Influenza

01Pathophysiology

Influenza virus belongs to the Orthomyxoviridae family and contains a segmented, single-stranded, negative-sense RNA genome. The two surface glycoproteins that drive both pathogenesis and immune evasion are hemagglutinin (HA) and neuraminidase (NA).

Hemagglutinin binds to sialic acid residues on respiratory epithelial cells, allowing the virus to attach and enter the host cell. This is the primary target of neutralizing antibodies and the key antigen in influenza vaccines. Once the virus replicates inside the cell, neuraminidase cleaves sialic acid on the host cell surface, releasing newly formed virions to infect neighboring cells. This is the enzymatic target of oseltamivir and zanamivir.

The virus causes direct cytopathic damage to the respiratory epithelium, leading to cell death and denudation of the mucosal surface. This explains the dry, nonproductive cough and sore throat. The denuded epithelium also creates a vulnerability to secondary bacterial infection, most commonly by Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae.

The systemic symptoms (high fever, myalgia, headache, profound malaise) are driven by a robust cytokine response, particularly IL-6, TNF-alpha, and interferons. This is why influenza feels disproportionately severe compared to other upper respiratory infections. The abrupt, dramatic onset of systemic symptoms is the hallmark that distinguishes influenza from the common cold.

Understanding the segmented genome is critical for exam purposes. Because the genome is divided into 8 segments, reassortment can occur when two different influenza strains co-infect the same cell (commonly in pigs or birds). This produces a completely novel HA or NA subtype, which the population has no pre-existing immunity against. This is called antigenic shift and is responsible for pandemics. In contrast, antigenic drift refers to gradual point mutations in HA and NA genes during normal replication, producing minor antigenic changes that cause seasonal epidemics and necessitate annual vaccine reformulation.

02Classification and Clinical Manifestation

Influenza Virus Types

Clinical Manifestation Spectrum

03Diagnostic Workup

In most outpatient settings during influenza season, the diagnosis is clinical. A patient presenting with abrupt onset of fever, myalgia, and cough during a known influenza outbreak does not routinely require laboratory confirmation to initiate treatment.

However, when testing is needed (hospitalized patients, immunocompromised hosts, atypical presentations, or infection control purposes), the approach is as follows:

The RIDT is the best initial test because it is fast and widely available at the point of care. It detects viral nucleoprotein antigens via immunoassay. The key limitation to remember is its low sensitivity (50 to 70%), meaning a negative RIDT cannot be used to exclude influenza. If clinical suspicion remains high despite a negative RIDT, proceed to RT-PCR.

RT-PCR is the most accurate test and serves as the gold standard for confirmation. It detects viral RNA with greater than 99% sensitivity and can subtype influenza A (e.g., H1N1, H3N2). This is the preferred test for hospitalized patients and outbreak investigations.

Viral culture is no longer used for clinical diagnosis due to slow turnaround but remains important for public health surveillance and antiviral resistance testing.

Obtain a chest X-ray only when you suspect a complication such as pneumonia. In primary viral pneumonia, expect bilateral diffuse interstitial or ground-glass infiltrates. In secondary bacterial pneumonia, expect focal lobar consolidation, often appearing after a brief period of clinical improvement (the "biphasic pattern").

04Management and Treatment

Acute Stabilization

For uncomplicated influenza, the mainstay of treatment is oseltamivir 75 mg orally twice daily for 5 days, ideally initiated within 48 hours of symptom onset. The 48-hour window is the most commonly tested concept. Oseltamivir is a neuraminidase inhibitor that prevents release of new virions from infected cells, thereby limiting viral spread.

However, for hospitalized patients, severely ill patients, or those with progressive disease, antiviral therapy should be initiated regardless of the duration of symptoms. Do not withhold treatment in a critically ill patient simply because more than 48 hours have passed.

Zanamivir is an inhaled neuraminidase inhibitor and serves as an alternative. The key contraindication to remember is underlying airway disease (asthma or COPD), because the inhaled formulation can trigger bronchospasm.

Baloxavir marboxil is a newer agent that inhibits the cap-dependent endonuclease, blocking viral mRNA synthesis. It is given as a single oral dose and is approved for uncomplicated influenza in patients aged 5 and older, or for high-risk patients aged 12 and older. This is increasingly appearing on exams as an alternative option.

Supportive care includes antipyretics (acetaminophen or ibuprofen), adequate hydration, and rest. In children with influenza, avoid aspirin due to the risk of Reye syndrome.

Secondary Bacterial Pneumonia Management

If a patient with influenza develops a biphasic illness with recurrence of fever and new productive cough with lobar consolidation, treat for secondary bacterial pneumonia. Empiric coverage should target S. aureus (including MRSA in high-risk settings), S. pneumoniae, and H. influenzae. This typically requires a beta-lactam plus coverage for MRSA if risk factors are present (e.g., vancomycin or linezolid).

Prevention

Annual influenza vaccination is the single most important preventive measure. The inactivated influenza vaccine (IIV) is recommended for all individuals aged 6 months and older, including pregnant women. The live attenuated influenza vaccine (LAIV), given intranasally, is contraindicated in pregnancy, immunocompromised patients, children younger than 2 years, and children with asthma or wheezing.

Post-exposure chemoprophylaxis with oseltamivir (75 mg once daily for the duration of exposure or at least 7 days) is indicated for unvaccinated high-risk individuals exposed to a confirmed case.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students miss influenza questions is by not recognizing the clinical scenario as influenza at all. Test-writers rarely hand you the word "flu." Instead, they describe a patient with abrupt-onset fever, severe myalgia, dry cough, and headache during December or January, and expect you to connect that pattern to influenza without being told the diagnosis.

The second major trap is the 48-hour treatment window. Many students know oseltamivir must be started within 48 hours but then incorrectly withhold it in a hospitalized or severely ill patient who presents beyond that window. The guideline is clear: for hospitalized patients and those with severe or progressive illness, start antiviral therapy at any time point regardless of symptom duration.

A classic distractor involves zanamivir. When the vignette mentions a patient with asthma or COPD who needs antiviral therapy, zanamivir will appear as an answer choice. The correct answer is oseltamivir (oral), because zanamivir (inhaled) is contraindicated in patients with reactive airway disease.

Another high-yield trap involves children. If a child with a viral illness is given aspirin and subsequently develops vomiting, hepatomegaly, and altered mental status, this is Reye syndrome, not worsening influenza.

Finally, be sure you understand the distinction between antigenic shift and antigenic drift. Drift causes seasonal epidemics and is the reason for annual vaccine updates. Shift causes pandemics and involves reassortment of genome segments between different influenza A subtypes, typically involving an animal reservoir. Influenza B does not undergo antigenic shift because it lacks a significant animal reservoir. This is a foundational virology concept that appears repeatedly across different question formats.

The core competency being tested across all influenza questions is the ability to recognize the classic acute presentation, initiate timely antiviral therapy, and identify complications (especially secondary bacterial pneumonia with its biphasic course) before they progress.