Update: Influenza Activity in the United States During the 2016–17 Season and Composition of the 2017–18 Influenza Vaccine

During the 2016-17 influenza season (October 2, 2016-May 20, 2017) in the United States, influenza activity* was moderate. Activity remained low through November, increased during December, and peaked in February nationally, although there were regional differences in the timing of influenza activity. Influenza A(H3N2) viruses predominated through mid-March and were predominant overall for the season, but influenza B viruses were most commonly reported from late March through May. This report summarizes influenza activity in the United States during October 2, 2016-May 20, 2017† and updates the previous summary (1).

US Department of Health and Human Services/Centers for Disease Control and Prevention in persons aged 5-24 years; influenza B viruses accounted for 28% of the viruses reported for that age group.

Novel Influenza A Viruses
Three human infections with novel influenza A viruses were reported to CDC during the 2016-17 influenza season.The first was an infection with an influenza A(H1N2) variant (H1N2v) virus ¶ reported by Iowa public health officials during the week ending November 19, 2016 (week 46).The patient was not hospitalized and fully recovered.
The second case, a human infection with a North American lineage avian influenza A(H7N2) virus, was reported to CDC during the week ending December 24, 2016 (week 51).The patient reported close, prolonged unprotected exposure to the respiratory secretions of infected, sick cats at a New York City animal shelter.This is the first avian influenza A(H7N2) virus infection in humans identified in the United States since 2003 and the first known human infection with an influenza A virus acquired through exposure to an ill cat.The patient was mildly ill, not hospitalized, and recovered completely.
The third case, an infection with an influenza A(H3N2) variant (H3N2v) virus, was detected through the Department of Defense Global Laboratory-based Influenza Surveillance Program and reported by Texas during the week ending April 29, 2017 (week 17).The patient reported contact with swine at an agricultural event the week preceding illness onset, was not hospitalized, and fully recovered.

Antigenic and Genetic Characterization of Influenza Viruses
WHO collaborating laboratories in the United States are requested to submit a subset of influenza-positive respiratory

Antiviral Susceptibility of Influenza Viruses
CDC tested 2,569 influenza virus specimens (304 influenza A(H1N1)pdm09, 1,303 influenza A(H3N2), and 962 influenza B viruses) collected in the United States during October 1, 2016-May 20, 2017 for resistance to the influenza neuraminidase inhibitor antiviral medications oseltamivir, zanamivir, and peramivir, which are currently recommended for use against seasonal influenza.All 2,569 influenza viruses tested were found to be susceptible to all three of these antiviral medications.An additional 34 influenza A(H1N1)pdm09 viruses were tested for resistance to oseltamivir and peramivir and an additional 1,083 influenza A(H3N2) viruses were tested for resistance to oseltamivir and zanamivir; all were found to be susceptible to these antiviral medications.

Composition of the 2017-18 Influenza Vaccine
The Food and Drug Administration's Vaccines and Related Biologic Products Advisory Committee (VRBPAC) has recommended that the 2017-18 influenza trivalent vaccine to be used in the United States contain an A/Michigan/45/2015 (H1N1) pdm09-like virus, an A/Hong Kong/4801/2014 (H3N2)-like virus, and a B/Brisbane/60/2008-like (B/Victoria lineage) virus.It is recommended that quadrivalent vaccines, which have two influenza B viruses, contain the viruses recommended for the trivalent vaccines, as well as a B/Phuket/3073/2013like (B/Yamagata lineage) virus (2).This represents an update in the influenza A(H1N1) component compared with the composition of the 2016-17 influenza vaccines.The recommended Northern Hemisphere 2017-18 vaccine viruses are the same as the vaccine viruses recommended for inclusion in the 2017 Southern Hemisphere influenza vaccines.These vaccine recommendations were based on a number of factors, including global influenza virologic and epidemiologic surveillance, genetic and antigenic characterization, human serology studies, antiviral susceptibility, and the availability of candidate influenza viruses.

