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The Pharmaceutical Journal Vol 265 No 7115 p446
September 23, 2000 Special Feature

Influenza prevention and treatment - an update

By Alex Elliot, BSc, and Joanna Ellis, PhD

Each year, increased hospitalisation rates and between 10,000 and 13,000 deaths (in an average AH3N2 year*) are attributable to influenza in the UK.1 Annual vaccination continues to be the main strategy for the control of influenza. Current influenza vaccines contain inactivated (killed) virus and new vaccines containing attenuated live viruses are undergoing clinical trials. The recent development of neuraminidase inhibitors provides a new tool for both the prevention and treatment of influenza infection.

Clinical features of influenza
Influenza is a highly infectious viral infection, which occurs mainly in the winter months of October to March in the northern hemisphere. It is characterised by a rapid onset with fever, chills, myalgia, headache and a non-productive cough. The virus is transmitted by breathing in the tiny droplets from the breath of infected people.
Influenza for most people is a mild illness, which is resolved in one to two weeks. However, complications associated with influenza infection can occur in both the upper and lower respiratory tract. Otitis media is a common complication in children and conjunctivitis occurs in both adults and children. Serious respiratory complications, such as bronchitis and pneumonia, which may be fatal, can occur.2 The potential for developing complications is higher in certain risk groups, such as the elderly and individuals with chronic medical conditions.

Influenza diagnosis
Several different pathogens can produce respiratory illnesses with similar clinical symptoms, making an accurate diagnosis of influenza by a physician difficult. A number of laboratory tests are available to confirm the diagnosis of influenza. These are based on either the detection of viral antigens, viral nucleic acid, virally infected cells or infectious virus particles in respiratory secretions. In addition, several rapid (less than 15 minutes to perform) near-patient tests are being developed.3-5 The impact of such tests on the speed of influenza diagnosis has yet to be assessed. Furthermore, attempts must still be made to isolate virus for further detailed analysis and strain characterisation, for the purpose of vaccine strain selection. Serological tests, which detect a rise in specific antibody to the virus in a patient’s serum, are also performed where confirmation of recent influenza infection is required.

The virus
Influenza viruses are divided into three types, designated A, B and C.6 Influenza A viruses are further classified into subtypes based on differences in their surface glycoproteins, haemagglutinin (HA) and neuraminidase (NA) (Figure 1).

Figure 1: Diagrammatic representation
of an influenza virus particle

To date, 15 HA and nine NA subtypes have been identified.7,8 All influenza A subtypes have been found in aquatic and domestic birds, but only a few subtypes have been found to infect mammals and humans. Two subtypes of influenza A are currently circulating in humans, H1N1 and H3N2. Influenza types B and C exclusively infect humans. Influenza A and B are responsible for epidemics of respiratory illness that occur almost annually and are often associated with increased rates of hospitalisation and death. Studies suggest that H3N2 infections are more severe than H1N1, and that influenza B is intermediate between the two.9 Influenza C viruses usually cause a mild or asymptomatic infection limited to the upper respiratory tract. Therefore, efforts to control the impact of influenza on public health are aimed at types A and B.

Evading immunity
The ability of influenza viruses to cause repeated infections is due to the constantly changing structure of the virus. Both the HA and NA proteins are antigenic, inducing the production of antibodies in the host. Spontaneous mutations continually occur in HA and NA molecules, causing a gradual change in the virus. This ongoing process occurs in both influenza A and B and is called antigenic drift. The emergence, through drift, of new strains of influenza virus leads to a reduction in the effectiveness of antibodies to previous infections, and results in the ability of the virus to reinfect individuals and cause annual epidemics of the disease. Worldwide epidemics, or pandemics, of influenza have occurred at less frequent and irregular intervals. Pandemics are the consequence of more radical changes brought about when genes from different strains or subtypes of influenza A viruses infect the same host. The influenza viral genome is segmented, making possible the interchange of genes from different viruses. This may result in the emergence of a new influenza variant with a novel pairing of HA and NA proteins, to which existing antibodies are ineffective.10 This process of genetic reassortment, leading to a completely novel antigenic subtype in humans, is termed antigenic shift.

