Healthy children under five in England will receive the H1N1 vaccine next month. (11/20, The Guardian, London)

The WHO’s representative in China acknowledges the country’s H1N1 outbreak is much larger than official numbers show. (11/19, AP)

The 2009 H1N1 virus is susceptible to neuraminidase inhibitors (oseltamivir, zanamivir, peramivir) but resistant to the adamantanes (amantadine, rimantadine). Therefore, neuraminidase inhibitors are recommended for antiviral treatment of 2009 H1N1.^3,4 Emergence of oseltamivir-resistant 2009 H1N1 virus during or following treatment has been rarely identified.^5,6 Patients with infection caused by oseltamivir-resistant virus with the H275Y mutation in the neuraminidase should be treated with zanamivir.^5,6

No randomized trials of neuraminidase-inhibitor treatment of hospitalized influenza patients have been conducted. However, three observational studies suggest that oseltamivir treatment of hospitalized patients with seasonal influenza may reduce mortality. In one prospective Canadian study among hospitalized patients with seasonal influenza, (N=327; mean age, 77 years), in which 71% began oseltamivir treatment >48 hours after illness onset, oseltamivir treatment was significantly associated with a reduced risk of death (OR, 0.21; P=0.03) within 15 days after hospitalization as compared with untreated patients.⁷ In a subanalysis, in a Hong Kong study of hospitalized seasonal influenza patients (N=356; mean age, 70.2 years), oseltamivir treatment initiated within <96 hours after illness onset was independently associated with decreased mortality as compared with untreated patients (OR, 0.26; P=0.001).⁸ A retrospective chart review of hospitalized seasonal influenza patients in Thailand (N=445; mean age, 22 years), including 35% with radiographically confirmed pneumonia, reported that any oseltamivir treatment was significantly associated with survival (OR, 0.11; 95% CI, 0.04 – 0.30) as compared with untreated patients.⁹

Observational data from the United States and Mexico suggest that neuraminidase inhibitor treatment (primarily oseltamivir) of hospitalized patients with 2009 H1N1 may reduce disease severity and mortality. Starting treatment with a neuraminidase inhibitor within 2 days after symptom onset was significantly associated with a lower risk of ICU admission or death in hospitalized 2009 H1N1 patients (N=272; median age, 21 years), as compared with later treatment (P <0.05).¹⁰ In ICU patients with 2009 H1N1 (N=58; median age, 44 years), survivors were more likely to have received neuraminidase inhibitor treatment than nonsurvivors (OR, 8.5; P=0.04).¹¹

Although most hospitalized 2009 H1N1 patients have been treated with oral oseltamivir, including critically ill persons, parenteral neuraminidase inhibitors (peramivir,^12,13 zanamivir¹⁴) might be beneficial for some patients.¹³ Some critically ill 2009 H1N1 patients have been treated for twice the standard 5 days, and some have received higher oseltamivir dosing.¹ There are no head-to-head clinical trials of oral or intravenous oseltamivir, inhaled or intravenous zanamivir, intravenous peramivir, or combination treatment with other antivirals among hospitalized 2009 H1N1 patients to inform clinicians, but clinical trials may be available for enrollment.¹⁵ There is an urgent need for additional clinical, virologic, time-to-treat, and pharmacokinetic studies to assess neuraminidase inhibitor effectiveness, to inform dosing and duration, and to inform optimal clinical management for hospitalized 2009 H1N1 patients.

Taken together, although data are limited, findings of observational studies all point in the same direction, suggesting benefit of early neuraminidase inhibitor treatment for hospitalized influenza patients as well as for patients presenting >48 hours after illness onset. In the setting of 2009 pandemic influenza A (H1N1) virus activity in a community, empiric neuraminidase inhibitor treatment should be started as soon as possible for any hospitalized patient who presents with influenza that is suspected (e.g., acute respiratory illness, acute exacerbation of chronic conditions, or other complications) or confirmed, in addition to initiating antibiotic treatment as indicated for suspected bacterial coinfection.

Tim Uyeki, M.D., M.P.H.
Influenza Division
Centers for Disease Control and Prevention
Atlanta, Georgia

No potential conflict of interest relevant to this article was reported.

This article (10.1056/NEJMopv0910738) was published on November 18, 2009, at NEJM.org.

References

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Rapid antigen tests, as compared with RT-PCR assays, had poor sensitivity to the 2009 influenza A (H1N1) virus in 21 patients with severe infection and acute lung injury that required mechanical ventilation.

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Dr. Margaret Hamburg last week defended the FDA’s decision to not use adjuvanted H1N1 vaccines. (11/12, Reuters)