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L4
Influenza virus
D. Haider
Overview of influenza virus
• The influenza virus group consists of three enveloped RNA members of the family
Orthomyxoviridae: influenza A, B, and C. Influenza A viruses are further classified into
hemagglutinin (H) and neuraminidase (N) subtypes based on activity of their surface
envelope glycoproteins. Influenza A viruses infect humans and other animal (for
example, avian and swine) hosts. Influenza B viruses infect only humans. Influenza C
viruses are rarely recognized in humans.
• H and N surface envelope glycoproteins (antigenic variation) change frequently. Minor
changes (antigenic drift) are partly responsible for seasonal outbreaks or epidemics that
occur almost yearly. Major changes (antigenic shift) are alterations in the virus caused by
reassortment of genes between human and animal influenza A strains. Emergence of a
new influenza virus for which humans have no previous protective immunity may result
in severe worldwide pandemics.
• Influenza A outbreaks generally occur in winter, with attack rates of 10% to 20%.
Seasonal influenza disproportionately affects persons 65 years of age or older. Very old
persons, very young children, persons with chronic medical conditions, and pregnant
women often develop severe disease, which accounts for most hospitalizations and deaths
attributable to this infection. In pandemics, disease extends beyond the usual season with
higher attack rates and increased mortality in all age groups, especially otherwise healthy
young adults
• The highly pathogenic H5N1 avian influenza virus predominantly affects children and
young adults recently exposed to infected birds and poultry in Europe and Asia. Person-
to-person human transmission appears to be limited. The novel H7N9 influenza virus in
China is also believed to result from exposure to infected poultry or contaminated
environments. To date, no evidence of sustained person-to-person spread has been found.
Clinical features and evaluation:
Influenza virus is transmitted by sneezing and coughing. After an incubation period of 1 to 4
days, patients develop fever, headache, myalgia, pharyngeal irritation, and respiratory symptoms
(dry cough and nasal discharge). Mild or asymptomatic infections occur, particularly with
influenza B. Viral shedding begins 24 to 48 hours before symptom onset and may continue for
5 to 10 days.
Patients with uncomplicated infection improve within 2 to 5 days. The most common
complications are primary influenza pneumonia and secondary bacterial pneumonia, which are
mainly responsible for increased morbidity and mortality in patients aged 65 years and older
During a confirmed local influenza outbreak, infection can be reliably diagnosed on the basis of
clinical criteria alone. When confirmation is needed, rapid antigen tests of respiratory samples
from nasopharyngeal swabs detect both influenza A and B. Positive test results are highly
specific. However, sensitivity ranges from 40% to 80%.
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Detection of viral nucleic acid by polymerase chain reaction (PCR) is rapid, has high sensitivity
and specificity, and can determine the type and subtype of influenza virus.
Serologic assays are useful only for diagnosing infection retrospectively. Whether to test is based
on how the result will influence management; testing is generally reserved for patients at high
risk for complications, including adults older than 65 years, immunocompromised patients,
pregnant and postpartum women, and health care workers
Antiviral therapy begun within 48 hours of symptom onset reduces symptom duration, decreases
hospitalization rates, and reduces the incidence and severity of complications; however, adults
younger than 65 years without high-risk conditions are unlikely to benefit from antiviral therapy
begun later. When treatment is required, the neuraminidase inhibitors oseltamivir and zanamivir
are active against influenza A and B.
Oseltamivir or zanamivir is recommended for patients with confirmed or highly suspected
influenza infection who have an increased risk for complications. Peramivir, an intravenous
neuraminidase inhibitor, was approved for use in adults in 2014. All hospitalized patients should
receive a neuraminidase inhibitor promptly, even if 48 hours or more has elapsed since disease
onset. Treatment duration is generally 5 days but may be longer in immunocompromised or
severely ill patients.
Prevention:
Vaccine and antiviral agents.
VACCINE: inactivated vaccines are the main control measures.
Recommendation of vaccination:
1. More than 65 year old.
2. All patients aged 6 months or older in a clinical risk group(chronic respiratory disease,
chronic heart disease, chronic kidney disease, chronic liver disease, diabetes,
immunosuppressed people, asplenia, pregnancy, morbid obesity.
3. All children aged between 2-16 years not in clinical risk group
Vaccination is also offered to: household contact to of immunocompromised people,
health care workers, others as clinical judgment suggests (e.g: other chronic illnesses,
long-term care home residents).
4. Two doses are required in children under 9 years who have been not previously
vaccinated. Otherwise single dose is sufficient, usually given in October
5. The main contraindication to vaccination is hypersensitivity to hen's eggs.
6. Protection is around 70% and lasts for 1 year. Diminished responses are seen in organ
transplant recipients receiving immunosuppressive therapy. Protection is reduced in
elderly.
Mubark A. Wilkins