza viral neuraminidase strips protective sialic acid residues from respiratory mucosal epithelial cells, denuding the tracheobronchial basement membrane, exposing fibronectin receptors, paralyzing mucociliary escalators, and inducing apoptosis of alveolar macrophages and neutrophils. This creates a primed microenvironment for secondary bacterial adherence and parenchymal invasion.

Microbial Pathogens & Antimicrobial Selection

Streptococcus pneumoniae : Most frequent causative organism overall (~50% of secondary bacterial pneumonias). Causes classical dense lobar consolidation, rusty sputum, and prominent leukocytosis with bandemia.

Staphylococcus aureus (MSSA and Community-Acquired MRSA) : Particularly virulent and notorious following influenza. Often secretes the Panton-Valentine leukocidin (PVL) cytotoxin, causing severe necrotizing pneumonia, hemoptysis, multiple cavitary lung abscesses, pneumatoceles, empyema, profound leukopenia, and rapid progression to septic shock.

Group A Streptococcus (Streptococcus pyogenes) : Causes rapidly progressive necrotizing pneumonia and massive, early-accumulating parapneumonic empyema.

Empiric Inpatient Regimen : Requires immediate coverage for both S. pneumoniae and MRSA: Intravenous Ceftriaxone (1 to 2 g daily) PLUS Intravenous Vancomycin (15-20 mg/kg Q8-12H targeted to trough 15-20 mcg/mL or AUC 400-600) or Linezolid (600 mg IV Q12H) .

Outpatient COVID-19 Management & Therapeutics

SARS-CoV-2 presentation spans asymptomatic infection to life-threatening acute respiratory distress syndrome (ARDS). In ambulatory care, risk stratification identifies individuals at high risk for progression to severe disease who benefit from early targeted antiviral therapeutics.

High-Risk Criteria for Severe COVID-19
Age ≥65 years (the single strongest prognostic predictor of hospitalization and death);
Unvaccinated or incompletely vaccinated status;
Chronic kidney disease (especially stages 3-5);
Chronic cardiopulmonary disease (COPD, moderate-to-severe asthma, heart failure, CAD);
Diabetes mellitus (type 1 or type 2);
Obesity (BMI ≥30 kg/m², with exponential risk at BMI ≥40 kg/m²);
Moderate-to-severe immunocompromise (solid organ transplant, active malignancy, immunosuppressive therapies);

Advanced liver disease, sickle cell disease, or neurodevelopmental disorders.

Outpatient Therapeutic Regimens
1. Nirmatrelvir-Ritonavir (Paxlovid) - Preferred First-Line Oral Therapy

Mechanism : Nirmatrelvir is an orally active peptidomimetic inhibitor of the SARS-CoV-2 main protease (Mpro / 3CLpro), blocking viral polyprotein cleavage. Ritonavir has no intrinsic anti-SARS-CoV-2 activity; it acts as a potent, irreversible CYP3A4 inhibitor (pharmacokinetic booster) that prevents the hepatic metabolism of nirmatrelvir, maintaining therapeutic plasma levels.

Efficacy : Demonstrates an ~86% to 89% relative reduction in COVID-19-related hospitalization and death when initiated within 5 days of symptom onset.

Standard Dosing : Nirmatrelvir 300 mg (two 150-mg pink tablets) PLUS Ritonavir 100 mg (one 100-mg white tablet) taken together orally twice daily for 5 days.

Timing Window : Must be started within 5 days of symptom onset .

Renal Dosage Adjustment :

eGFR ≥60 mL/min: Standard dose (300/100 mg PO BID);

eGFR 30 to 59 mL/min (moderate impairment) : Reduce nirmatrelvir to 150 mg (one tablet) with ritonavir 100 mg (one tablet) PO BID for 5 days;

eGFR <30 mL/min or ESRD on dialysis : CONTRAINDICATED . Drug accumulation and safety data are lacking; alternative therapies (IV remdesivir) must be used.

Critical CYP3A4 Drug-Drug Interactions (Paxlovid)

Ritonavir profoundly inhibits CYP3A4, causing dangerous elevations of co-administered medications with narrow therapeutic indices:

Statins : Simvastatin and lovastatin are absolutely contraindicated (severe rhabdomyolysis risk). Atorvastatin and rosuvastatin should be temporarily held during the 5-day treatment course and resumed 3 to 5 days after completion.

Direct Oral Anticoagulants (DOACs) : Rivaroxaban is absolutely contraindicated (dramatic elevation of drug levels and life-threatening hemorrhage). Apixaban levels increase significantly; consider temporary switch to enoxaparin/unfractionated heparin, or choose IV remdesivir.

