What is Influenza ("Superflu")
Influenza is an acute viral respiratory infection caused by influenza A and B viruses that spreads through respiratory droplets and characteristically presents with abrupt onset of fever, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis. 1
Viral Biology and Classification
Influenza A viruses are categorized into subtypes based on two surface antigens: hemagglutinin (H) and neuraminidase (N), which are critical for viral attachment and release. 2
Since 1977, influenza A (H1N1) and A (H3N2) viruses, along with influenza B viruses, have been in global circulation. 1
Influenza A undergoes frequent antigenic changes through antigenic drift (point mutations during viral replication) and antigenic shift (major changes through genetic reassortment between different strains), making it the type most responsible for pandemics. 2, 3
Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses. 1
Transmission Dynamics
The virus spreads person-to-person primarily through coughing and sneezing of infected persons. 1, 2
The incubation period ranges from 1-4 days, with an average of 2 days. 1, 4
Adults are infectious from the day before symptom onset through approximately 5-6 days after illness begins. 4
Children can be infectious for >10 days and may shed virus for <6 days before illness onset. 1, 4
Severely immunocompromised persons can shed virus for weeks or months. 1, 4
Clinical Presentation
Uncomplicated influenza is characterized by abrupt onset of:
- Fever (though not always present, especially in vaccinated elderly patients) 1
- Myalgia (muscle aches) 1
- Headache 1
- Severe malaise 1
- Nonproductive cough 1
- Sore throat 1
- Rhinitis (nasal symptoms) 1
Pediatric-specific presentations include:
- Otitis media, nausea, and vomiting are commonly reported in children. 1, 4
- Young children may present with initial symptoms mimicking bacterial sepsis with high fevers. 1, 4
- <20% of hospitalized children can have febrile seizures. 1
Disease Course and Resolution
Uncomplicated illness typically resolves after 3-7 days in most individuals. 4
Cough and malaise frequently persist for >2 weeks even after other symptoms resolve. 1, 4
Diagnostic Limitations
Respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens based on symptoms alone. 1, 4
Clinical definitions including fever and cough have sensitivity of only 63-78% and specificity of 55-71% compared with viral culture. 1, 4
Rapid molecular assays are the preferred diagnostic tests because they can be done at the point of care, are highly accurate, and have fast results. 3
Complications
Influenza can lead to serious complications including:
- Exacerbation of underlying medical conditions (pulmonary or cardiac disease) 1, 2
- Secondary bacterial pneumonia (particularly with Staphylococcus aureus, including MRSA) 5
- Primary influenza viral pneumonia 1, 5
- Encephalopathy, transverse myelitis, Reye syndrome 1, 4
- Myositis, myocarditis, and pericarditis 1, 4
High-Risk Populations
The following groups face significantly elevated risks of severe outcomes:
- Adults aged >65 years (account for >90% of influenza-related deaths) 1, 4
- Infants aged 0-1 years (hospitalization rates comparable to elderly at 200-1,000 per 100,000) 4
- Pregnant women 6, 5
- Individuals with chronic cardiopulmonary conditions 1, 4
- Children aged 0-4 years with high-risk conditions (hospitalization rate ~500 per 100,000) 1, 4
Disease Burden
The WHO estimates that annual epidemics result in ~1 billion infections, 3-5 million cases of severe illness, and 300,000-500,000 deaths globally. 7
In the United States, annual influenza-associated hospitalizations average approximately 114,000, with 57% occurring in persons <65 years. 4
During severe epidemic years, more than 20,000 influenza-associated deaths occur in the United States. 4
Pediatric deaths range from 44-68 annually in recent seasons, with 94% occurring in unvaccinated children. 4