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Antivirals are essential lines of defence in the management of H1N1 (swine flu), especially in the early stages of an outbreak or pandemic and before a vaccine is available.
Researchers segregate influenza into three genera. Aquatic birds are the natural host of influenza A, but some strains spread between species, potentially causing epidemics and pandemics. Influenza B infects only humans and usually causes less severe illness and more limited outbreaks than influenza A. Influenza C is one cause of the common cold.
Influenza A contains eight genes that encode 10 proteins, including haemagglutinin and neuraminidase. Haemagglutinin allows influenza to bind to and invade the host’s cells. Neuraminidase, an enzyme, helps release new virus particles from infected cells (Bouvier, 2008). Oseltamivir (Tamiflu) and zanamivir (Relenza) target influenza A and B (Matheson, 2007).
Oseltamivir is recommended as the first-line treatment in the current swine flu pandemic. Treatment should begin as soon as possible and within two days of symptom onset. The recommended oral dose for adolescents aged 13-17 and adults is 75mg twice daily for five days to treat infection and 75mg once daily for 10 days as post-exposure prophylaxis. The dose is adapted for younger children and HPA (2009a) have summarised prescribing information.
Oseltamivir is not prescribed for prophylaxis in the current outbreak. However, the HPA (2009b) recommends the clinical judgement should be used when a risk is identified to a vulnerable individual, for example, someone who has long term lung disease and has been in close proximity to an infected individual (Zanamivir id recommended for some people with long-term kidney disease and for pregnant women)..
Oral oseltamivir is successful as a prophylaxis against symptomatic influenza in 61% of people with 75mg daily. In adults, 150mg of oseltamivir daily reduces the risk of lower respiratory tract complications by 68%. Adults taking oseltamivir are 20-30% more likely to have their symptoms alleviated than those taking placebo (Jefferson, 2006).
In children with confirmed influenza, oseltamivir reduces the duration of illness by 26%. It also reduced the risk of complications. The reduction in acute otitis media was especially marked: a 44% decline among in children aged 1-2 years and 56% in those aged 1-5 years during a 28-day follow-up (Matheson, 2007).
Care should be taken when prescribing oseltamivir for pregnant women and those with renal impairment. It is important that HPA guidance is followed (HPA, 2009a). An alternative antiviral zanamivir (Relenza) is recommended for these patients as it is an inhaled medicine that reaches low concentrations in the blood (HPA, 2009)
Oseltamivir is a generally well tolerated drug. Gastrointestinal adverse reactions appear to be twice as common following oseltamivir compared with placebo. Vomiting was a complication for children treated with oseltamivir (Matheson, 2007). In adults, oseltamivir induced nausea and the risk was especially marked with 150 mg daily (Jefferson, 2006).
Nurses need to encourage the judicious antiviral use to limit resistance, which is beginning to emerge. During the 2007-2008 season, 12% of H1N1 cases in the US showed oseltamivir resistance. Preliminary data from 2008-2009 suggest a marked increase in resistance (Dharan et al, 2009).
Oseltamivir is not a magic bullet for influenza. However, a combination of the ‘catch it, bin it, kill it’ public health messages, vaccines and antivirals should mean that society is in a better positioned than ever before to counter the threat from pandemic flu.
- Neuraminidase, an enzyme, helps release new virus particles from infected cells. Oseltamivir inhibits neuraminidase and targets influenza A and B.
- Treatment with oseltamivir should begin within two days of onset of symptoms of influenza.
- Oseltamivir shortens disease duration, reduces the risk that influenza will spread and lowers the likelihood of complications.
- Oseltamivir is generally well-tolerated, although gastrointestinal adverse reactions appear to be twice as common following oseltamivir compared with placebo.
- Nurses need to encourage the judicious use of antivirals to limit resistance.
Bouvier, N.M., Palese, P. (2008) The biology of influenza viruses. Vaccine;.26: Suppl 4, D49-53.
Dharan, N.J. et al (2009) Infections with oseltamivir-resistant influenza A(H1N1) virus in the United States Journal of the American Medical Association; 301: 10, 1034–41.
Heath Protection Agency (2009a) Summary of Prescribing Guidance for the Treatment and Prophalyxis of Influenza Like Illness. Treatment Phase. London: HPA.
Health Protection Agency (2009b) Guidance on the Use of Prophylaxis in the Treatment Phase of the H1N1v Pandemic. London: HPA.
Jefferson, T. et al (2006) Neuraminidase inhibitors for preventing and treating influenza in healthy adults. Cochrane Database of Systematic Reviews. DOI:10.1002/14651858.CD001265.pub2.
Matheson, N.J. et al (2007) Neuraminidase inhibitors for preventing and treating influenza in children. CochraneDatabase of Systematic Reviews. DOI: 10.1002/14651858.CD002744.pub2