Update: Influenza Activity—United States, 2011-12 Season and Composition of the 2012-13 Influenza Vaccine FREE

MMWR. 2012;22:414-420.

3 figures omitted.

During the 2011-12 influenza season in the United States, influenza activity* occurred at low levels during October through December and increased in January and February before peaking in mid-March. Influenza A (H3N2) viruses predominated overall, but influenza A (H1N1)pdm09 (pH1N1) and influenza B viruses also circulated widely. This influenza season was mild compared with recent years, with a lower percentage of outpatient visits for influenza-like illness (ILI),† lower rates of hospitalizations, and fewer deaths attributed to pneumonia and influenza. This report summarizes influenza activity in the United States during the 2011-12 influenza season (October 2, 2011-May 19, 2012) and reports the recommendations for the components of the 2012-13 Northern Hemisphere influenza vaccine.

During October 2, 2011-May 19, 2012, World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 169,453 specimens for influenza viruses; 22,417 (13%) were positive (Figure). Of the positive specimens, 19,285 (86%) were influenza A viruses, and 3,132 (14%) were influenza B viruses. Among the influenza A viruses, 14,968 (78%) were subtyped; 11,002 (74%) were influenza A (H3N2) viruses, and 3,966 (26%) were pH1N1 viruses.

The proportion of specimens testing positive for influenza during the 2011-12 season first exceeded 10% (indicating higher levels of viral circulation) during the week ending February 4, 2012, and peaked at 32% during the week ending March 17, 2012.

Although influenza A (H3N2) viruses predominated, pH1N1 and influenza B viruses also circulated widely, and the relative proportion of each type and subtype varied by geographic region and week. From October through December 2011, fewer than 5% of specimens tested for influenza were positive. Of those that were positive, 81% were influenza A and 19% were influenza B viruses. As activity increased in January 2012, the proportion of influenza A viruses increased, accounting for 88%-95% of viruses reported each week from January 1 to March 17. Although pH1N1 viruses accounted for only 4% of influenza A viruses reported from October through December, that proportion increased to 22% from January through mid-March. The largest number of both influenza A (H3N2) and pH1N1 viruses were reported for the week ending March 17. As influenza A activity declined, the number of influenza B viruses increased, with the largest number of influenza B viruses reported for the week ending April 21.

Regional§ differences were observed in the timing of influenza activity and the relative proportions of circulating viruses. Using the percentage of specimens testing positive for influenza to determine the peak of influenza activity, activity peaked in regions 2, 3, 4, 5, 6, 7, 8, and 9 during March 4-24 (weeks 10-12), but peak activity was not observed in regions 1 and 10 until the weeks ending April 21 (week 16) and March 31 (week 13), respectively. The highest proportion of influenza B viruses was observed in region 10 (40%). The proportion of influenza B viruses in the other regions ranged from 3% in regions 7 and 8 to 21% in region 2. Among influenza A viruses, regions 5 and 7 were strongly influenza A (H3N2) predominant, with A (H3N2) accounting for 93% and 90%, respectively, of subtyped influenza A viruses. In contrast, pH1N1 viruses accounted for 42% of subtyped influenza A viruses in region 2, 60% in region 6, and 41% in region 8.

Thirteen cases of human infection with a novel swine-origin influenza A (H3N2) variant (H3N2v) virus have been reported since August 2011.¹ These H3N2v viruses had the M gene from the pH1N1 virus. The thirteen cases were identified in six states: Indiana (two cases), Iowa (three), Maine (two), Pennsylvania (three), Utah (one), and West Virginia (two). One of the 13 cases occurred in an adult, and 12 occurred in children. Three cases resulted in hospitalization; all three patients have recovered fully from their illness. Six of the 13 cases were in persons who reported no recent exposure to swine. In addition, two other novel viruses were identified during the 2011-12 season: one case of influenza A (H1N2) variant (H1N2v) was identified in Minnesota, and one case of influenza A (H1N1) variant (H1N1v) was identified in Wisconsin. One case was in a person who reported close contact with swine preceding symptom onset; both patients are fully recovered.

Since October 1, 2011, CDC has antigenically characterized 1,887 influenza viruses submitted by U.S. laboratories including 527 pH1N1 viruses, 1,058 influenza A (H3N2) viruses, and 302 influenza B viruses. Of the 527 pH1N1 viruses tested, 503 (95%) were characterized as A/California/7/2009-like, the pH1N1 component of the 2011-12 influenza vaccine. Twenty-four viruses (5%) of the 527 tested showed reduced titers with antiserum produced against A/California/7 /2009. Of the 1,058 influenza A (H3N2) viruses, 864 (82%) were characterized as A/Perth/16/2009-like, the influenza A (H3N2) component of the 2011-12 influenza vaccine for the Northern Hemisphere. A total of 194 (18%) of the 1,058 tested showed reduced titers with antiserum produced against A/Perth/16/2009.

