Antiviral Drug-Resistance among Influenza Viruses

Oseltamivir and Zanamivir (Neuraminidase Inhibitors)

Oseltamivir or zanamivir are the primary antiviral agents recommended for the prevention and treatment of influenza [28, 51, 105].

Because currently circulating influenza A (H3N2) and 2009 H1N1 viruses are resistant to adamantanes, these medications are not recommended for use against influenza A virus infections. However, influenza A and B virus strains are, with rare exception, susceptible to oseltamivir and zanamivir [110]. Sporadic oseltamivir-resistant 2009 H1N1 virus infections have been identified, including with rare episodes of limited transmission [111--115], but the public health impact has been limited to date. However, additional sporadic cases of oseltamivir-resistant 2009 H1N1 virus infection can be expected, and ongoing surveillance for oseltamivir resistance among influenza viruses is essential for public health because oseltamivir is the most widely used antiviral medication.

Development of resistance to zanamivir or oseltamivir also has been identified during treatment of seasonal influenza [116--120]. One study reported that oseltamivir-resistant seasonal influenza A viruses were isolated from nine (18%) of 50 Japanese children during treatment with oseltamivir [121]. Transmission of neuraminidase-inhibitor--resistant influenza B viruses has been reported among household contacts [122]. Development of resistance to oseltamivir during treatment was more common among seasonal influenza A (H1N1) virus infections (27%) compared with seasonal influenza A (H3N2) (3%) or B (0) viruses in another study [123]. Sporadic cases of resistance to oseltamivir have been observed among persons with 2009 H1N1 virus infection (e.g., immunosuppressed patients with prolonged viral replication during oseltamivir treatment and persons who developed illness while receiving oseltamivir chemoprophylaxis) [114, 124]. Emergence of oseltamivir-resistant 2009 H1N1 virus strains within 48 hours after initiation of treatment has been reported [125]. Transmission of oseltamivir-resistant influenza B virus strains or 2009 H1N1 virus strains acquired from persons treated with oseltamivir is rare but has been documented [112, 122]. Isolation of influenza A viruses with reduced susceptibility to zanamivir have been reported rarely, although the number of posttreatment isolates tested is limited [117--119, 126]. Clinical isolates with reduced susceptibility to zanamivir have been obtained occasionally from immunocompromised children on prolonged therapy [118, 127]. Prolonged shedding of oseltamivir- or zanamivir-resistant virus by severely immunocompromised patients, even after cessation of oseltamivir treatment, has been reported [118, 127--129]. Rare cases of infection with 2009 H1N1 virus resistant or with reduced susceptibility to multiple neuraminidase inhibitors in severely immunosuppressed pediatric patients with prolonged viral replication have been reported [130, 131].

Top

During 2007--2008, increased resistance to oseltamivir associated with a specific mutation causing a histidine to tyrosine substitution (H275Y) in neuraminidase was reported among seasonal influenza A (H1N1) virus strains in many countries and became prevalent worldwide [132--134]. Most persons infected with oseltamivir-resistant seasonal influenza A (H1N1) virus strains had not received oseltamivir treatment previously and were not known to have been exposed to a person receiving oseltamivir treatment or chemoprophylaxis [133, 135]. Influenza caused by oseltamivir-resistant seasonal influenza A (H1N1) virus strains appears to be similar to illness caused by oseltamivir-sensitive virus strains [133, 134, 136]. Since the recent emergence of 2009 H1N1 virus, oseltamivir-resistant seasonal influenza A (H1N1) virus has been of less clinical concern because very few seasonal influenza A (H1N1) virus strains have been circulating [113]. Nearly all sporadic cases of oseltamivir-resistant 2009 H1N1 virus infections identified to date also have been associated with the H275Y mutation in neuraminidase; these oseltamivir-resistant H275Y virus infections are susceptible to zanamivir. As of December 2010, no evidence existed of ongoing transmission of oseltamivir-resistant 2009 H1N1 virus strains worldwide.

Amantadine and Rimantadine (Adamantanes)

Adamantane resistance among circulating influenza A viruses increased rapidly worldwide beginning during 2003--2004. The percentage of influenza A virus isolates submitted from throughout the world to the World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza at CDC that were adamantane-resistant increased from 0.4% during 1994--1995 to 12.3% during 2003--2004 [137]. During the 2005--06 influenza season, CDC determined that 193 (92%) of 209 influenza A (H3N2) viruses isolated from patients in 26 states demonstrated a change at amino acid 31 in the M2 gene that confers resistance to adamantanes [138]. Resistance to adamantanes remains high among influenza A isolates, with resistance detected among all tested influenza A (H3N2) and 2009 H1N1 viruses tested [113]. Therefore, amantadine and rimantadine are not recommended for antiviral treatment or chemoprophylaxis of currently circulating influenza A virus strains.

Top

Treatment Issues for Patients Hospitalized with Suspected or Confirmed Influenza

Treatment of patients with severe influenza (e.g., those requiring hospitalization) presents multiple challenges. The effect of specific antiviral strategies in serious or life-threatening influenza is not established from clinical trials conducted to support licensure of oseltamivir and zanamivir, as those studies were conducted primarily among previously healthy outpatients with uncomplicated illness. However, a number of more recent observational studies have reported that oseltamivir treatment up to 96 hours after illness onset of patients hospitalized with suspected or confirmed influenza is associated with lower risk for severe outcomes [12, 23, 27, 65, 150]. For this reason, recommendations in this report do not necessarily represent FDA-approved uses of antiviral products but are based on published expert opinion and observational studies and are subject to change as the developmental status of investigational products and the epidemiologic and virologic features of influenza change over time.

