Update: Influenza Activity --- United States, October 3, 2010--February 5, 2011

This report summarizes U.S. influenza activity* since the beginning of the 2010--11 influenza season (October 3, 2010) and updates the previous report (1). From October through early December 2010, influenza activity remained low in most regions of the United States. Activity increased beginning in mid-December 2010 and continued to increase during January and early February 2011. Influenza B, 2009 influenza A (H1N1), and influenza A (H3N2) viruses all have been identified thus far this influenza season, and most viruses in circulation are antigenically similar to strains included in the 2010--11 vaccine.

Viral Surveillance

During October 3, 2010--February 5, 2011, approximately 140 World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 116,255 respiratory specimens for influenza viruses; 22,641 (19.5%) were positive (Figure 1). Of these, 16,496 (73%) were influenza A viruses, and 6,145 (27%) were influenza B viruses. A total of 11,094 (67%) of the influenza A viruses were subtyped; 7,845 (71%) were influenza A (H3) viruses, and 3,249 (29%) were 2009 influenza A (H1) viruses.

Influenza virus--positive test results have been reported from all 50 states and the District of Columbia. The percentage of specimens testing positive for influenza first exceeded 10% during the week ending November 27, 2010, increased through the week ending January 29, 2011, when 34% of specimens tested positive, and decreased slightly in the week ending February 5, 2011, when 32% of specimens tested positive.

Although influenza A (H3N2) viruses have predominated this season, 2009 influenza A (H1N1) and B viruses also have circulated widely. The relative proportion of each type or subtype has varied by date and U.S. Department of Health and Human Services region.† From early November through mid-December, influenza B viruses accounted for 40%--49% of influenza viruses reported in the United States, with the largest numbers reported from Region 4, the southeastern states. Influenza B viruses were predominant in Region 4 through the end of December. During November and December, influenza A viruses predominated in all other regions and have predominated in all regions during January and early February. More than 80% of subtyped influenza A viruses from November and December were A (H3N2). However, the proportion of 2009 influenza A (H1N1) viruses began to increase during January and accounted for 50% of all subtyped influenza A viruses for the week ending February 5, 2011.

Outpatient Illness Surveillance

Since October 3, 2010, the weekly percentage of outpatient visits for influenza-like illness (ILI)§ reported by approximately 1,700 U.S. Outpatient ILI Surveillance Network (ILINet) providers in 50 states, New York City, Chicago, and the District of Columbia that comprise ILINet has ranged from 1.1% to 4.6%. Since December 19, 2010, the percentage has exceeded the national baseline of 2.5% (Figure 2). On a regional level,¶ the percentage of outpatient visits for ILI ranged from 1.8% to 7.3% during the week ending February 5, 2011. Nine of the 10 regions (Regions 1--8 and 10) reported ILI above region-specific baseline levels. Data collected in ILINet are used to produce a measure of ILI activity** by state. During the week ending February 5, 2011, 19 states (Alabama, Georgia, Illinois, Indiana, Kansas, Louisiana, Maryland, Mississippi, Missouri, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, South Carolina, Tennessee, Texas, and Virginia) experienced high ILI activity, nine states experienced moderate ILI activity, New York City and 10 states experienced low ILI activity, 12 states experienced minimal ILI activity, and data from the District of Columbia were insufficient to calculate an ILI activity level.

State-Specific Levels of Influenza Activity

For the week ending February 5, 2011, the level of influenza activity†† was reported as widespread by 37 states and regional in nine states. The District of Columbia reported local activity, and four states, as well as Puerto Rico, Guam, and the U.S. Virgin Islands, reported sporadic activity.

Widespread influenza activity was first reported in Georgia during the week ending December 18; an additional 13 states reported regional spread of influenza activity for that week. By the week ending January 22, widespread influenza activity had been reported by at least one state in each region.

Influenza-Associated Hospitalizations

CDC monitors hospitalizations associated with laboratory-confirmed influenza infections using the FluSurv-NET surveillance system. FluSurv-NET§§ is a population-based surveillance network that was created during the 2009--10 influenza season, when surveillance in six states was added to ongoing surveillance for influenza-associated hospitalizations in the 10 Emerging Infections Program (EIP) states. Based on EIP surveillance data, the cumulative hospitalization rate (per 100,000 population) for October 3, 2010--February 5, 2011, was 14.5 among children aged 0--4 years, 2.5 among children aged 5--17 years, 3.5 among adults aged 18--49 years, 6.3 among adults aged 50--64 years, and 18.8 among adults aged ≥65 years. The cumulative incidence for all age groups since October 3, 2010, was 6.3 per 100,000. Based on FluSurv-NET data, the cumulative hospitalization rate (per 100,000) for October 3, 2010--February 5, 2011, was 18.5 among children aged 0--4 years, 3.2 among children aged 5--17 years, 4.2 among adults aged 18--49 years, 7.5 among adults aged 50--64 years, and 21.3 among adults aged ≥65 years. The cumulative incidence for all age groups since October 3, 2010, was 7.6 per 100,000 (Figure 3).