Surveillance and selection of vaccine strains
As a result of the constantly changing make-up of influenza viruses, the World Health Organisation (WHO) global influenza surveillance network monitors the viruses causing outbreaks of influenza throughout the year in different parts of the world. The network comprises four international centres located in London, Atlanta, Melbourne and Tokyo, and 110 national laboratories in 83 countries.11 The aim of this surveillance is to collect, analyse and distribute information on influenza activity, which will aid the prevention and control of influenza infection and its complications. Three types of information are assessed. First, epidemiological data, such as consultation rates with physicians for influenza-like illness, illness levels among schoolchildren and death rates from respiratory illnesses, are estimated. Secondly, antigenic and genetic characteristics of the HA and NA proteins of viruses isolated from respiratory secretions in laboratories are analysed. These are compared with viruses isolated from previous years and those in the current vaccine. Thirdly, the ability of currently available vaccines to evoke antibody responses in children, adults and the elderly to newly detected strains is also analysed in clinical trials. All data from around the world are collated by the WHO in Geneva, which, every year, recommends to vaccine manufacturers which strains to include in the vaccine.

Panel 1: Recommendations for influenza vaccine composition 2000/2001

1. An A/Moscow/19/99 (H3N2)-like virus
2. An A/New Caledonia/20/99 (H1N1)-like virus
3. A B/Beijing/184/93-like virus

Recommendations are made in February for vaccines to be used in the northern hemisphere the following year (Panel 1), and in September for use in the southern hemisphere.

Current influenza vaccines
Every year, new influenza vaccine has to be produced. The vaccine normally contains three components: two subtypes of influenza A (H1N1 and H3N2) and one of influenza B. The information on which strains have been dominant over the winter and any evidence of the emergence and potential spread of new strains is carefully analysed. The effectiveness of the vaccine will depend on how close the vaccine strains match the influenza viruses that circulate in the following winter. Influenza vaccines currently available in the UK contain highly purified inactivated (killed) virus, grown in specific pathogen-free fertile hen’s eggs. Both split virion and surface antigen vaccines (purified from chemically disrupted viruses) are available, containing split particles, or HA and NA proteins, respectively. The timing of the WHO recommendations is critical since it may take six to nine months from when production begins in March to prepare and test the new vaccine ready for the new influenza season. In Europe, the first step is the approval of strains by the European Medicines Evaluation Agency in March. Standardisation of the vaccine then takes place, in which potency and safety tests are performed. The immunogenicity of the newly manufactured vaccine is then assessed in clinical trials.

Immunisation policy

The aim of the Department of Health influenza immunisation policy is to reduce the level of morbidity and mortality due to influenza. This year, the recommendations for those to receive the vaccine have been extended to include immunisation of all people aged 65 and over,12 rather than 75 years and over as previously recommended (Panel 2).

Panel 2: Recommended groups to receive influenza vaccination

1. All people aged 65 years and over
2. People with chronic respiratory disease, including asthma
3. People with chronic cardiovascular disease or renal disease
4. Individuals with diabetes mellitus
5. Those who are immunocompromised, either due to disease, or treatment such as steroid medication or cancer treatment
6. Those living in residential long-stay accommodation, where influenza, once introduced, may spread rapidly

The decision follows an assessment showing that immunising the otherwise fit 65-74 year age group offers benefits in life expectancy as well as reductions in complications and hospital admissions. Despite efforts at targeting the vaccine, not all individuals in key target groups are receiving the vaccine.13,14 For the first time, a target of 70 per cent uptake in people aged 65 years and over is being set for health authorities, with the aim of a minimum 60 per cent uptake in this first year. The number of doses of influenza vaccine available to general practitioners in the UK will be increased from eight to 10 million.

