RNA Viruses: Influenza and SARS-CoV-2 – Structure, Pathogenesis, Epidemiology, and Prevention

RNA Viruses: Classification and Structure

Types and Genomic Organization

RNA viruses are a diverse group of pathogens classified by their genomic structure, presence of an envelope, and capsid morphology. Understanding these features is essential for studying their replication and pathogenicity.

• Genomic Structure: RNA viruses may have single-stranded or double-stranded RNA genomes.

• Envelope: Some RNA viruses possess a lipid envelope derived from the host cell membrane, which contains viral glycoproteins essential for host cell entry.

• Capsid: The protein shell (capsid) varies in size and shape, influencing virus stability and infectivity.

Major Types of RNA Viruses

• Positive single-stranded RNA (+ssRNA): Genome can be directly translated by host ribosomes to produce viral proteins.

• Retroviruses: A subset of +ssRNA viruses that reverse transcribe their RNA genome into DNA (e.g., HIV).

• Negative single-stranded RNA (-ssRNA): Genome must be transcribed into mRNA before translation.

• Double-stranded RNA (dsRNA): Contains both sense and antisense RNA strands.

Key Point: Positive-sense RNA acts as mRNA, while negative-sense RNA requires transcription to mRNA for protein synthesis.

Pathogenic Mechanisms: Influenza vs. SARS-CoV-2

Influenza Virus Pathogenesis

Influenza viruses are enveloped, negative-sense single-stranded RNA viruses (Orthomyxoviridae) that primarily infect the respiratory tract.

• Attachment: Hemagglutinin (HA) binds to sialic acid receptors on upper respiratory tract (URT) cells.

• Penetration: Virus enters via endocytosis; M2 ion channel is critical for uncoating.

• Biosynthesis: Viral -ssRNA enters the nucleus, is copied to +ssRNA, which serves as a template for genome replication and transcription.

• Assembly: HA and neuraminidase (NA) accumulate in the host cell membrane; new virions assemble.

• Release: Virions bud from the cell, requiring NA; infected cells die by apoptosis.

Key Proteins: HA (attachment), NA (release), M2 (uncoating).

SARS-CoV-2 Pathogenesis

SARS-CoV-2 is an enveloped, positive-sense single-stranded RNA virus (Coronaviridae) responsible for COVID-19.

• Attachment: Spike (S) protein binds to ACE2 receptors on epithelial, alveolar, kidney, and heart cells.

• Entry: Virus enters via endocytosis or membrane fusion, facilitated by host proteases (e.g., TMPRSS2).

• Replication: +ssRNA genome is translated by host ribosomes; viral replication and transcription complexes form on the endoplasmic reticulum (ER).

• Assembly and Release: Structural proteins are assembled in the ER-Golgi intermediate compartment (ERGIC) and secreted.

Additional info: SARS-CoV-2 infection can cause systemic effects, including ARDS, myocarditis, kidney damage, and long-term sequelae.

Antigenic Variation: Shift and Drift

Mechanisms of Influenza Virus Evolution

Influenza viruses undergo frequent genetic changes, leading to new strains and necessitating annual vaccine updates.

• Antigenic Drift: Gradual accumulation of mutations in HA and NA genes, resulting in minor changes to viral antigens.

• Antigenic Shift: Abrupt genetic reassortment when two different influenza viruses co-infect a host (e.g., pigs), producing a novel virus with mixed genome segments. This can lead to pandemics.

Example: H5N1 (bird flu) is currently endemic in birds but has the potential to spread to humans.

SARS-CoV-2 Mutations

SARS-CoV-2 accumulates mutations in the spike protein, affecting transmissibility and immune escape. Common mutations are tracked to inform vaccine updates.

Diagnosis, Treatment, and Prevention

Influenza

• Diagnosis: Based on clinical symptoms during outbreaks; laboratory tests distinguish virus strains.

• Treatment: Antivirals (oseltamivir, zanamivir) inhibit neuraminidase; symptomatic treatment for mild cases.

• Prevention: Annual immunization with multivalent vaccines (trivalent or quadrivalent IM injection, or nasal spray with live attenuated virus). Vaccines protect only against included strains.