Of the 302 influenza B viruses tested, 147 (49%) belonged to the B/Victoria lineage, and 139 (95%) of these were characterized as B/Brisbane/60/2008-like, the influenza B component for the 2011-12 Northern Hemisphere influenza vaccine. Eight (5%) of the 147 viruses belonging to the B/Victoria lineage showed reduced titers with antisera produced against B/Brisbane/60/2008. A total of 155 (51%) viruses tested belonged to the B/Yamagata lineage.

Since October 1, 2011, a total of 2,756 influenza virus specimens have been tested for antiviral resistance. All 317 influenza B viruses tested were sensitive to both oseltamivir and zanamivir. Among 1,275 influenza A (H3N2) viruses tested, no resistance to oseltamivir or zanamivir was detected. Among the 1,164 pH1N1 viruses tested for resistance to oseltamivir, 16 (1.4%) were found to be resistant, and of the 518 viruses tested for resistance to zanamivir, all were found to be sensitive.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among pH1N1 and influenza A (H3N2) viruses currently circulating globally.

The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee recommended that the 2012-13 trivalent influenza vaccine for the United States contain A/California/7/2009-like (pH1N1), A/Victoria/361/2011-like (H3N2), and B/Wisconsin/1/2010-like (B/Yamagata lineage). This represents a change in the influenza A (H3N2) and influenza B components from the 2011-12 Northern Hemisphere influenza vaccine formulation. This recommendation was based on global influenza virus surveillance data related to epidemiology and antigenic characteristics, serologic responses to 2011-12 trivalent seasonal vaccines, and the availability of candidate strains and reagents.

Nationally, the weekly percentage of outpatient visits for ILI to health-care providers participating in the U.S. Outpatient Influenza-Like Illness Surveillance Network (ILINet) met, but did not exceed, the national baseline level¶ of 2.4% for 1 week (the week ending March 17, 2012) during the 2011-12 influenza season. This was the only season since ILINet began operating in its current configuration (i.e., since the 1997-98 season) that the percentage of outpatient visits for ILI did not exceed the baseline. For comparison, during the 2008-09 influenza season (the season preceding the 2009 pandemic), the peak percentage of outpatient visits for ILI was 3.6% and occurred in mid-February; during the 2009 pandemic, the peak percentage of outpatient visits for ILI was 7.7% and occurred in late October.² The peak percentage of outpatient visits for ILI during the most recent influenza season (2010-11) was 4.5% and occurred in early February. During the 2011-12 season, on a regional level, the percentage of visits for ILI failed to meet or exceed region-specific baselines in regions 1, 2, 3, 6, and 9. The percentage of outpatient visits for ILI met or exceeded its baseline levels for a single week in regions 4 and 8, for 3 weeks in region 10, 6 weeks in region 5, and 7 weeks in region 7. ILINet data are used to produce a weekly state-level measure of ILI activity** varying from minimal to high: the number of states experiencing high ILI activity peaked during the week ending March 17 (week 11) with four states.

State and territorial epidemiologists report the geographic distribution of influenza in their states through a weekly influenza activity code.†† The geographic distribution of influenza activity was most extensive during the week ending March 17, 2012 (week 11), when 20 states reported widespread influenza activity and 20 states reported regional influenza activity. During the week ending May 19, one state was still reporting widespread influenza activity. The number of states reporting widespread or regional activity during the peak week of activity has ranged from 49 to 50 states during the previous three influenza seasons (CDC, unpublished data, 2012).

CDC monitors hospitalizations associated with laboratory-confirmed influenza infections using the FluSurv-NET§§ surveillance system. Cumulative hospitalization rates (per 100,000 population) were calculated by age group based on 2,356 total hospitalizations during October 2, 2011-April 28, 2012, of which 274 occurred among persons aged 0-4 years, 195 among persons aged 5-17 years, 526 among persons aged 18-49 years, 423 among persons aged 50-64 years, and 938 among persons aged ≥65 years. The cumulative hospitalization rate (per 100,000 population) for this period was 14.2 among children aged 0-4 years, 4.2 among children aged 5-17 years, 4.1 among adults aged 18-49 years, 8.5 among adults aged 50-64 years, and 30.4 among adults aged ≥65 years. The cumulative incidence for all age groups since October 2, 2011, was 8.6 per 100,000. During the past three influenza seasons, age-specific hospitalization rates have ranged from 35.5 to 72.8 per 100,000 population for ages 0-4 years, 6.4 to 27.3 for ages 5-17 years, 3.6 to 23.1 for ages 18-49 years, 5.1 to 30.8 for ages 50-64 years, and 13.5 to 65.9 for ages ≥65 years.

