Influenza
Dr Ubaid N P
 Acute respiratory tract infection
 3 types – A, B & C
 Sudden onset of chills, fever, malaise, muscular pain and
cough
 International disease
 Occurs in several forms – subclinical, epidemics(2-3 years &
4-7 years), pandemics (10-40 years)
 Pandemics – 1918(Spanish influenza), 1957(Asian influenza),
1968 (Hong Kong influenza)
 Characteristics of epidemic : 10-50% attack rate
 At present three types of influenza viruses are circulating in
the world : A (H1N1), A (H3N2) and B viruses
 H5N1(1997 – Hong Kong) doesn't easily transmit between
humans
Epidemiological determinants
Agent factors
(a) AGENT : family Orthomyxoviridae, three viral subtypes influenza type A, type B and type C
 No cross immunity
 Influenza A and B responsible for epidemics of disease throughout
the world
 Influenza A virus has 2 distinct surface antigens - the haemagglutinin
(H) and the neuraminidase (N) antigens.
 H antigen initiates infection following attachment of the virus to
susceptible cells.
 The N antigen is responsible for the release of the virus from the
infected cell.
 16HA & 9NA subtypes have been identified currently
Antigenic shift:
 Occurs when the antigenic change is sudden complete or major
change
 result from genetic recombination of human with animal or avian
virus, providing a major antigenic change.
 Can cause a major epidemic or pandemic involving all age groups.
Antigenic drift:
 when the antigenic change is gradual over a period of time
 Antigenic drift involves "point mutation" in the gene
 Since the isolation of the virus A in 1933, major antigenic
changes have occurred twice - once in 1957 (H2N2) and
again in 1968 (H3N2).
 Strains occurring between 1946 and 1957 have been called
(H1N1) strains.
 The shift in 1968 involves only the H antigen.
 Antigenic changes occur to a lesser degree in the
B group influenza viruses.
 Influenza C appears to be antigenically stable.
(b) RESERVOIR OF INFECTION – Animals & birds
(c) SOURCE OF INFECTION: Usually a case or subclinical
case. The secretions of the respiratory tract are infective.
(d) PERIOD OF INFECTIVITY: Virus is present in the
nasopharynx from 1 to 2 days before and 1 to 2 days after
onset of symptoms.
Host factors
(a) AGE AND SEX :
Affects all ages and both sexes.
 Mortality higher in certain high-risk groups in the population
such as old people (generally over 65 years of age), children
under 18 months, and persons with diabetes or chronic heart
disease, kidney and respiratory ailments

(b) HUMAN MOBILITY: important factor in the spread of
infection.
(c) IMMUNITY:
 Immunity to influenza is subtype - specific.
 Antibodies against HA and NA are important in immunity to
influenza.
 Resistance to initiation of infection is related to antibody against
HA, which neutralizes the virus
 Decreased severity of disease and decreased ability to transmit virus
to contacts are related to antibody directed against the NA.
 Immunity can be incomplete as reinfection with the same virus can
occur.
Environmental factors:
 Season: epidemics usually occur in winter months in the
Northern Hemisphere and in the winter or rainy season in the
Southern Hemisphere. ln India, epidemics have often occurred
in summer.
 Over crowding: Enhances transmission. The attack rates are high
in close population groups, e.g., schools, institutions, ships, etc.
Mode of transmission:
 droplet infection or droplet nuclei - sneezing, coughing or
talking
Incubation period: 18 to 72 hours.
Pathogenesis and clinical features
The virus enters the respiratory tract
inflammation and necrosis of superficial epithelium of the tracheal and
bronchial mucosa
Secondary bacterial invasion.
Symptoms - fever, chills, aches, coughing and generalized weakness.
Fever lasts from 1-5 days, averaging 3 days in adults.
The most dreaded complication is pneumonia, which should be suspected if fever persists
beyond 4 or 5 days or recurs abruptly after convalescence
Other complications are acute sinusitis, otitis media, purulent bronchitis,
reye syndrome
Laboratory diagnosis
(a) VIRUS ISOLATION: Nasopharyngeal secretions - indirect
fluorescent antibody technique
(b) SEROLOGY: Haemagglutination Inhibition(HI), ELISA,
Paired Sera
Prevention of influenza
 Good ventilation of public buildings
 Avoidance of crowded places during epidemics.