New approaches to influenza vaccination
Inactivated influenza vaccine is highly effective in young adults. However, efficacy may be suboptimal in children and the elderly. Certain individuals are also allergic to egg proteins. Because of these limitations, efforts have been directed towards either improving the efficacy of the current vaccine, by the addition of new adjuvants together with intranasal administration, or developing new types of vaccines. Other types of vaccine currently being studied include:

Live attenuated vaccines An intranasal cold-adapted vaccine is in late clinical development in several countries.15 A donor virus, which has become attenuated by cold-adaptation, is mixed with a wild-type virus that contains the HA and NA segments against which protection is sought. Following reassortment of viral genes, the resulting vaccine strain contains an attenuated donor genome, but expresses the wild-type HA and NA surface proteins. Live attenuated vaccines have the advantage of stimulating stronger immune responses without causing illness and can be administered by spraying into the nose. Both live attenuated vaccines and recombinant vaccines (proteins produced by DNA technology) offer promise for the future and appear to be safe, with few severe reactions in vaccinated subjects, including very young infants.16

DNA vaccines The potential of immunisation with purified DNA as a powerful technique for inducing immune responses was first demonstrated in the early 1990s. The routes of inoculation used for influenza DNA immunisation include intramuscular, intradermal and delivery of DNA-coated gold beads directly to the skin. Although DNA vaccines have several advantages, such as no requirement for production of vaccine in eggs, the introduction of genetic information into mammalian hosts raises several safety concerns. These include the possible formation of anti-DNA antibodies and the potential for causing transformation from a normal cell to a cancer cell. Efficacy of DNA vaccination against influenza has been demonstrated in animals. Initial human trials are still in progress, so it is too early to predict if DNA vaccines may be used in human immunisation programmes.

Treatment of influenza infection
General treatment Influenza for most people is an unpleasant, but self-limiting, disease. The main symptoms may last for up to seven days. Treatment for most individuals is symptomatic and those affected are advised to stay at home, rest and drink plenty of fluids.

Antiviral drugs
There is normally a good match between circulating viruses and the vaccine and when this happens influenza vaccine has been shown to prevent illness in approximately 70-90 per cent of healthy individuals. This, however, leaves a window of vaccine failures and at-risk patients where drug prophylaxis and treatment can provide the protection needed.

Amantadine and rimantadine Until last year, the anti-matrix protein drugs amantadine (Symmetrel), and its close derivative rimantadine (Flumavine) were the only antiviral compounds licensed for the treatment of influenza infections.

Figure 2: Amantadine blocks the pore of the M2 ion channel located within the virus envelope preventing virus replication. (Figure from “Action of adamantanamines” by A. J. Hay in Seminars in Virology. Copyright © 1992 by Academic Press, reproduced by permission of the publisher)

The target of both drugs is the M2 protein which forms an ion channel and facilitates the release of viral genetic material into the infected cell. Amantadine and rimantadine inhibit viral replication by blocking this channel (Figure 2).

The effectiveness of amantadine and rimantadine for both the prophylaxis and treatment of influenza infections has been established through many studies over the past 30 years. The benefits of prophylactic treatment have been proven using experimental challenges in healthy individuals with influenza A.17 Vital dose-response studies have helped determine the optimum dose for protection, as both of the drugs have a low toxic-to-therapeutic ratio that makes dosing vitally important.18 In naturally occurring influenza infections in adults and children, amantadine and rimantadine have been shown to have an efficacy of up to 90 per cent in preventing influenza illness.19,20 Within closed populations, eg, boarding schools, nursing homes and prisons, the drugs have also been shown to be effective in preventing illness, and, in some cases, reducing mortality. 21,22
Amantadine and rimantadine have proved equally effective in treating acute influenza infections in healthy adults, children and the elderly.23-25 The drugs are equally as effective as the annual vaccine and are able to prevent illness or reduce influenza-like symptoms. However, it is thought that administration of amantadine and rimantadine must be initiated within 48 hours of the onset of symptoms if they are to be effective.