As of May 19, 2012, among the 1,237 (66%) of 1,887 FluSurv-NET adult patients for whom medical chart data were available for analysis, the most frequent underlying conditions were chronic lung disease (42%), cardiovascular disease (37%), and metabolic disorders (34%). Five percent of adult patients hospitalized with influenza were pregnant. Among 333 children hospitalized with laboratory-confirmed influenza, 47% did not have any known underlying conditions, and 19% had underlying asthma or reactive airway disease.

During the 2011-12 influenza season, the percentage of deaths attributed to pneumonia and influenza (P&I) exceeded the epidemic threshold¶¶ for 1 week, during the week ending January 21, 2012 (week 3) and peaked at 7.9%. From the 2008-09 season through the 2010-11 season, the peak percentage of P&I deaths ranged from 7.9% to 9.1%, and the total number of consecutive weeks at or above the epidemic threshold ranged from 3 to 13 (CDC, unpublished data, 2012).

For the 2011-12 influenza season, 26 laboratory-confirmed influenza-associated pediatric deaths were reported. These deaths were reported from 15 states: Arkansas (one case), Arizona (one), California (six), Florida (two), Hawaii (one), Missouri (one), North Carolina (two), New Jersey (one), Nevada (three), New York (one), Oklahoma (one), Texas (three), Virginia (one), Washington (one), and Wisconsin (one). Their mean and median ages were 7.3 and 6.5 years, respectively; three children were aged <6 months, six were aged 2-4 years, 12 were aged 5-11 years, and five were aged 12-17 years. Six of the 26 deaths reported were associated with influenza B viruses, five deaths were associated with influenza A (H3) viruses, seven were associated with pH1N1 viruses, seven were associated with an influenza A virus for which the subtype was not determined, and one was associated with an influenza virus with the type not determined.

For comparison, during the 2010-11 season, 122 pediatric deaths were reported. During the 2009 pandemic, 348 pediatric deaths were reported during April 15, 2009-October 2, 2010. Before the pandemic, 67 influenza-associated pediatric deaths were reported for the 2008-09 season.

World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza. Rahul Ganatra, Krista Kniss, MPH, Scott Epperson, MPH, Lenee Blanton, MPH, Desiree Mustaquim, MPH, Amber Bishop, MPH, Tiffany D’Mello, MPH, Alejandro Perez, MPH, Rosaline Dhara, MPH, Lynnette Brammer, MPH, Sandra Chaves, MD, Larisa Gubareva, MD, Teresa Wallis, MS, Xiyan Xu, MD, Joseph Bresee, MD, Alexander Klimov, PhD, Nancy Cox, PhD, Lyn Finelli, DrPH, Influenza Div, National Center for Immunization and Respiratory Diseases, CDC. Corresponding contributor: Rahul Ganatra, rganatra@cdc.gov, 404-639-3747.

The 2011-12 influenza season was one of the mildest and latest seasons on record. The peak percentage of outpatient visits for ILI (2.4%) was the lowest reported since the system began in its current format in 1997. The peak percentage of visits for ILI during those 14 seasons ranged from 3.2% for the 2002-03 season to 7.7% during the 2009 H1N1 pandemic. Hospitalization rates overall were lower than rates reported during the 2010-11 influenza season, but the relative impact by age group was similar (highest rates in the ≥65 and 0-4 year age groups); both seasons had influenza A (H3N2) viruses predominating and cocirculating with pH1N1 and influenza B viruses. The number of influenza-associated pediatric deaths reported to CDC for the 2011-12 season was the lowest reported since data collection began in the 2004-05 season (range for previous years: 46-348 pediatric deaths), and P&I mortality as reported through the 122 Cities Mortality Reporting System exceeded the epidemic threshold only slightly for a single week. Based on the percentage of specimens testing positive for influenza, the peak of influenza activity for the 2011-12 season, occurring during the week ending March 17, 2012, was the latest since the 1987-88 season, when activity peaked during the week ending March 26, 1988.

