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  • humans can be infected with influenza types A, B and C.
    • types A and B create seasonal and epidemic disease outbreaks.
  • influenza viruses also account for 5-15% of all "common cold" like illness
  • Pandemics are usually created by type A with H1N1, H2N2, and H3N2 subtypes responsible for the 3 pandemics of the 20th century. H2N2 circulated between 1957 and 1968 but currently does not. 
    • the biggest recent pandemic, that of 1918 which was caused by H1Ni and also infected pigs, killed perhaps 50 million people worldwide and unlike seasonal influenza which has a predilection for killing the frail elderly, 99% of the deaths in this pandemic were in those under 65 years of age, and almost 50% of deaths were aged 20-40 years suggesting that perhaps the elderly had been exposed to a similar virus and had developed immunity.
  • Only type A viruses infect birds, and all known subtypes can do so.
    • 15 subtypes of influenza virus are known to infect birds, thus providing an extensive reservoir of influenza viruses potentially circulating in bird populations. Migratory waterfowl, esp. wild ducks, are the natural reservoir of avian influenza viruses, and these birds are the most resistant to infection. Domestic poultry are particularly susceptible.
  • Bird flu viruses do not usually infect humans, however, in 1997 in Hong Kong, an outbreak of H5N1 avian influenza marked the 1st known direct transmission of avian influenza virus from birds to humans. Since then H5, H7 and H9 avian influenza subtypes have been shown to infect humans.
    • the H5N1 virus has become the most concerning With its Z genotype and two distinct clades, the virus has become highly pathogenic (HPAI) - increasingly adapted to the environment, pathogenic in poultry and expansive in its mammalian host range. It has become endemic in many bird species in south-east Asia with a continuing international rate of spread. It has caused human disease outbreaks in 3 successive waves beginning in 2003
  • the “swine flu” of 2009 which appears to have originated in Mexico or Southern California is H1N1 type A influenza
  • in 2020, China announced that they have been watching a recently emerged genotype 4 (G4) reassortant Eurasian avian-like (EA) H1N1 swine flu virus (G4 EA H1N1) which can infect human airway cells and there is a risk it could cause a pandemic.
  • in June 2021, it seems that the reduced viral transmission measures of the Covid-19 pandemic may have made the influenza B/Yamagata lineage and a clade of the influenza A H3N2 virus, known as 3c3, extinct.

influenza virus survival

  • 24-48hrs on hard, non-porous surfaces
  • 8-12hrs on cloth, paper & tissue
  • 5min on hands
  • inactivated by 70% alcohol, chlorine or heating to 56degC for at least 30min

influenza transmissibility

  • incubation period = 1-3 days
  • infectious period for current H1N1 is assumed to be:
    • from 24hours prior to onset of symptoms
    • until either 7 days after onset of symptoms or until acute respiratory symptoms have resolved, whichever is the later.
  • attack rate = 10-35%
  • case fatality rate = 1% for seasonal influenza and 50% for HPAI
  • reproduction number = avg. number of secondary cases of disease generated by a typical primary case in a susceptible population
  • reproduction numbers for selected infectious diseases:
    • pertussis = 16-18
    • measles = 10-15
    • polio = 8-12
    • rubella = 6-7
    • diphtheria = 6-7
    • smallpox = 5-7
    • mumps = 4-7
    • SARS = 3 (excluding superspreaders)
    • influenza seasonal = 1.5-3
    • influenza pandemic = 2

influenza pandemics

  • main pandemics have been 1729, 1781, 1830, 1898, 1918, 1957, 1967
  • pandemics are inevitable with a predicted global death burden from a moderate pandemic is 45 million people which is 75% of the current annual death burden of 58 million deaths per year.
  • the 2003 and 2004 HPAI outbreaks cost $US10billion including Asian poultry sector losses
  • the 2003 SARS virus outbreak costed $US30million
  • the cost of a avian influenza pandemic have been estimated to be $US330-4400 billion (cw. world's annual GDP of $US44,380billion)

planning assumptions for a pandemic influenza:

