Update on 2009 pandemic influenza A (H1N1) virus
ABSTRACTThe pandemic of a novel strain of swine-origin influenza A (H1N1) is expected to make this a difficult flu season. Fortunately, this strain is relatively mild, and the principles of prevention, diagnosis, and treatment remain the same. Physicians will have a number of complex decisions to make about when to test, when to treat, and when to simply reassure.
KEY POINTS
- Vaccination this season will require two vaccines: a trivalent vaccine for seasonal influenza and a monovalent vaccine for 2009 pandemic influenza A (H1N1).
- Recent studies indicate that the monovalent vaccine for 2009 pandemic influenza A (H1N1) may require only one injection.
- To date, 2009 pandemic influenza A (H1N1) virus has not been exceptionally virulent and differs from conventional influenza in that it seems to disproportionately affect children and young adults. Pregnant women are at a higher risk of complications.
- Most people with 2009 pandemic influenza A (H1N1) do not need to be tested, treated, or seen by a clinician.
- Antiviral drugs should be reserved only for those at high risk of influenza complications.
ANTIVIRAL TREATMENT
Since influenza test results do not specify whether the patient has seasonal or pandemic influenza, treatment decisions are a sticky wicket. Most patients with pandemic H1N1 do not need to be tested or treated.
Several drugs are approved for treating influenza and shorten the duration of symptoms by about 1 day. The earlier the treatment is started, the better: the time of antiviral initiation affects influenza viral load and the duration of viral shedding.3
The neuraminidase inhibitors oseltamivir and zanamivir (Relenza) block release of virus from the cell. Resistance to oseltamivir is emerging in seasonal influenza A, while most pandemic H1N1 strains are susceptible.
Oseltamivir resistance in pandemic H1N1
A total of 11 cases of oseltamivir-resistant pandemic H1N1 have been confirmed worldwide, including 3 in the United States (2 in immunosuppressed patients in Seattle, WA). Ten of the 11 cases occurred with oseltamivir exposure. All involved a histidine-to-tyrosine substitution at position 275 (H275Y) of the neuraminidase gene. Most were susceptible to zanamivir.
Supplies of oseltamivir and zanamivir are limited, so they should be used only in those who will benefit the most, ie, those at higher risk of influenza complications. These include children under 5 years old, adults age 65 and older, children and adolescents on long-term aspirin therapy, pregnant women, patients who have chronic conditions or who are immunosuppressed, and residents of long-term care facilities.