During the 2011-12 season, influenza activity peaked in mid-March, and influenza A (H3N2) viruses were most commonly reported during the season overall. The proportions of influenza viruses varied by region and week. The proportion of influenza B viruses reported was highest at the end of the season, with the majority of these viruses reported from the northwestern states. The proportion of pH1N1 viruses reported was highest mid-season, with the majority of these viruses reported from the southern states. The majority of all influenza viruses in specimens sent to CDC for further antigenic characterization were similar to the components of the 2011-12 Northern Hemisphere vaccine.

Testing for seasonal influenza and monitoring for novel influenza virus infections should continue year-round, as should specimen submission to CDC for further antigenic and genetic analysis and antiviral resistance monitoring. The detection of 13 cases of infection with H3N2v viruses and one case each of H1N1v and H1N2v viruses since August 2011 further emphasizes the importance of continuing to monitor for novel influenza A viruses. Although summer influenza activity in the United States typically is low, cases of influenza and even sporadic outbreaks commonly are detected in the United States throughout the summer. Health-care providers should remain vigilant and consider influenza as a potential cause of summer respiratory illnesses. Public health laboratories should send virus specimens to CDC that they cannot type or subtype using standard methods immediately and submit all specimens that are otherwise unusual, including all summer specimens, as soon as possible after identification.

Since 2010, CDC has recommended that everyone aged ≥6 months receive an influenza vaccine each year, preferably in the fall before the U.S. influenza season begins.³ However, during other times of the year, persons who have not received the vaccine for the current season and are traveling to parts of the world where influenza activity is ongoing should receive an influenza vaccine to protect themselves while traveling. This is particularly important for persons at high risk for influenza-related complications.*** This recommendation also applies to persons who are traveling within the temperate regions of the Southern Hemisphere or as part of large tourist groups (e.g., on cruise ships) that might include persons from other parts of the world where influenza activity is ongoing.⁴ Persons should be vaccinated at least 2 weeks before travel because it takes 2 weeks for vaccine immunity to develop after vaccination. Travelers also should be aware that all influenza vaccine manufactured for the 2011-12 season expires by June 30, 2012, after which influenza vaccines will not be available in the United States until the 2012-13 vaccine is available in the fall.

As a supplement to influenza vaccination, antiviral drugs are an important adjunct to reduce the impact of influenza. Based on recommendations of the Advisory Committee on Immunization Practices, 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-related complications.⁵ 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 the majority of currently circulating influenza viruses are sensitive to these medications. Amantadine and rimantadine should not be used because of sustained high levels of resistance to these drugs among circulating influenza A viruses.

Participating state and territorial health departments and state public health laboratories. U.S. World Health Organization collaborating laboratories. National Respiratory and Enteric Virus Surveillance System collaborating laboratories. U.S. Outpatient ILI Surveillance Network. FluSurv-NET. Influenza Associated Pediatric Mortality Surveillance System. 122 Cities Mortality Reporting System.

CDC collects, compiles, and analyzes data on influenza activity year-round in the United States. The influenza season generally begins in the fall and continues through the winter and spring months; however, the timing and severity of influenza activity varies by geographic location and season.

What is added by this report?

During the 2011-12 influenza season, influenza A (H3N2), influenza A (H1N1)pdm09, and influenza B viruses cocirculated. In addition, 15 cases of infection with novel influenza A viruses were reported. Compared with recent influenza seasons, this season had a lower percentage of outpatient visits for influenza-like illness, lower rates of hospitalizations, and fewer deaths attributed to pneumonia and influenza.

What are the implications for public health practice?

All unvaccinated persons aged ≥6 months should be offered influenza vaccine throughout the influenza season. In addition, timely empiric antiviral treatment is recommended for patients with severe, complicated, or progressive influenza illness, those at higher risk for influenza complications, or those for whom treatment can be started within 48 hours of illness onset.

*The CDC influenza surveillance system collects five categories of information from eight data sources: 1) viral surveillance (World Health Organization collaborating laboratories, the National Respiratory and Enteric Virus Surveillance System, and novel influenza A virus case reporting); 2) outpatient illness surveillance (U.S. Outpatient Influenza-Like Illness Surveillance Network); 3) mortality (122 Cities Mortality Reporting System, and influenza-associated pediatric mortality reports); 4) hospitalizations (FluSurv-NET which includes the Emerging Infections Program and surveillance in four additional states); and 5) summary of geographic spread of influenza (state and territorial epidemiologist reports).

†Defined as a temperature of ≥100.0°F (≥37.8°C), oral or equivalent, and cough or sore throat, in the absence of a known cause other than influenza.