SARS-CoV-2

• Diagnosis: PCR and antigen tests detect viral RNA or proteins.

• Treatment: Symptomatic treatment for mild cases; antiviral therapy (Paxlovid, Remdesivir) and immunomodulators (dexamethasone, baricitinib) for severe cases; anticoagulants for hospitalized patients.

• Prevention: Vaccination is the most effective method; hybrid immunity (infection plus vaccination) provides robust protection.

Additional info: Booster vaccine efficacy against Omicron infection wanes over time, but protection against severe disease persists longer.

Epidemiology and Clinical Manifestations

Influenza

• Incubation Period: 1-2 days.

• Symptoms: Fever, malaise, headache, myalgia, cough.

• Complications: Secondary bacterial infections due to epithelial cell death.

SARS-CoV-2

• Incubation Period: 2-14 days.

• Symptoms: Fever, cough, aches, shortness of breath, loss of taste/smell, fatigue, headache, sore throat, congestion, nausea, diarrhea.

• Complications: ARDS, myocarditis, kidney damage, long-term effects (lung, brain, organ damage).

Table: Comparison of Respiratory Infections

Ailment	Manifestations
Common Cold (viral)	Sneezing, rhinorrhea, congestion, sore throat, headache, malaise, cough
Influenza (viral)	Fever, rhinorrhea, headache, body aches, fatigue, dry cough, pharyngitis, congestion
Strep Throat (bacterial)	Fever, red and sore throat, swollen lymph nodes in neck
Viral Pneumonia	Fever, chills, mucus-producing cough, headache, body aches, fatigue
Bacterial Pneumonia	Fever, chills, congestion, cough, chest pain, rapid breathing, and possible nausea and vomiting
Bronchitis (viral or bacterial)	Mucus-producing cough, wheezing
Inhalation Anthrax (bacterial)	Fever, malaise, cough, chest discomfort, vomiting
Severe Acute Respiratory Syndrome (SARS)	High fever (>38°C), chills, shaking, headache, malaise, myalgia
Middle East Respiratory Syndrome (MERS)	Fever, cough, shortness of breath
Severe Acute Respiratory Syndrome 2 (SARS-CoV-2)	Fever, chills, headache, myalgia, shortness of breath, cough, loss of sense of smell

Table: Influenza vs. SARS-CoV-2

Feature	Influenza	SARS-CoV-2
Symptoms	Fever, cough, aches, pneumonia (rare)	Fever, cough, aches, pneumonia
Death	May cause death, mortality about 0.1%	May cause death, mortality around 1-4% worldwide
Transmission	Droplet, fomite	Droplet, fomite (rare), airborne
Asymptomatic spread	Rare	Apparently common
Treatment	Antivirals, symptomatic	Symptomatic, Paxlovid and other drugs approved to treat
Duration of illness (severe cases)	1-2 weeks	2-4 weeks or more

Key Terms and Definitions

• Hemagglutinin (HA): Viral glycoprotein responsible for binding to host cell receptors.

• Neuraminidase (NA): Enzyme that facilitates viral release from host cells.

• Antigenic Drift: Minor genetic changes in viral antigens due to point mutations.

• Antigenic Shift: Major genetic reassortment resulting in new viral subtypes.

• ARDS (Acute Respiratory Distress Syndrome): Severe lung inflammation and damage, often requiring mechanical ventilation.

• ACE2: Angiotensin-converting enzyme 2, a receptor for SARS-CoV-2 entry.

Formulas and Equations

• Viral Replication Error Rate:

$\text{RNA-dependent RNA polymerase has } 1/10,000 \text{ bp error rate}$

Summary and Applications

• Annual influenza vaccination is necessary due to antigenic drift and shift.

• SARS-CoV-2 vaccines and boosters are essential for preventing severe disease; hybrid immunity offers enhanced protection.

• Diagnosis and treatment strategies differ between influenza and SARS-CoV-2, reflecting their unique pathogenesis and epidemiology.

Additional info: New vaccine production methods (cell culture) and ongoing research into long-term effects of SARS-CoV-2 are critical for public health.