The above trials and studies prove that amantadine and rimantadine are potent inhibitors of influenza and reduce disease, but their use in the UK has been limited. The drugs are restricted in that they are only specific against influenza A because their target protein, M2 is present only in influenza A viruses. There has been concern about the side effects associated with treatment. It has been reported that these occur in approximately 6 per cent of patients, more often in the elderly, and can consist of neurological reactions, including light-headedness and an inability to concentrate, and gastrointestinal complaints.26, 27 Due to the lower incidence of neurological side effects associated with rimantadine treatment, it is now perceived to be the drug of choice in some countries, although it can still cause gastrointestinal complaints, nausea and vomiting.18 In the UK, amantadine has been rebranded (Lysovir) and the recommended dosage of the drug has been reduced. The overall efficacy has been retained at this lower dose but the occurrence of side effects has been reduced.
In vitro and in vivo studies have shown that cross-resistant viruses to both drugs can be generated very rapidly.26 Furthermore, transmission of resistant viruses between humans has been observed in families undergoing prophylactic drug treatment28 and can cause typical influenza.29
In general, there seems to be a lack of awareness of the potential of these drugs for preventing or limiting influenza A infections. With the advent of novel inhibitors, amantadine and rimantadine may still play an important role in the control of influenza through combined drug therapies.

Neuraminidase inhibitors The NA molecule located on the surface of the influenza virus particle (Figure 1) is an enzyme that is essential for virus replication and infectivity. The enzyme cleaves terminal sialic acid residues (the receptor molecules for the viral haemagglutinin) from neighbouring glycoconjugates. The active site of the molecule is highly conserved in NA species of both influenza A and B viruses, making it an ideal target for antiviral therapy.

Figure 3: Diagrammatic representation of NI compounds blocking the active site of the NA molecule. Green lines represent bonds formed between different groups on the drugs and amino acids within the site. Zanamivir and oseltamivir bind to the site more readily than sialic acid through a guanidine and hydrophobic group, respectively, increasing their antiviral potency (Adapted from “Disarming flu viruses,” by W. Graeme Laver, Norbert Bischofberger and Robert G. Webster. Copyright © 1998 by Scientific American, Inc.)


The principle behind neuraminidase inhibitors (NI) is to provide a synthetic sialic acid residue that will block the active site of the enzyme and prevent it from functioning (Figure 3). There has been much research carried out in this field which has encouraged the development of new sialic acid derivatives with potent antiviral properties. One of these compounds, GG167 (zanamivir), was discovered in 199330 and has been developed by Glaxo Wellcome under licence from the Australian company Biota Holdings. Phase I and II clinical trials proved its potency and phase III trials have proved its efficacy and safety in human subjects.
Results from randomised, double-blind placebo-controlled studies of large groups of subjects in both the northern31-35 and the southern36 hemispheres have demonstrated the clinical usefulness of the drug. The main conclusions were that in subjects treated with zanamivir, symptoms of headache, sore throat, fever, cough, weakness and muscle ache were alleviated faster than in placebo groups, a difference of one to 2.5 days. However, for the drug to be clinically effective it had to be administered less than 30-40 hours after the onset of symptoms. The incidence of complications and use of antibiotics were also reported to be lower in treated groups. Side effects were described in approximately 3 per cent of subjects, the most common being sinusitis, diarrhoea and nausea, but this was comparable to placebo groups. Zanamivir is applied topically to the respiratory tract, via inhalation using a Diskhaler,33 but it has been reported that this can cause bronchospasm and/or a decline in lung function in some patients with underlying respiratory disease.
The application for licensing zanamivir, under the trade name Relenza, was initially turned down by the US Food and Drug Administration (FDA) on the grounds that the US trials did not produce as good results as other trials around the world.37 The FDA was eventually persuaded after further trials and more conclusive results relating to the clinical efficacy and safety of the drug. Zanamivir was licensed in the US in July, 1999, and in the UK in September, 1999. Controversially, the National Institute for Clinical Excellence (NICE) in the UK, after agreeing to set-up a “fast track” assessment of the drug to enable its use, recommended that zanamivir should not be prescribed in the 1999/2000 winter season. The NICE stated that further studies needed to be completed to fully assess clinical benefits, especially in at-risk groups.38
Another promising NI, oseltamivir was first described in 199739 and was licensed in the US in late 1999, under the trade name Tamiflu. The main advantages oseltamivir has over zanamivir is that its bioavailability is much greater and it can be taken orally. The efficacy and safety of oseltamivir has been proven and the drug is effective at reducing the duration and severity of illness.40-44 Oseltamivir significantly reduced the duration of illness in infected patients by 25 per cent. Where treatment was initiated within 24 hours of the onset of symptoms this reduction was 37 per cent. It was reported that the overall health of treated patients was improved in respect of symptoms, activity and sleep quality, and secondary complications, including sinusitis, bronchitis and pneumonia, were also reduced. Overall conclusions show that oseltamivir is well tolerated and treatment reduces the severity and duration of acute influenza infection.