 Encouraging sufferers to cover their faces with a handkerchief when
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coughing and sneezing,
To stay at home at the first sign influenza
Hygienic practices during handling of poultry products, including
handwashing and prevention of cross – contamination
Thorough cooking, to more than 70°C, of poultry products
The vaccine is not recommended to control spread in the general
population.
To be effective the vaccine must be administered at least two weeks
before the onset of an epidemic, or preferably 2 io 3 months before
influenza is expected.
 Since epidemics of influenza are unpredictable, the hope of
preventing influenza epidemics by prophylactic mass vaccination
is remote.
 Since influenza vaccines will not control epidemics, they are
recommended only in certain selected population groups
 E.g. in industry to reduce absenteeism, and in public servants to
prevent disruption of critical public services, such as the police,
fire protection, transport and medical care.
 Also, certain groups e.g. the elderly and individuals in any age
group who have a known underlying chronic or debilitating
disease are selectively immunized
Influenza vaccines
(a) KILLED VACCINES
 Formulated in aqueous or saline suspension
 Subcutaneous or intramuscular route.
 A single inoculation (0.5 ml for adults and children over 3
years and 0.25 ml for children from 6 months to 36 months
of age)
 In unprimed individuals, 2 doses of the vaccine, separated by
an interval of 3 to 4 weeks
 The protective value : 70-90%
 immunity lasts for only 6-12 months.
 Revaccination on an annual basis is recommended.
 Can produce fever, local inflammation at the site of injection, and
very rarely Guillain-Barre syndrome (an ascending paralysis).
 Persons allergic to eggs may develop symptoms and signs of
hypersensitivity
(b) LIVE ATTENUATED VACCINES:
 Administered as "nose drops" into the respiratory tract.
 Stimulate local as well as systemic immunity
Antiviral drugs
 Prophylactic & therapeutic
 Zanamivir & Oseltamivir – Neuraminidase inhibitors
 amantadine and rimantidine
Avian influenza
 large group of different influenza viruses that primarily affect
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birds
On rare occasions - pigs and humans
Avian H5N1, strain with pandemic potential
Ultimately adapt into a strain that is contagious among
humans - human influenza virus.
The H5N1, strain first infected humans in Hong Kong in
1997, causing 18 cases including six deaths.
Since then – 478 cases, 286 deaths, 15 countries
Most cases have occurred in previously healthy children and
young adults.
Pandemic Influenza A(H1N1) 2009
 Can infect lower respiratory tract and cause pneumonia
 Emergence – March 2009, declared pandemic by WHO on
11th June 2009
 Incubation period : 2-3 days, could range upto 7 days
Case definitions
Suspected case: A person with acute febrile illness (Fever
≥38°C) with onset
(a) Within 7 days of close contact with a person who is a
confirmed case of influenza A(H1N1) 2009 virus infection;
or
(b) Within 7 days of travel to areas where are confirmed cases;
or
(c) resided in a community where there are confirmed
influenza A(H1N1) 2009 cases
Probable case: A person with acute febrile illness who
(a) Is positive for influenza A, but unsubtypable; or
(b) Is positive for influenza A by an influenza rapid test or an
influenza Immunoflurescence Assay (IFA) & meets the
criteria for a suspected case; or
(c) Person who died of an unexplained acute respiratory illness
whos is considered to be epidemiologically linked to a
probable or confiremd case
Confirmed case: A person with acute febrile respiratory
illness with lab confirmed influenza A(H1N1) 2009 virus
infection at WHO approved lab by one or more of the
following test
(a) Real Time PCR
(b) Viral Culture
(c) Four fold rise in influenza A (H1N1) virus specific
neutralizing antibodies
Clinical features
(a) Uncomplicated influenza
(i) ILI symptoms – no shortness of breath
(ii) GI symptoms - no dehydration
(b) Complicated influenza
(i) RT, CNS involvement, severe dehydration, renal failure, multi
organ failure, septic shock etc
(ii) Exacerbation of underlying chronic disease – COPD, Asthma,
hepatic or renal failure, diabetes etc
(iii) Hospitalization requiring presentations
(iv) Any of the signs of progressive illness
 Risk factors for severe disease
 Laboratory diagnosis : Need for it?
RT – PCR, sample collection
 Infection control : Community, Hospital,
Immunocompromised patients
Vaccine