Excerpts:
This report summarizes U.S. influenza activity* from September 28, 2008, the start of the 2008--09 influenza season, through April 4, 2009, and reports on the 2009--10 influenza vaccine strain selection. Low levels of influenza activity were reported from October through early January. Activity increased from mid-January and peaked in mid-February. Influenza A (H1N1) viruses have predominated overall this season, but influenza B viruses have been isolated more frequently than influenza A viruses since mid-March. Widespread oseltamivir resistance was detected among circulating influenza A (H1N1) viruses and a high level of adamantane resistance was identified among influenza A (H3N2) viruses.
Antigenic Characterization
WHO collaborating laboratories in the United States are requested to submit a subset of their influenza virus isolates to CDC for further antigenic characterization. CDC has antigenically characterized 945 influenza viruses collected by U.S. laboratories during the 2008--09 season, including 594 influenza A (H1N1), 88 influenza A (H3N2), and 263 influenza B viruses. All 594 influenza A (H1N1) viruses are related to the influenza A (H1N1) component of the 2008--09 influenza vaccine (A/Brisbane/59/2007). All 88 influenza A (H3N2) viruses are related to the influenza A (H3N2) vaccine component (A/Brisbane/10/2007). Influenza B viruses currently circulating can be divided into two distinct lineages represented by the B/Yamagata/16/88 and B/Victoria/02/87 viruses. Among the 263 influenza B viruses tested, 50 (19.0%) belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006); the remaining 213 (81.0%) belong to the B/Victoria lineage and are not related to the vaccine strain.
Composition of the 2009--10 Influenza Vaccine
WHO recommended that the 2009--10 Northern Hemisphere trivalent influenza vaccine contain A/Brisbane/59/2007-like (H1N1), A/Brisbane/10/2007-like (H3N2), and B/Brisbane/60/2008-like (B/Victoria lineage) viruses. The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee recommended these same vaccine strains be included in the 2009--10 influenza vaccine for the United States (1). Only the influenza B component represents a change from the 2008--09 vaccine formulation. These recommendations were based on antigenic and genetic analyses of recently isolated influenza viruses, epidemiologic data, post-vaccination serologic studies in humans, and the availability of candidate vaccine strains and reagents.
Antiviral Resistance of Influenza Virus Isolates
CDC conducts surveillance for resistance of circulating influenza viruses to licensed influenza antiviral medications: adamantanes (amantadine and rimantadine) and neuraminidase inhibitors (zanamivir and oseltamivir). Since October 1, 2008, of the 699 influenza A (H1N1) viruses from 44 states tested for neuraminidase inhibitor resistance, 694 (99.3%) were resistant to oseltamivir; all were sensitive to zanamivir (Table). All 103 influenza A (H3N2) and all 274 influenza B viruses tested were sensitive to oseltamivir and zanamivir. Three influenza A (H1N1) viruses (0.4%) and all 100 (100%) influenza A (H3N2) viruses tested were resistant to adamantanes (amantadine and rimantadine). The adamantanes are not effective against influenza B viruses. None of the influenza A (H1N1) viruses tested were resistant to both oseltamivir and adamantanes.
Novel Influenza A Viruses
A case of human infection with a novel influenza A virus was reported by the Iowa Department of Public Health during the week ending February 28, 2009. A male aged 3 years was infected with a swine influenza A (H1N1) virus. An investigation revealed that the child had close contact with ill pigs. The child has fully recovered from the illness, and no additional cases were identified among the child's contacts or other persons exposed to the ill pigs. This is the third human infection with swine influenza virus identified in the United States this influenza season. None of the cases were related to occupation. The other two human infections with swine influenza identified during the 2008--09 influenza season occurred in a person aged 14 years from Texas and a person aged 19 years from South Dakota (2,3).
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Editorial Note:
During this influenza season, a high level of resistance to the antiviral drug oseltamivir was detected among circulating influenza A (H1N1) viruses. Since October 1, 2008, 99.3% of influenza A (H1N1) viruses tested were resistant to oseltamivir. To date, influenza A has accounted for 67.3% of all influenza viruses identified, and influenza A (H1N1) has accounted for 89.8% of the influenza A viruses that were subtyped. No oseltamivir resistance has been detected among influenza A (H3N2) or B viruses currently circulating in the United States; however, all the influenza A (H3N2) viruses tested were resistant to adamantanes. The adamantanes are not effective against influenza B viruses. None of the influenza A (H1N1) viruses tested were resistant to both oseltamivir and the adamantanes, and all influenza viruses tested this season have been susceptible to zanamivir. CDC issued interim guidelines for the use of influenza antiviral medications on December 19, 2008. Health-care providers should review their local surveillance data if available to determine which types (A or B) and subtypes of influenza A (H1N1 or H3N2) are most prominent in their community and consider using diagnostic tests to distinguish influenza A from influenza B. When an influenza A (H1N1) virus infection or exposure is suspected, zanamivir is the preferred medication; combination therapy of oseltamivir and rimantidine is an acceptable alternative (6).
Since early February, the relative proportion of influenza B viruses has been increasing each week, and more than half of influenza viruses identified since the week ending March 14, 2009, were influenza B. Approximately 80% of influenza B viruses tested have not been related to the influenza B vaccine strain. However, all influenza B viruses this season have been susceptible to oseltamivir and zanamivir. Health-care providers should be aware of these recent increases in influenza B viruses and of the differences in antiviral resistance patterns compared with influenza A (H1N1) viruses. When an influenza B infection or exposure is detected, treatment with oseltamivir or zanamivir is recommended. However, when the type or subtype is unknown, zanamivir is the preferred medication; combination therapy of oseltamivir and rimantidine also is acceptable (6).
-snip-FIGURE 1. Number (N = 24,793) and percentage of respiratory specimens testing positive for influenza reported by World Health Organization and National Respiratory and Enteric Virus Surveillance System collaborating laboratories, by type, and surveillance week - United States, September 28, 2008-April 4, 2009
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