see also:

• viruses
• oseltamivir (Tamiflu)
• zanamivir (Relenza)
• Western Health influenza pandemic information
• Timeline of spread of H1N1 pandemic 2009
• Vic. DHS Chief Medical Officer's influenza alerts
• CDC website for latest updates on Swine influenza A (H1N1) infection outbreak
• Aust. Govt responses to the potential pandemic phase
• Victorian DHS influenza pandemic site
• Australian flu pandemic site
• WHO information on swine flu
• UpToDate.com - Epidemiology, clinical manifestations, and diagnosis of swine H1N1 influenza A
• UpToDate.com - Treatment and prevention of swine H1N1 influenza A
• The Lancet - Nov 2005 - Review - Understanding the symptoms of the common cold and influenza (pdf)
• MJA 2009: ASID (HICSIG) position statement: infection control guidelines for patients with influenza-like illnesses, including pandemic (H1N1) influenza 2009, in Australian health care facilities

• 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.
• bird flu:
  • 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
    • Dec 2022, domestic cat dies from highly pathogenic avian influenza A (H5N1) clade 2.3.4.4b viral infection presumably acquired from nearby duck infections in France but had the E627K mutation in polymerase basic protein two which confers mammalian susceptibility
    • in 2023, WHO reported low risk concerns of a potential human H5N1 pandemic as the global bird flu outbreaks have spread to mammals with evidence of it appearing to spread amongst minks from weasel to weasel. In the past year, there have been 10 human cases but no human-to-human spread as yet.
    • in April 2023, a woman in China is the 3rd person infected with H3N8 and the 1st to die from it
    • 2022 USA study announced Nov 2023, reports on HPAI H5N1 infections in non-avian mammals in US ¹⁾
    • human case of H5N1 brought to Victoria from India in Mar 2024
    • global H7 bird flu outbreak arrived in Vic and WA in May 2024 and has spread to 67 dairy herds in nine US states and a 3rd dairy worker but currently has low transmissibility combined with mild symptoms although the 3rd case did have a more flu-like respiratory illness
    • Apr 2024: 1st lab confirmed human death from H5N2 bird flu - a 59yr old man in Mexico
• the “swine flu” of 2009 which appears to have originated in Mexico or Southern California is H1N1 type A influenza
  • see timeline and links for 2009 H1N1 "swine flu" here
  • of concern is the 2008 seasonal influenza virus which was resistant to Tamiflu antiviral and if the H1N1 flu virus comes in contact with that virus, it could adopt its antiviral character.
• 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.

• 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

• 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 in those aged > 65yrs (0.1% overall for all ages), 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
  • Covid-19 Omicron BA.2 = 12
  • measles = 10-15
  • polio = 8-12
  • Covid-19 Delta = 6
  • rubella = 6-7
  • diphtheria = 6-7
  • smallpox = 5-7
  • mumps = 4-7
  • SARS = 3 (excluding superspreaders)
  • influenza seasonal = 1.5-3
  • influenza pandemic = 2

• immunocompetent patients:
  • until after 72hrs of patient commencing anti-influenza medications (at least 6 doses of oseltamivir), OR,²⁾
  • if not received anti-influenza medications, duration of illness or 10 days after symptom onset
• immunocompromised patients:
  • duration of illness

• risk factors include:
  • young children
  • elderly
  • pregnant women
  • individuals with co-morbidities
  • indigenous people
• however, among patients hospitalized with influenza-like illness during 2015–2016, >60% were previously healthy with no underlying conditions
  • it seems an important marker early in the infection to indicate potential progression to severe illness is high oleoyl-ACP-hydrolase (OLAH) gene expression levels which mainly produce oleic acid but seems to drive severe outcomes in viral lung infections and Olah^−/− mice are protected from lethal influenza disease and excessive inflammation ³⁾

• 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 an avian influenza pandemic have been estimated to be $US330-4400 billion (cw. world's annual GDP of $US44,380billion)

• 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.

• 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.

• 5-15% of patients requiring admission to hospital from COVID-19 coronavirus (2019-nCoV / SARS-CoV-2) or influenza had auto-immune impaired type 1 interferon responses due to the presence of autoantibodies that bind type 1 interferons - these occur in 2% of the population during their life time, usually at age 60-65yrs, and once developed are usually lifelong ⁵⁾

• 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)

• 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

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

• oseltamivir (Tamiflu)
• 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

• cannot use amantadine nor rimantadine, thus need to use either zanamir or oseltamivir, with zanamir being easier to administer