Outpatient Illness Surveillance
Nationally, the weekly percentage of outpatient visits for influenza-like illness § § (ILI) to heath care providers participating in the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) was at or above the national baseline ¶ ¶ level of 2.2% from the week ending December 17, 2016 (week 50) and remained at or above baseline for 17 consecutive weeks during the 2016-17 season (Figure 3).Nationally, the peak percentage of outpatient visits for ILI was 5.1% and occurred during the week ending February 11, 2017 (week 6).During the 2011-12 through 2015-16 seasons, peak weekly percentages of outpatient visits for ILI ranged from 3.6% to 6.1% and remained at or above baseline levels for an average of 13 weeks (range = 1-20 weeks).
ILINet data are used to produce a weekly jurisdiction-level measure of ILI activity,*** ranging from minimal to high.

Geographic Spread of Influenza Activity
State and territorial epidemiologists report the geographic distribution of influenza in their jurisdictions † † † through a weekly influenza activity code.§ § § The geographic distribution of influenza activity was most extensive during the week ending February 11, 2017 (week 6), when 47 jurisdictions reported widespread influenza activity.From 2011 to 2016, the peak number of jurisdictions reporting widespread influenza activity ranged from 20 during the 2011-12 season to 48 during the 2012-13 season.

Influenza-Associated Hospitalizations
CDC monitors hospitalizations associated with laboratoryconfirmed influenza infections in adults and children through the Influenza Hospitalization Surveillance Network (FluSurv-NET), ¶ ¶ ¶ which covers approximately 27 million persons (9% of the U.S. population).During October 1, 2016-April 30, 2017, a total of 18,184 laboratory-confirmed influenza-related hospitalizations were reported, with a cumulative incidence for all age groups of 65.0 per 100,000 population.The hospitalization rate was highest among persons aged ≥65 years, who accounted for approximately 60% of reported influenzaassociated hospitalizations.§ § Defined as a fever (temperature ≥100.0°F[≥37.8°C],oral or equivalent) and cough and/or sore throat, without a known cause other than influenza.¶ ¶ The national and regional baselines are the mean percentages of visits for influenza-like illness (ILI) during noninfluenza weeks for the previous three seasons plus two standard deviations.Noninfluenza weeks are defined as periods of ≥2 consecutive weeks during which each week accounted for <2% of the season's total number of specimens that tested positive for influenza.National and regional percentages of patient visits for ILI are weighted based on 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 jurisdiction 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, corresponding to ILI activity from outpatient clinics at or below the average, to high, corresponding to ILI activity from outpatient clinics 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 clearer picture of influenza activity in the United States.† † † For this surveillance component, 54 jurisdictions participate: the 50 states, the District of Columbia, Guam, Puerto Rico, and U.S. Virgin Islands.§ § § 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 two or more institutional outbreaks (ILI or laboratory-confirmed influenza) in one region of the state, with recent laboratory evidence of influenza in that region; virus activity no greater than sporadic in other regions; 4) regional: increased ILI activity or institutional outbreaks (ILI or laboratory-confirmed influenza) in two or more outbreaks, 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.The cumulative hospitalization rate during October 1, 2016-April 30, 2017, was 44.1 per 100,000 population among children aged 0-4 years, 16.7 among children and adolescents aged 5-17 years, 19.8 among adults aged 18-49 years, 65.1 among adults aged 50-64 years, and 290.5 among adults aged ≥65 years.Among all hospitalizations, 14,185 (78.0%) were associated with influenza A virus infections, 3,873 (21.2%) with influenza B virus infections, 62 (0.3%) with influenza A virus and influenza B virus co-infections, and 64 (0.4%) with an influenza virus for which the type was not determined.Among the 5,383 patients for which influenza A subtype information was available, 5,276 (98.0%) were infected with influenza A(H3N2) viruses and 107 (2.0%) were infected with influenza A(H1N1)pdm09 viruses.
Complete medical chart abstraction data were available for 7,315 (40.2%) hospitalized adults and children with laboratory-confirmed influenza as of June 9, 2017.Among 6,838 hospitalized adults with complete medical chart abstraction, 6,434 (94.1%) had at least one reported underlying medical condition that placed them at high risk**** for influenza-associated complications.The most commonly reported underlying medical conditions among adults were **** Persons at higher risk include 1) children aged <2 years; 2) adults aged ≥65 years; 3) persons with chronic pulmonary conditions (including asthma), cardiovascular disease (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 muscles, such as cerebral palsy, epilepsy [seizure disorders], stroke, intellectual disability [mental retardation], moderate to severe developmental delay, muscular dystrophy, or spinal cord injury); 4) persons with immunosuppression, including that caused by medications or by human immunodeficiency virus infection; 5) women who are pregnant or postpartum (within 2 weeks after delivery); 6) persons aged ≤18 years who are receiving long-term aspirin therapy; 7) American Indians/Alaska Natives; 8) persons with extreme obesity (i.e., body mass index ≥40); and 9) residents of nursing homes and other chronic care facilities.† † † † The seasonal baseline proportion of pneumonia and influenza (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 National Center for Health Statistics Mortality Surveillance System during the preceding 5 years.The epidemic threshold is set at 1.645 standard deviations above the seasonal baseline.cardiovascular disease (51.8%), metabolic disorders (44.4%), obesity (34.8%), and chronic lung disease (30.1%).Among 477 hospitalized children with complete medical chart abstraction, 269 (56.4%) had at least one underlying medical condition; the most commonly reported of these were asthma (26.4%) and neurologic disorder (23.2%).Among 447 women of childbearing age (15-44 years) hospitalized with laboratoryconfirmed influenza infections, 119 (26.6%) were pregnant.