  • susceptibility to the virus subtype will be universal
  • incubation period will be 2 days
  • infected patients can shed virus for 0.5-1 day before onset of clinical illness
  • clinical disease attack rate will be 30% overall
  • of those who become ill, 50% will seek outpatient medical care
  • each ill patient will yield 2 secondary infections
  • an epidemic wave will last 6-8wks in a community and recur at least twice in the course of a pandemic
  • hospitalisation and death rates will depend on viral virulence as well as pre- and post-exposure immuno/chemophrophylaxis but projections are 1-10% hospitalised in developed countries with 1/3rd of these dying.

influenza vs common cold

  • Studies on the symptoms generated by different common cold viruses indicate that it is not possible to identify the virus on the basis of the symptoms, since similar symptoms are caused by different viruses.
  • However, there are some suggestive differentiating features between influenza and the common cold
  • The best predictors for influenza are cough and fever, since this combination of symptoms has been shown to have a positive predictive value of around 80% in differentiating influenza from a population suffering from flu-like symptoms.
clinical feature influenza common cold
fever > 38 deg C usual, lasts 3-4 days uncommon
dry cough common early unusual early unless asthmatic, common late perhaps due to post-nasal drip
headache usual early and can be severe common but usually mild unless develops sinusitis
aches and pains usual and can be severe rare
fatigue and weakness usual and can last 2-3 weeks or more after acute illness sometimes but mild
debilitating fatigue usual, early onset can be severe rare
nausea, vomiting & diarrhoea in children < 5 yrs age rare
watering of eyes rare usual
runny, snuffly nose less significant a significant symptom
sneezing rare, and in early stages usual
sore throat usual usual
chest discomfort usual and can be severe sometimes but usually mild
complications respiratory failure; can worsen cardiac failure and other chronic conditions otitis media; sinusitis
fatalities 1% of seasonal cases not reported
prevention influenza vaccine; frequent hand washing; cover cough; frequent hand washing; cover cough/sneezing

influenza testing

  • usual test is influenza PCR via nasal and throat swab added together into viral medium (if not available, use dry swab and add 2 drops NSaline to each to keep them moist)
  • during a pandemic, the State Government may pay for such testing IF the patient meets their criteria (eg. contact with confirmed case of pandemic virus, not just seasonal virus - see govt web sites above for details)
  • otherwise the test may be charged to the patient for a fee of ~$A150 in 2009.
  • alternatively, rapid in-office tests are more than 70 percent sensitive and 90 percent specific for viral antigens. The assays vary in complexity, specificity, sensitivity, time to obtain results, specimen analyzed, and cost.

drug therapy for human influenza

  • neuraminidase inhibitor antiviral therapies for influenza appear to only have minimal benefit and are probably not worth the expense for healthy adults and children 1)

potential indications for prophylactic Rx

  • unvaccinated people at high risk
  • people at high risk given vaccine after onset of the epidemic ( for 2wks for those >9yrs old & 6wks for < 9yrs age)
  • vaccinated people at high risk when vaccine virus & epidemic are a poor antigenic match
  • people with immunodeficiency
  • unvaccinated persons caring for or living with people at high risk
  • all residents & staff in long-term care institutions where there are people at high risk during an institutional outbreak (for >2wks)
  • consider for:
    • people exposed in the household
    • vaccinated people at high risk to ensure optimal prophylaxis

potential indications for Rx:

  • all people at high risk in whom influenza develops
  • persons with severe influenza
  • consider for others who wish to shorten duration of illness

selection of anti-viral drug:

influenza A:

  • Relenza
  • amantadine or rimantadine are approved for prophylaxis & efficacy & risk is similar for both drugs although rimantadine is safer but more expensive
  • zanamir & oseltamivir are more expensive & have no clear benefits

influenza B:

  • cannot use amantadine nor rimantadine, thus need to use either zanamir or oseltamivir, with zanamir being easier to administer
influenza.txt · Last modified: 2021/06/05 06:22 by gary1