§The 10 U.S. Department of Health and Human Services regions include the following states and territories: Region 1: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont; Region 2: New Jersey, New York, Puerto Rico, and the U.S. Virgin Islands; Region 3: Delaware, District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia; Region 4: Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee; Region 5: Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin; Region 6: Arkansas, Louisiana, New Mexico, Oklahoma, and Texas; Region 7: Iowa, Kansas, Missouri, and Nebraska; Region 8: Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming; Region 9: Arizona, California, Hawaii, Nevada, American Samoa, Commonwealth of the Northern Mariana Islands, Federated States of Micronesia, Guam, Marshall Islands, and Republic of Palau; Region 10: Alaska, Idaho, Oregon, and Washington.

¶The national and regional baselines are the mean percentage of visits for ILI during noninfluenza weeks for the previous three seasons plus two standard deviations. A noninfluenza week is a week during which fewer than 10% of specimens tested positive for influenza. National and regional percentages of patient visits for ILI are weighted on the basis of state population. Use of the national baseline for regional data is not appropriate.

**Activity levels are based on the percentage of outpatient visits in a state attributed to ILI and are compared with the average percentage of ILI visits that occur during weeks with little or no influenza virus circulation. Activity levels range from minimal, which would correspond to ILI activity from outpatient clinics being at or below the average, to high, which would correspond to ILI activity from outpatient clinics being much higher than the average. Because the clinical definition of ILI is nonspecific, not all ILI is caused by influenza; however, when combined with laboratory data, the information on ILI activity provides a useful picture of influenza activity in the United States.

††Levels of activity are 1) no activity ; 2) sporadic: isolated laboratory-confirmed influenza cases or a laboratory-confirmed outbreak in one institution, with no increase in activity; 3) local: increased ILI, or at least two institutional outbreaks (ILI or laboratory-confirmed influenza) in one region of the state, with recent laboratory evidence of influenza in that region, with virus activity no greater than sporadic in other regions; 4) regional: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least two but less than half of the regions in the state with recent laboratory evidence of influenza in those regions; and 5) widespread: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in at least half the regions in the state, with recent laboratory evidence of influenza in the state.

§§FluSurv-NET conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in children aged <18 years (since the 2003-04 influenza season) and adults aged ≥18 years (since the 2005-06 influenza season). The FluSurv-NET covers approximately 80 counties in the 10 Emerging Infections Program states (California, Colorado, Connecticut, Georgia, Maryland, Minnesota, New Mexico, New York, Oregon, and Tennessee) and additional Influenza Hospitalization Surveillance Project (IHSP) states. IHSP began during the 2009-10 season to enhance surveillance during the 2009 H1N1 pandemic. IHSP sites included Iowa, Idaho, Michigan, Oklahoma, and South Dakota during 2009-10 season; Idaho, Michigan, Ohio, Oklahoma, Rhode Island, and Utah during the 2010-11 season; and Michigan, Ohio, Rhode Island, and Utah during the 2011-12 season. Incidence rates are calculated using National Center for Health Statistics population estimates for the counties included in the surveillance catchment area. Laboratory confirmation is dependent on clinician-ordered influenza testing, and testing for influenza often is underutilized because of the poor reliability of rapid test results and greater reliance on clinical diagnosis for influenza. As a consequence, cases identified as part of influenza hospitalization surveillance likely are an underestimation of the true number of persons hospitalized with influenza.

¶¶The seasonal baseline proportion of P&I deaths is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I that were reported by the 122 Cities Mortality Reporting System during the preceding 5 years. The epidemic threshold is set at 1.645 standard deviations above the seasonal baseline.

***Children aged <5 years (especially those aged <2 years); adults aged ≥50 years; persons with chronic pulmonary (including asthma), cardiovascular (except hypertension alone), renal, hepatic, hematologic (including sickle cell disease), metabolic disorders (including diabetes mellitus) or neurologic and neurodevelopmental conditions (including disorders of the brain, spinal cord, peripheral nerves, and muscle such as cerebral palsy, epilepsy [seizure disorders], stroke, intellectual disability [mental retardation], moderate to severe developmental delay, muscular dystrophy, or spinal cord injury); persons with immunosuppression, including that caused by medications or by human immunodeficiency virus infection; women who are pregnant or postpartum (within 2 weeks after delivery); persons aged ≤18 years who are receiving long-term aspirin therapy; American Indians/Alaska Natives; persons who are morbidly obese (i.e., body mass index ≥40 kg/m²); and residents of nursing homes and other chronic-care facilities.