Panel 3: Useful links containing influenza-related information

Previous studies of amantadine and rimantadine showed how resistance could be generated in patients undergoing treatment.23 There is similar concern with NIs. With the predicted demand for the drugs, will the isolation of resistant viruses increase? In vitro studies have demonstrated that growing the virus repeatedly in the presence of drug does lead to resistance but to date there have been few resistant viruses isolated from clinical cases. As the demand for NIs increases, it will be important to monitor the generation and genetic composition of resistant viruses.
The targeted use of NI drugs in prophylaxis, in special situations, would be a beneficial alternative or addition to vaccination. When used prophylactically, zanamivir has been shown to be highly effective in preventing infection with clinical illness.34
New NI drugs are continually being developed. One such compound, RWJ-270201, produced by Biocryst Pharmaceuticals, is currently undergoing phase II clinical trials. Preliminary results from these early trials, suggest that human volunteers infected with influenza showed significant reductions in virus titres after treatment.
Further trials will be required to determine the full clinical effectiveness of the drug before it appears on the market.
Antiviral therapy has the potential to reduce the burden of influenza infection on the general population. Effective treatment requires accurate diagnosis of influenza infection within 48 hours of onset of symptoms. This is often very difficult to achieve and therefore cost-effective use of drugs may rely upon near-patient testing to confirm infection.

Summary: Influenza prevention and treatment

  • Influenza is a seasonal, self-limiting disease causing respiratory tract infections resulting in significant morbidity and mortality
  • Annual vaccination provides the most effective treatment against influenza, preventing illness in approximately 70-90 per cent of healthy individuals
  • Current research is developing novel vaccines with improved efficacy and longer lasting immunity
  • Antiviral drug chemotherapy provides prophylactic protection against and treatment of influenza
  • The potential threat of future influenza pandemics and the continuing generation of drug-resistant viruses to current compounds leaves no doubt that the development of new antiviral drugs against influenza is required
  • New NIs (zanamivir and oseltamivir) are proven to reduce the duration and severity of illness in healthy adults l NIs require further trials to evaluate the safety and efficacy in the young (<12 years) and in at-risk groups
  • The ability of the virus to mutate and evolve rapidly makes protecting the population from influenza a constant battle between man and virus