Initiation of antiviral treatment as early as possible is recommended for hospitalized patients. However, antiviral treatment might be effective in reducing morbidity and mortality in hospitalized patients even if treatment is not started until >48 hours after onset of illness. Data from observational studies indicates the benefit of antiviral treatment for hospitalized persons even when treatment is delayed [12, 23, 26--28, 150]. Careful attention to ventilator and fluid management and to the prevention and treatment of secondary bacterial pneumonia (e.g., S. pneumoniae, S. pyogenes, and S. aureus, including MRSA) also is critical for severely ill patients [66, 158--161].

Top

Treatment regimens might need to be altered to fit the clinical circumstances. For example, clinical judgment should be the guide regarding the need to extend treatment regimens longer than 5 days for patients whose illness is prolonged. No data are available to evaluate the effectiveness of larger doses of antivirals to treat severe influenza illness (e.g., viral pneumonia requiring admission to an intensive care unit), and one study indicated that enteric absorption among critically ill patients was adequate [162]. However, doubling the dose of oseltamivir (e.g., 150 mg twice daily in adults) has been advocated as an appropriate strategy in the treatment of severely ill patients with influenza A (H5N1), and limited data suggest this dosage is well tolerated [163] and might be beneficial [164].

Limited data indicate that administering oseltamivir via a gastric tube can provide systemic absorption in some critically ill patients with 2009 H1N1 or H5N1 [162, 165, 166], and these findings might be applicable to severe illness with other influenza virus infections. However, gastric stasis or bleeding can make this administration route problematic because of the potential for reduced absorption of medication [165, 167]. For these patients, parenteral medications might be preferable, but no clinical trials have demonstrated increased benefit, and none are FDA-approved. Clinical trials are needed to better understand optimal treatment approaches, and clinicians interested in enrolling patients in clinical trials of experimental intravenous antivirals and combination antiviral treatment should consult the National Institutes of Health (available at http://www.clinicaltrials.gov). For patients who are not eligible for clinical trial enrollment, physicians might wish in some instances to pursue single-patient emergency Investigational New Drug (IND) protocols for antivirals that are not licensed. Clinicians may contact the study sponsor or manufacturer to explore this possibility and obtain information about implementing an IND protocol; contact information is available at http://www.clinicaltrials.gov.

Patients receiving antiviral medications who do not respond to treatment might have an infection with an antiviral-resistant influenza virus. Oseltamivir resistance, sometimes within 1 week of treatment initiation, has been reported particularly among immunocompromised patients with 2009 H1N1 virus infection who were receiving treatment with oseltamivir [114, 168--170]. Infection-control measures are especially important for patients who are immunocompromised to reduce the risk for transmission of oseltamivir-resistant viruses [104, 105].

Top

Investigational parenterally administered products that can be obtained via IND or other protocols in clinical trials include peramivir and zanamivir. Peramivir is an investigational neuraminidase inhibitor medication that has variable activity against influenza A and B viruses as reported in human and animal studies with small sample sizes [171, 172]. Investigational preparations of zanamivir that can be administered parenterally have been reported to reduce the likelihood of infection in a challenge model of experimental infection with influenza A virus [171, 173]. Intravenous zanamivir has been used with success in clinical settings [169, 170]. Intravenous zanamivir is the recommended antiviral treatment for severely ill patients with highly suspected or confirmed oseltamivir-resistant 2009 H1N1 virus infection [51, 169, 174].

For patients who are intubated, use of the zanamivir disc inhaler is not possible. Suboptimal delivery to sites of infection in patients with pneumonic or extrapulmonary disease is also of concern for patients with severe respiratory illness [171]. Limited experimental use of an unlicensed nebulized formulation of zanamivir has been well tolerated [175], but use of the nebulized preparation of the licensed powder formulation contained in the disc inhaler is not recommended because it has been demonstrated to clog ventilator tubing [176].

Concerns about influenza viruses with pandemic potential, the appearance and widespread transmission of 2009 pandemic influenza A (H1N1), and the limited treatment options available for severely ill patients has prompted renewed interest in development of additional antiviral drugs with activity against influenza viruses [171, 177]. Clinicians should be alert to the future availability of new therapeutic options and recommendations. In addition, careful attention to infection-control measures is recommended [104, 105], particularly in hospital areas that house immunocompromised patients.

Top

For current information on antiviral resistance among circulating influenza viruses in the United States, please see the FluView Weekly U.S. Influenza Surveillance Report Report.

For information on antiviral resistance among influenza viruses circulating globally, see the World Health Organization (WHO) influenza virological update page. (Note: WHO periodically updates its antiviral resistance information in association with its latest influenza virological update.)

For global updates on the susceptibility of 2009 H1N1 viruses to treatment with oseltamivir, see the WHO surveillance and monitoring influenza update page. (Note: WHO provides weekly updates on oseltamivir resistance and links to this information from its latest influenza surveillance and monitoring update).

For additional guidance on antiviral treatment of patients with suspected or documented antiviral resistant influenza virus infection, see the World Health Organization (WHO) Guidelines for Pharmacological Management of Influenza Viruses PDF Version [540 KB, 32 pages] . Per WHO guidance: "Zanamivir is the treatment of choice for all patients where oseltamivir resistance is demonstrated or highly suspected. Intravenous zanamivir may be considered where available."

Review the references cited in this guidance.

Top