Update: Influenza Activity --- United States, 2009--10 Season [CDC - MMWR]

Update: Influenza Activity --- United States, 2009--10 Season
Weekly
July 30, 2010 / 59(29);901-908


During the 2009--10 influenza season, the second wave of influenza activity from 2009 pandemic influenza A (H1N1) occurred in the United States; few seasonal influenza viruses were detected. Influenza activity* peaked in late-October and was associated with higher pediatric mortality and higher rates of hospitalizations in children and young adults than in previous seasons. The proportion of visits to health-care providers for influenza-like illness (ILI), as reported in the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet), was among the highest since ILI surveillance began in 1997 in its current form. This report summarizes influenza activity in the United States during the 2009--10 influenza season (August 30, 2009--June 12, 2010).

U.S. Viral Surveillance

Since April 2009, the beginning of the 2009 H1N1 pandemic, through June 12, 2010, approximately 740,000 influenza specimens were tested for influenza, and the number of laboratory-confirmed positives was approximately four times the average of the previous four seasons. Two peaks in percentage of specimens testing positive for influenza occurred: 43.1% in June during the initial pandemic wave, and 38.2% in October during the second wave. During August 30, 2009--June 12, 2010, the 2009--10 influenza season, World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 468,218 specimens for influenza viruses; 91,152 (19.5%) were positive (Figure 1). The proportion of specimens testing positive for influenza during the 2009--10 season exceeded 20% during the week ending August 30, 2009, peaked at 38.2% during the week ending October 24, and declined to less than 10% during the week ending December 12. Of the 91,152 positive specimens from 2009--10 season, 90,758 (99.6%) were influenza A viruses and 394 (0.4%) were influenza B viruses. Among the influenza A viruses, 67,022 (73.8%) were subtyped; 66,916 (99.8%) were 2009 pandemic H1N1, 72 (0.1%) were influenza A (H3N2), and 34 (0.1%) were seasonal influenza A (H1N1) viruses.

Antigenic Characterization

Since September 1, 2009, CDC has antigenically characterized two seasonal influenza A (H1N1), 14 influenza A (H3N2), 43 influenza B, and 1,904 of the 2009 pandemic H1N1 viruses. Of those 2009 pandemic H1N1 viruses tested, 1,895 (99.5%) were related to the A/California/07/2009 (H1N1) reference virus selected by WHO as the monovalent 2009 pandemic H1N1 vaccine virus used during the 2009--10 season, and as a component in the 2010--11 Northern Hemisphere seasonal influenza vaccine.

Both seasonal influenza A (H1N1) viruses tested were related to A/Brisbane/59/2007, the influenza A (H1N1) component of the 2009--10 Northern Hemisphere influenza vaccine. The 14 influenza A (H3N2) viruses tested showed reduced titers with antisera produced against A/Brisbane/10/2007, the 2009--10 Northern Hemisphere influenza A (H3N2) vaccine component, and were antigenically related to A/Perth/16/2009, the WHO recommended influenza A (H3N2) component of the 2010 Southern Hemisphere and 2010--11 Northern Hemisphere vaccine formulations. Of the 43 influenza B viruses from the United States tested, 38 (88.4%) belonged to the B/Victoria lineage and were related to B/Brisbane/60/2008, the influenza B vaccine component for the 2009--10 and 2010--11 Northern Hemisphere influenza vaccine. Five (11.6%) viruses tested belonged to the B/Yamagata lineage.

U.S. Novel Influenza Cases

Early identification and investigation of novel influenza A cases is critical to evaluate possible human-to-human transmission. CDC conducts surveillance for human infections with novel influenza A viruses year-round and carries out extensive epidemiologic investigations on each case. During the 2009--10 season, in addition to the pandemic strain virus infections, three cases of human infection with novel influenza A viruses were identified and then characterized at CDC. These three cases, identified in Kansas, Iowa, and Minnesota, were isolated cases of human infections with contemporary North American swine-lineage influenza A (H3N2) viruses currently circulating in swine herds. No additional human cases were linked to these three patients. Although the Minnesota patient reported visiting a live animal market in the days preceding illness onset (May 8, 2010), only the Kansas patient specifically reported contact with pigs in the week preceding symptom onset (July 28, 2009). The Iowa patient had onset of symptoms in September 2009. The Kansas and Iowa patients did not require hospitalization; the Minnesota patient was hospitalized, and recovered fully.

Resistance to Antiviral Medications

In the United States, two classes of antiviral drugs are approved by the Food and Drug Administration for use in treating or preventing influenza virus infections: neuraminidase inhibitors (oseltamivir and zanamivir) and adamantanes (amantadine and rimantidine). During the 2009--10 influenza season, testing of the 2009 pandemic H1N1 viruses found that 1.1% of 4,811 tested viruses were resistant to oseltamivir. All of the oseltamivir-resistant 2009 pandemic H1N1 viruses shared a single genetic mutation conferring oseltamivir resistance.

Since September 1, 2009, one seasonal influenza A (H1N1) virus was tested and found to be resistant to oseltamivir. No oseltamivir resistance was identified among the 19 influenza A (H3N2) or the 36 influenza B viruses tested. All tested viruses retained their sensitivity to zanamivir. Adamantane resistance continued to be high among influenza A (H3N2) viruses, with 100% of the 18 influenza A (H3N2) viruses tested resistant to the adamantanes. Adamantane resistance among seasonal influenza A (H1N1) viruses was not detected in the single virus tested. However, among 2009 pandemic H1N1 viruses tested, 1,895 (99.8%) of 1,899 were resistant to adamantanes.

open here to see the full-text (large):
Update: Influenza Activity --- United States, 2009--10 Season