Antiarrhythmics : Amiodarone, flecainide, propafenone, dronedarone, quinidine are contraindicated due to fatal ventricular arrhythmias.

Anticonvulsants : Carbamazepine, phenytoin, phenobarbital, and primidone are potent CYP inducers that drastically reduce nirmatrelvir/ritonavir plasma concentrations, causing viral therapeutic failure; combination is contraindicated.

Immunosuppressants : Calcineurin inhibitors (tacrolimus, cyclosporine) and mTOR inhibitors (sirolimus, everolimus) experience marked, unpredictable level spikes causing acute renal shutdown and toxicity; requires specialized transplant team management or switching to remdesivir.

Sedatives/Hypnotics : Midazolam, triazolam (prolonged sedation, respiratory depression).

COVID-19 Rebound : A recurrence of mild symptoms or a new positive antigen test 2 to 8 days following completion of a 5-day course. Rebound is self-limited, does not progress to severe disease or hospitalization, and does not warrant a repeat course of Paxlovid .

2. Intravenous Remdesivir (Veklury) - Preferred Alternative

Mechanism : Adenosine nucleotide analog that inhibits viral RNA-dependent RNA polymerase (RdRp), inducing premature chain termination.

Regimen : 3-day outpatient intravenous infusion: 200 mg IV on Day 1, followed by 100 mg IV on Days 2 and 3 .

Timing Window : Initiated within 7 days of symptom onset (87% relative reduction in hospitalization).

Clinical Indications : First-line alternative when Paxlovid is contraindicated due to severe renal failure (eGFR <30 mL/min), severe drug-drug interactions, or inability to swallow oral medication.

3. Molnupiravir (Lagevrio) - Secondary Oral Option

Dosing : 800 mg (four 200-mg capsules) PO twice daily for 5 days, initiated within 5 days of onset.

Efficacy : Modest efficacy (~30% relative reduction in hospitalization), significantly inferior to Paxlovid and Remdesivir. Reserved strictly for non-hospitalized adults when neither Paxlovid nor Remdesivir is clinically accessible.

Contraindications : Strictly contraindicated in pregnancy (teratogenic, embryofetal lethality) and in patients aged <18 years (cartilage toxicity, bone growth plate malformations).

Explicitly Harmful / Non-Recommended Therapies in Outpatient COVID-19

Systemic Corticosteroids (Dexamethasone) : The landmark RECOVERY trial proved that dexamethasone reduces mortality in patients requiring supplemental oxygen or mechanical ventilation, but causes net harm (increased mortality and prolonged viral clearance) in patients who do NOT require supplemental oxygen ! Never prescribe dexamethasone for ambulatory outpatients with SpO2 ≥94% on room air!

Antibiotics (Azithromycin) : Ineffective against viral infection; randomized trials demonstrate zero benefit while driving bacterial resistance.

Ineffective Therapies : Hydroxychloroquine, ivermectin, colchicine, high-dose zinc, and vitamin C have failed to show benefit in randomized trials.

Respiratory Syncytial Virus (RSV) in Adults

RSV is increasingly recognized as a major pathogen in older adults (≥65 years) and individuals with chronic cardiopulmonary disorders (COPD, congestive heart failure, asthma) or severe immunocompromise, accounting for 60,000 to 160,000 hospitalizations annually in the United States.

Clinical Presentation : Manifests as lower respiratory tract disease with persistent cough, wheezing, dyspnea, copious sputum production, and low-grade fever. It is a frequent precipitant of acute exacerbations of COPD (AECOPD) and acute decompensated heart failure (ADHF) .

Diagnostic Confirmation : Multiplex RT-PCR respiratory viral panel (nasopharyngeal swab).

Management : Primarily supportive care: supplemental oxygen, judicious intravenous hydration, and inhaled short-acting bronchodilators (albuterol/ipratropium) for objective bronchospasm. Oral or inhaled ribavirin is reserved exclusively for select severely immunocompromised hosts (e.g., allogeneic hematopoietic cell transplant recipients) in specialty consultation.

RSV Vaccines in Adults : Recombinant protein subunit vaccines (Arexvy, Abrysvo) and mRNA vaccines (mResvia) are recommended as a single lifetime dose for all adults aged ≥75 years, and for adults aged 60 to 74 years with chronic medical conditions (COPD, CHF, CKD, diabetes, immunocompromise) via shared clinical decision-making.