Weekly
June 3, 2011 / 60(21);705-712
During the 2010--11 influenza season, influenza activity* first began to increase in the southeastern United States, and peaked nationally in early February. Compared with the previous pandemic year (2009--10), higher rates of hospitalization were observed for persons aged ≥65 years during the 2010--11 season, whereas lower hospitalization rates were observed in younger populations than during the pandemic year. Overall, the percentages of outpatient visits for influenza-like illness (ILI) were lower during the 2010--11 season than the 2009--10 pandemic influenza season. In the United States, influenza A (H3N2) remained the predominant virus throughout the season; however, 2009 influenza A (H1N1) and influenza B viruses also circulated, and the predominant virus varied by U.S. Department of Health and Human Service (HHS) region and week. This report summarizes influenza activity in the United States during the 2010--11 influenza season (October 3, 2010--May 21, 2011) and describes the components of the 2011--12 Northern Hemisphere influenza vaccine.
Viral Surveillance
During October 3, 2010--May 21, 2011, World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 246,128 specimens for influenza viruses; 54,226 (22%) were positive (Figure 1). Of the positive specimens, 40,282 (74%) were influenza A viruses, and 13,944 (26%) were influenza B viruses. Among the influenza A viruses, 28,545 (71%) were subtyped; 17,599(62%) were influenza A (H3N2) viruses, and 10,946 (38%) were 2009 influenza A (H1N1) viruses.
The proportion of specimens testing positive for influenza during the 2010--11 season first exceeded 10%, indicating higher levels of virus circulation, during the week ending November 27, 2010. The proportion peaked at 36% during the week ending February 5, 2011, and declined to <10% during the week ending April 16, 2011.
Although influenza A (H3N2) viruses predominated, 2009 influenza A (H1N1) and influenza B viruses also circulated widely. The relative proportion of each type and subtype of influenza virus varied by region and week. From early November though early December, influenza B viruses accounted for 40%--49% of influenza viruses reported nationally, with the largest numbers reported from the southeastern states (HHS Region 4).† Influenza B viruses were predominant in Region 4 from early November through late December. The proportion of 2009 influenza A (H1N1) viruses increased nationally, beginning in January, and peaked during the week ending February 20, 2011, when 49% of all subtyped influenza A viruses were 2009 influenza A (H1N1) viruses. Although during this time influenza A (H3N2) viruses still predominated nationally, 2009 influenza A (H1N1) predominated in five of the 10 regions (Regions 3, 4, 5, 8, and 9) for 5--7 consecutive weeks, ranging from the week ending January 15 to the week ending April 2, 2011.
Novel Influenza A Viruses
Five cases of human infection with a novel influenza A virus were reported during the 2010--11 influenza season from three states. All five cases were infected with swine-origin influenza A (H3N2) viruses. Two cases occurred in September (Pennsylvania and Wisconsin), one case in October (Pennsylvania), and two cases in November (Minnesota). Two of the five cases occurred in adults, and three occurred in children. Two of the five cases were hospitalized; all five have recovered fully from their illness. The two cases in Pennsylvania were not related. The cases in Wisconsin and Pennsylvania had direct contact with swine or lived in areas close to swine farms. The two cases from Minnesota occurred in a father (index case) and child. The father had a nasopharyngeal swab positive for swine-origin influenza A (H3N2) virus and had direct swine exposure 6 days before illness onset. The child, whose infection with influenza A (H3N2) virus was confirmed several weeks later by serologic testing, did not have direct swine exposure, and most likely acquired infection from close contact with her father. Other persons in the same household also had ILI during the same period, but serologic results were either negative or inconclusive.
Antigenic Characterization
Since October 1, 2010, CDC has antigenically characterized 2,494 influenza viruses submitted by U.S. laboratories. Those have included 613 2009 influenza A (H1N1) viruses, 1,139 influenza A (H3N2) viruses, and 742 influenza B viruses. Of the 613 2009 influenza H1N1 viruses tested, 612 (99.8%) were characterized as A/California/7/2009-like, the 2009 influenza A (H1N1) component of the 2010--11 influenza vaccine. One virus (0.2%) of the 613 tested showed reduced titers with antiserum produced against A/California/7/2009. Of the 1,139 influenza A (H3N2) viruses, 1,103 (96.8%) were characterized as A/Perth/16/2009-like, the influenza A (H3N2) component of the 2010--11 influenza vaccine for the Northern Hemisphere. Of the 1,139 tested, 36 (3.2%) showed reduced titers with antiserum produced against A/Perth/16/2009.
Of the 742 influenza B viruses tested, 699 (94%) belonged to the B/Victoria lineage and 698 (99.9%) of these were characterized to be B/Brisbane/60/2008-like, the influenza B vaccine component for the 2010--11 Northern Hemisphere influenza vaccine. One (0.1%) of the 699 viruses belonging to the B/Victoria lineage showed reduced titers with antisera produced against B/Brisbane/60/2008. Of the 742 viruses tested, 43 (5.8%) belonged to the B/Yamagata lineage.
Resistance to Antiviral Medications
Since October 1, 2010, a total of 5,758 influenza virus specimens have been tested for antiviral resistance. All 723 influenza B viruses tested were sensitive to both oseltamivir and zanamivir. Among the 806 influenza A (H3N2) viruses tested for resistance to oseltamivir, two (0.2%) were found to be resistant. All 784 influenza A (H3N2) viruses tested for resistance to zanamivir were sensitive. Among the 4,229 2009 influenza A (H1N1) viruses tested for resistance to oseltamivir, 39 (0.9%) were found to be resistant, and all of the 771 viruses tested for resistance to zanamivir were found to be sensitive. High levels of resistance to the adamantanes (i.e., amantadine and rimantadine) persist among 2009 influenza A (H1N1) and influenza A (H3N2) viruses currently circulating globally.
Composition of the 2011--12 Influenza Vaccine
The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee recommended that the 2011--12 trivalent influenza vaccine for the United States contain A/California/7/2009-like (H1N1), A/Perth/16/2009-like (H3N2), and B/Brisbane/60/2008-like viruses. This represents no change to any of the three components from the 2010--11 influenza vaccine formulation used in the United States or from the current formulation of the 2011 Southern Hemisphere influenza vaccines. These recommendations were based on antigenic analyses of influenza viruses that circulated in the United States and worldwide during 2010--11, epidemiologic data, postvaccination serologic studies in humans, and the availability of candidate vaccine strains and reagents.
Outpatient Illness Surveillance
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Update: Influenza Activity --- United States, 2010--11 Season, and Composition of the 2011--12 Influenza Vaccine
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