References

  1. Fleming DM. The impact of three influenza epidemics on primary care in England and Wales. Pharmacoeconomics 1996;9:38-45.
  2. Nicholson KG. Human influenza. In: Nicholson KG, Webster RG, Hay AJ, editors. Textbook of influenza. Oxford: Blackwell Science Ltd; 1998. p219-64.
  3. Johnson SLG, Bloy H. Evaluation of a rapid immunoassay for the detection of influenza A virus. J Clin Microbiol 1993;31:142-3.
  4. Todd SJ, Minnich L, Waner JL. Comparison of rapid immunofluorescence procedure with Test Pack RSV and Directigen Flu A for diagnosis of respiratory syncytial virus and influenza A virus. J Clin Microbiol 1995;33:1650-1.
  5. Waner JL, Todd SJ, Shalaby N, Murphy P, Wall LV. Comparison of Directigen Flu-A with viral isolation and direct immunofluorescence for the rapid detection and identification of influenza A virus. J Clin Microbiol 1991;29:479-82.
  6. WHO. A revision of the system of nomenclature for influenza viruses: a WHO memorandum. Bull World Health Org 1980;58:585-91.
  7. Schild GC, Newman RW, Webster RG, Major D, Hinshaw VS. Antigenic analysis of influenza A surface antigens. Considerations for the nomenclature of influenza virus. Arch Virol 1980;63:171-84.
  8. Rohm C, Zhou N, Suss J, Mackenzie J, Webster RG. Characterisation of a novel influenza haemagglutinin, H15; criteria for determination of influenza A subtypes. Virology 1996;
    217:508-16.
  9. Monto AS, Koopman JS, Longini IM. The Tecumesh study of illness. XIII. Influenza infection and disease, 1976-81. Am J Epidemiol 1985;121:811-22.
  10. Webster RG, Laver WG. Pandemic variation of influenza viruses. In: Kilbourne ED, editor. The influenza viruses and influenza. New York: Academic Press; 1975. p269-314.
  11. WHO. WHO influenza surveillance. Wkly Epidemiol Rec 1996;71:353-7.
  12. WHO. Recommendations for influenza vaccine composition for use in 2000-2001. Wkly Epidemiol Rec 2000;75(8):61-68.
  13. Irish C, Alli M, Gilham C, Joseph C, Watson J. Influenza vaccine uptake and distribution in England and Wales, July 1989-June 1997. Health Trends 1998;30:51-5.
  14. Watkins J. Effectiveness of influenza vaccination policy at targeting patients at high risk of complications during winter 1994-5: cross sectional survey. BMJ 1997;25:1069-70.
  15. Belshe RB, Mendelman PM, Treanor J, King J, Gruber WC, Piedra P et al. The efficacy of live attenuated, cold-adapted, trivalent, intranasal influenza virus vaccine in children. N Engl J Med 1998;338:1405-12.
  16. Murphy BR. Use of live attenuated cold-adapted influenza A reassortant virus vaccines in infants, children, young adults and elderly adults. Inf Dis Clin Prac 1993;2:174-81.
  17. Reuman PD, Bernstein DI, Keefer MC, Young EC, Sherwood JR, Schiff GM. Efficacy and safety of low dosage amantadine hydrochloride as prophlyaxis for influenza A. Antiviral Res 1989;11:27-40.
  18. Aoki FY. Amantadine and rimantadine. In: Nicholson KG, Webster RG, Hay AJ, editors. Textbook of influenza. Oxford: Blackwell Sciences; 1998. p457-76.
  19. Crawford SA, Clover RD, Abell TD, Ramsey CN, Glezen P, Couch RB. Rimantadine prophylaxis in children: a follow-up study. Pediatr Infect Dis J 1988;7:379-83.
  20. Brady MT, Sears SD, Pacini DL, Samorodin R, DePamphilis J, Oakes M et al. Safety and prophylactic efficacy of low-dose rimantadine in adults during an influenza A epidemic. Antimicrob. Agents Chemother 1990;34:1633-6.
  21. Arden NH, Patiarea PA, Fasano MB, Lui KJ, Harmon MW, Kendal AP et al. The roles of vaccination and amantadine prophylaxis in controlling an outbreak of influenza A (H3N2) in a nursing home. Arch Int Med 1988;148:865-8.
  22. Payler DK, Purdham PA. Influenza A prophylaxis with amantadine in a boarding school. Lancet 1984;i:502-4.
  23. Hayden FG, Sperber SJ, Belshe RB, Clover RD, Hay AJ, Pyke S. Recovery of drug-resistant influenza A virus during therapeutic use of rimantadine. Antimicrob Agents Chemother 1991;35:1741-7.