As of February 5, 2011, among the 628 FluSurv-NET adult patients for whom medical chart data were available for analysis, the most frequent underlying conditions were metabolic disorders (32%), cardiovascular disease (30%), and asthma or reactive airway disease (19%). Among 226 children hospitalized with laboratory-confirmed influenza, 47% did not have any underlying conditions, and 20% had underlying asthma or reactive airway disease.

Pneumonia and Influenza-Related Mortality

For the week ending February 5, 2011, pneumonia and influenza (P&I) was reported as an underlying or contributing cause of death for 8.0% of all deaths reported to the 122 Cities Mortality Reporting System. This percentage is at the epidemic threshold of 7.97% for that week.¶¶ Since October 3, 2010, the weekly percentage of deaths attributed to P&I ranged from 6.0% to 8.4%, and first exceeded the epidemic threshold during the week ending January 29, 2011 (Figure 4). Peak weekly percentages of deaths attributed to P&I previously were as follows: 8.2 for the week ending January 23, 2010, during the 2009--10 season; 7.9 for the week ending April 11, 2009, during the 2008--09 season; 9.1% for the week ending March 15, 2008, during the 2007--08 season; and 7.7% for the week ending February 24, 2007, during the 2006--07 season.

Influenza-Related Pediatric Mortality

As of February 5, 2011, a total of 30 influenza-related pediatric deaths from 18 states (Arizona, Colorado, Florida, Georgia, Illinois, Indiana, Louisiana, Michigan, New Jersey, New York, North Carolina, North Dakota, Oklahoma, Pennsylvania, Texas, Utah, Virginia, and West Virginia) and New York City have been reported to CDC for the 2010--11 season. Nine deaths were associated with influenza A (H3N2) virus infection, 12 deaths were associated with influenza B virus infection, three deaths were associated with influenza A (H1N1), and six were associated with an influenza A virus for which the subtype was not determined. Twenty of these deaths occurred during January 16--February 5, 2011. During the 2009 pandemic, 329 pediatric deaths were reported during April 15, 2009--January 23, 2010. Before the pandemic, 65 influenza-related pediatric deaths were reported for the 2008--09 season (through the week ending April 11, 2009), 88 pediatric deaths were reported for the 2007--08 season, and 77 pediatric deaths were reported for the 2006--07 season.

Antigenic Characterization

WHO collaborating laboratories in the United States are requested to submit a subset of their influenza-positive respiratory specimens to CDC for further antigenic characterization. Since October 1, 2010, CDC has antigenically characterized 564 influenza viruses submitted by U.S. laboratories: 82 were 2009 influenza A (H1N1), 300 influenza A (H3N2), and 182 influenza B viruses. All 82 of the 2009 influenza A (H1N1) viruses were characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2010--11 influenza vaccine. Of 300 influenza A (H3N2) viruses, 298 (99%) were characterized as A/Perth/16/2009-like, the influenza A (H3N2) component of the 2010--11 influenza vaccine. Two viruses (1%) of the 300 tested showed reduced titers with antiserum produced against A/Perth/16/2009. Of the 182 influenza B viruses tested, 170 (93%) belong to the B/Victoria lineage of viruses: 169 (99.4%) were characterized as B/Brisbane/60/2008-like, the recommended influenza B component for the 2010--11 influenza vaccine, and one (0.6%) showed reduced titers with antisera produced against B/Brisbane/60/2008. Twelve (7.0%) of the 182 influenza B viruses were identified as belonging to the B/Yamagata lineage of viruses.

Novel Influenza A Viruses

Four cases of human infection with a novel influenza A virus have been reported this influenza season. Three cases were reported during November and December 2010 and are described in a previous update (1). On January 25, 2011, a fourth case of human infection with swine origin influenza A (H3N2) was identified in a female child in Pennsylvania. She developed symptoms of fever, headache, and lethargy on September 6, 2010. She did not require hospitalization and has since fully recovered. The patient reported contact with swine in the week preceding symptom onset.

Antiviral Resistance of Influenza Virus Isolates

Since October 1, 2010, a total of 364 influenza virus isolates have been tested for antiviral resistance. Of the 158 influenza A (H3N2) and 119 influenza B viruses tested, 100% were sensitive to both oseltamivir and zanamivir. Among the 2009 influenza A (H1N1) viruses, the 87 tested for resistance to oseltamivir were 100% sensitive, and the 33 tested for resistance to zanamavir were 100% sensitive. High levels of resistance to the adamantanes (amantadine and rimantadine) persist among 2009 influenza A (H1N1) and A (H3N2) viruses currently circulating.

Reported by

WHO Collaborating Center for the Surveillance, Epidemiology, and Control of Influenza. L Brammer MPH, S Epperson, MPH, M Jhung, MD, K Kniss, MPH, D Mustaquim, MPH, A Bishop, MPH, R Dhara, MPH, T Wallis, MS, L Finelli, DrPH, L Gubareva, PhD, J Bresee, MD, A Klimov, PhD, N Cox, PhD, Influenza Div, National Center for Immunization and Respiratory Diseases; S Garg, MD, EIS Officer, CDC.

