Antiviral drugs (zanamivir or oseltamivir) may be appropriate for people who are at risk of complications
Elderly people and people with chronic co-morbidities who are frail, are at increased risk of influenza-related complications.
It may be appropriate to treat these people with antivirals such as zanamivir and oseltamivir. Treatment is more effective
the sooner it is given and must be initiated within 48 hours of the onset of symptoms.6 Laboratory testing
is unlikely to be useful in the decision to use antivirals, as results may take longer than 48 hours to be reported.
Antivirals can shorten the duration of influenza symptoms by one to three days if initiated within 48 hours of the onset
of symptoms.6 There is also some evidence that they can reduce the severity and incidence of complications
of influenza, as well as shorten the length of hospital stay, and reduce mortality in patients with severe influenza.6
Recommended treatment doses
Oseltamivir (Tamiflu) is available as a tablet and a suspension. It is not subsidised for seasonal influenza
and one course costs approximately $70. Note that as part of containment measures patients with suspected swine-origin
influenza A are currently offered funded antiviral therapy.
The recommended dose of oseltamivir for the treatment of influenza in adults and children aged 13 years and older is
75 mg twice daily for five days. A suspension is available for children aged one year and older and doses are based on
weight. For patients with renal impairment (creatinine clearance less than 30 mL/min), the dose should be reduced to 75
mg once daily.7
Zanamivir (Relenza) is available as an inhaled powder. It is not subsidised and one course costs approximately $65.
The recommended dose of zanamivir for the treatment of influenza in adults and children five years and older is 10 mg
(two inhalations) twice daily for five days.8
Adverse effects and precautions with neuraminidase inhibitors
Adverse effects commonly associated with oseltamivir include nausea and vomiting (approximately 5 to 10%). This
can be minimised by taking oseltamivir with food. Other adverse effects include abdominal pain and headache.9
Zanamivir has been reported to cause bronchospasm and a reduction in respiratory function, particularly in patients
who have underlying respiratory disease. These people should be informed of the risk of bronchospasm and advised to have
a fast-acting bronchodilator available, or if they are taking maintenance bronchodilator therapy, to use this before taking
Amantadine is not usually recommended for the treatment of influenza because of adverse effects and high rates
Amantadine, more often used for Parkinson’s disease, is also licensed for the prophylaxis and treatment of influenza.
However amantadine has significant CNS adverse effects including anxiety, insomnia, confusion and light-headedness. These
adverse effects are particularly common in elderly people.14
There are also high rates of amantadine-resistance in influenza isolates and for this reason it is no longer recommended
for the treatment of influenza. One exception is the treatment of oseltamivir-resistant influenza in those whom zanamivir
Antivirals for prophylaxis
Annual influenza immunisation is recommended to prevent influenza infection in people at high risk of complications.
Antivirals are not routinely recommended for prophylaxis against influenza, however they may be useful in some situations,
e.g. in inadequately vaccinated high-risk communities such as an outbreak of influenza in a residential care facility.9
In this situation, antivirals must be started within 48 hours after exposure to a person with influenza (i.e. close
contact with an infected person).
Doses used for prophylaxis:
- Oseltamivir, adults, 75 mg once daily for ten days
- Zanamivir, 10 mg (2 inhalations) once daily for ten days
When exposure to influenza is ongoing, oseltamivir prophylaxis can be continued for up to six weeks or zanamivir for
up to four weeks.8
Oseltamivir and zanamivir
Oseltamivir and zanamivir are neuraminidase inhibitors
Neuraminidase inhibitors prevent the release of newly replicated virions from infected cells, therefore preventing
the spread of infection.11 Neuraminidase enables infection to spread by cleaving the sialic acid residues
on receptors that bind virions to cells and to one another. Neuraminidase inhibitors bind to the active site, preventing
the enzyme from cleaving the host-cell receptors.12
Resistance to neuraminidase inhibitors
During the 2008–2009 influenza season, high rates of oseltamivir resistant strain (H1N1) of influenza were detected
in the United States, Europe, Australia and South Africa. Oseltamivir resistant strains have recently been detected in
Resistance occurs when amino acid substitutions occur in the active site preventing oseltamivir from binding. While
resistance to oseltamivir is concerning, this particular strain continues to be susceptible to zanamivir and amantadine.12
Local ESR surveillance data reports on which influenza strains are currently circulating and may be used to assist
in the choice of an appropriate antiviral agent (available from: http://www.surv.esr.cri.nz/virology/influenza_annual_report.php).13