  24. Hall CB, Dolin R, Gala CL, Markovitz DM, Zhang YQ, Madore PH et al. Children with influenza A infection: treatment with rimantadine. Pediatrics 1987;80:275-82.
  25. Betts RF, Treanor JJ, Graman PS, Bently DW, Dolin R. Antiviral agents to prevent or treat influenza in the elderly. J Resp Dis 1987;8:S56-9.
  26. Belshe RB, Smith MH, Hall CB, Betts R, Hay AJ. Genetic basis of resistance to rimantadine emerging during treatment of influenza virus infection. J Virol 1988;62:1508-12.
  27. Hay AJ. The action of adamantanamines against influenza A viruses: inhibition of the M2 ion channel protein. Semin Virol 1992;3:21-30.
  28. Hayden FG, Belshe RB, Clover RD, Hay AJ, Oakes MG, Soo W. Emergence and apparent transmission of rimantadine-resistant influenza A virus in families. N Engl J Med 1989;321:1696-702.
  29. Belshe RB, Burk B, Newman F, Cerruti RL, Sim IS. Resistance of influenza A virus to amantadine and rimantadine:
    results of one decade of surveillance. J Infec Dis 1989;
    159:430-5.
  30. von Itzstein M, Wu WY, Kok GB, Pegg MS, Dyason JC, Jin B et al. Rational design of potent sialidase-based inhibitors of influenza virus replication. Nature 1993;363:418-23.
  31. Mäkelä MJ, Pauksens K, Rostile T, Flemming DM, Man CY, Keene ON et al. Clinical efficacy and safety of the orally inhaled neuraminidase inhibitor zanamivir in the treatment of influenza: a randomized, double-blind, placebo-controlled European study. J Infect 2000;40:42-8.
  32. Monto AS, Flemming DM, Henry D, de Groot R, Makela M, Klein T et al. Efficacy and safety of the neuraminidase inhibitor zanamivir in the treatment of influenza A and B virus infections. J Infec Dis 1999;180:254-61.
  33. Monto AS, Robinson DP, Herlocher ML, Hinson JM, Elliott MJ, Crisp A. Zanamivir in the prevention of influenza among healthy adults: a randomized controlled trial. JAMA 1999;
    282:31-5.
  34. Hayden FG, Treanor JJ, Betts RF, Lobo M, Esinhart JD, Hussey EK. Safety and efficacy of the neuraminidase inhibitor GG167 in experimental human influenza. JAMA 1996;275:295-9.
  35. Hayden FG, Osterhaus AD, Treanor JJ, Fleming DM, Aoki FY, Nicholson KG et al. Efficacy and safety of the neuraminidase inhibitor zanamivir in the treatment of influenzavirus infections. N Engl J Med 1997;337:874-80.
  36. Mist SG. Randomised trial of efficacy and safety of inhaled zanamivir in treatment of influenza A and B virus infections. Lancet 1998;352:1877-81.
  37. Ault A. New influenza therapy voted down by FDA panellists. Lancet 1999;353:816.
  38. NICE’s guidance on the use of zanamivir (Relenza) for the treatment of influenza. CMO’s Update 1999;24:3.
  39. Kim CU, Lew W, Williams MA, Liu H, Zhang L, Swaminathan S et al. Influenza neuraminidase inhibitors possessing a novel hydrophobic interaction in the enzyme active site: design, synthesis and structural analysis of carbocyclic sialic acid analogues with potent anti-influenza activity. J Am Chem Soc 1997;119:681-90.
  40. Hayden FG, Treanor JJ, Fritz RS, Lobo M, Betts RF, Miller M et al. Use of the oral neuraminidase inhibitor oseltamivir in experimental human influenza: randomized controlled trials for prevention and treatment. JAMA 1999;282:1240-6.
  41. Hayden FG, Atmar RL, Schilling M, Johnston C, Poretz D, Paar D et al. Use of the selective oral neuraminidase inhibitor oseltamivir to prevent influenza. N Engl J Med 1999;341:1336-43.
  42. Mendel DB, Tai CY, Escarpe PA, Li W, Sidwell RW, Huffman JH et al. Oral administration of a prodrug of the influenza virus neuraminidase inhibitor GS 4071 protects mice and ferrets against influenza infection. Antimicrob Agents Chemother 1998;42:640-6.
  43. Treanor JJ, Hayden FG, Vrooman PS, Barbarash R, Bettis R, Riff D et al. Efficacy and safety of the oral neuraminidase inhibitor oseltamivir in treating acute influenza: a randomized controlled trial. JAMA 2000;283:1016-24.
  44. Nicholson KG, Aoki FY, Osterhaus ADME, Trottier S, Carewicz O, Mercier CH et al. Efficacy and safety of oseltamivir in treatment of acute influenza: a randomised controlled trial. Lancet 2000;355:1845-50.