Editorial Note

Influenza activity, as measured across all CDC influenza surveillance systems in the United States, began to increase in mid-December and continued to increase through the week ending February 5, 2011. Although the timing of peak activity is not predictable, peak activity in the United States most commonly occurs in February; however, substantial activity can occur as late as May (2). Vaccination remains the most effective method to prevent influenza and its complications. Health-care providers should continue to offer vaccine to all unvaccinated persons aged ≥6 months throughout the influenza season.

Influenza A (H3N2), 2009 A (H1N1), and B viruses have cocirculated this influenza season, with the predominant influenza virus varying over time and by region. Influenza A (H3N2) has been the predominant influenza virus in circulation in all regions except Region 4, where influenza B predominated early in the season. Although a small number of 2009 influenza A (H1N1) viruses were found to be circulating early in the season, the proportion of influenza A viruses that are 2009 influenza A (H1N1) has increased over the past few weeks in several regions. Thus far this season, all of the 2009 influenza A (H1N1) viruses and the majority of influenza A (H3N2) and B viruses in circulation that were tested are closely related to components included in the 2010--11 influenza vaccine.

According to 2010 recommendations of the Advisory Committee on Immunization Practices (ACIP), health-care providers should offer influenza vaccination to all persons aged ≥6 months throughout the influenza season (2). All children aged 6 months--8 years who receive a seasonal influenza vaccine for the first time should receive 2 doses. Children who received only 1 dose of a seasonal influenza vaccine in the first influenza season that they were vaccinated should receive 2 doses in the following influenza season. In addition, for the 2010--11 influenza season, children aged 6 months--8 years who did not receive at least 1 dose of an influenza A (H1N1) 2009 monovalent vaccine should receive 2 doses of a 2010--11 seasonal influenza vaccine, regardless of previous vaccination history (2).

Higher overall and age-specific rates of hospitalization often are observed during influenza A (H3N2)--predominant seasons (3). Based on FluSurv-NET surveillance data thus far, rates of hospitalization among patients with laboratory-confirmed influenza are increasing. Rates of influenza-associated hospitalization are highest in children aged 0--4 years and adults aged ≥65 years. This trend is similar to that seen in 2007--08, the last season in which influenza A (H3N2) was predominant. In influenza seasons before the 2009 pandemic, cumulative end-of-season hospitalization rates per 100,000 persons obtained from EIP surveillance data ranged from 7.7 in 2008--09 to 18.1 in 2007--08.

Since the beginning of this season, 30 influenza-related pediatric deaths have been reported. More than half of the pediatric deaths this season have occurred since January 16, 2011. Health-care providers are asked to notify their local or state health department as soon as possible when deaths associated with laboratory-confirmed influenza occur among children.

Antiviral medications continue to be an important adjunct to vaccination for reducing the health impact of influenza. On January 21, 2011, new ACIP recommendations on use of antiviral agents for treatment and chemoprophylaxis of influenza were released (4). Antiviral treatment is recommended as soon as possible for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization, or who are at higher risk for influenza complications (4--7). Antiviral treatment also may be considered for outpatients with confirmed or suspected influenza who do not have known risk factors for severe illness if treatment can be initiated within 48 hours of illness onset. Recommended antiviral medications include oseltamivir and zanamivir; recent viral surveillance and resistance data indicate that >99% of currently circulating influenza virus strains are sensitive to these medications. Amantadine and rimantadine should not be used because of the high levels of resistance to these drugs among circulating influenza A viruses (4).

Influenza surveillance reports for the United States are posted online weekly during October--May and are available at http://www.cdc.gov/flu/weekly/fluactivity.htm. Additional information regarding influenza viruses, influenza surveillance, influenza vaccine, influenza antiviral medications, and novel influenza A infections in humans is available at http://www.cdc.gov/flu.

Acknowledgments

This report is based, in part, on data contributed by participating state and territorial health departments and state public health laboratories, World Health Organization collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, the U.S. Outpatient ILI Surveillance Network, the Aggregate Hospitalization and Death Reporting Activity, the Influenza Associated Pediatric Mortality Surveillance System, and the 122 Cities Mortality Reporting System.

References

1. CDC. Update: influenza activity---United States, October 3, 2010--December 11, 2011.MMWR 2010;59:1651--5.
2. CDC. Prevention and control of influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2010;59(No. RR-8).
3. Dao CN, Kamimoto L, Nowell M, et al. Adult hospitalizations for laboratory-positive influenza during the 2005--2006 through 2007--2008 seasons in the United States. J Infect Dis 2010;202:881--8.
4. CDC. Antiviral agents for the treatment and chemoprophylaxis of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2011;60(No. RR-1):1--24.
5. CDC. Deaths related to 2009 pandemic influenza A (H1N1) among American Indian/Alaska Natives---12 states, 2009. MMWR 2009;58:1341--4.
6. Jain S, Kamimoto L, Bramley AM, et al. Hospitalized patients with 2009 H1N1 influenza in the United States, April--June 2009. N Engl J Med 2009;361:1935--44.
7. Morgan OW, Bramley A, Fowlkes A, et al. Morbid obesity as a risk factor for hospitalization and death due to 2009 pandemic influenza A(H1N1) disease. PLoS ONE 2010; 5:e9694.