Pneumonia and Influenza-Associated Mortality
CDC tracks pneumonia and influenza (P&I)-attributed deaths through the National Center for Health Statistics (NCHS) Mortality Reporting System.The percentages of deaths attributed to P&I are released 2 weeks after the week of death to allow for collection of sufficient data to produce a stable P&I mortality percentage.Weekly mortality surveillance data include a combination of machine-coded and manually coded causes of death collected from death certificates.During the 2016-17 season, there was a backlog of data requiring manual coding within the NCHS mortality surveillance data.Work is underway to reduce and monitor the number of records awaiting manual coding.The percentages of deaths attributable to P&I are higher among manually coded records than the more rapidly available machine coded records and might result in initially reported P&I percentages that are lower than percentages calculated from final data.
During the 2016-17 season, based on data from NCHS, the proportion of deaths attributed to P&I was at or above the epidemic threshold † † † † for 12 consecutive weeks from the week ending December 31, 2016 through the week ending March 18, 2017 (weeks 52-11).Mortality attributed to P&I peaked twice, once at 8.2% of all deaths during the week ending January 21, 2017 (week 3) and once at 8.1% during the week ending February 25, 2017 (week 8).During the 2011-12 through 2015-16 seasons, the peak weekly percentages of deaths attributable to P&I ranged from 8.7% during the 2011-12 season to 11.1% during the 2012-13 season.

Influenza-Associated Pediatric Mortality
CDC monitors pediatric influenza deaths through the Influenza-Associated Pediatric Mortality Surveillance System.As of June 9, 2017, a total of 98 laboratory-confirmed influenzaassociated pediatric deaths occurring during the 2016-17 season had been reported to CDC from Chicago, New York City, and 28 states.Of these 98 deaths, 46 were associated with an influenza A(H3N2) virus infection, three with an influenza A(H1N1)pdm09 virus infection, 14 with an influenza A virus for which no subtyping was performed, 34 with an influenza B virus infection, and one with an influenza virus for which the type was not determined.Since influenza-associated pediatric mortality became a nationally notifiable condition in 2004, the total number of influenza-associated pediatric deaths per season has ranged from 37 to 171, excluding the 2009 pandemic, during which 358 pediatric deaths were reported to CDC during April 15, 2009-October 2, 2010.

Discussion
The 2016-17 influenza season was notable for the predominant circulation of influenza A(H3N2) viruses.Nationally, influenza activity peaked in mid-February, with influenza A(H3N2) viruses predominant early in the season through the week ending March 25, 2017 (week 12).Influenza B viruses became the predominant virus starting during week 13 (the week ending April 1, 2017) and remained the predominant virus through the end of May.The timing of peak influenza activity varied across the United States.Influenza activity peaked at least 1 month earlier (week 52 to week 2) in the western United States (regions 8, 9, and 10) than in the rest of the country.During the 2016-17 season, severity indicators (e.g., hospitalization and mortality rates) were within the range that has been observed during previous seasons when influenza A(H3N2) viruses predominated.Previous influenza A(H3N2)-predominant seasons have been associated with increased hospitalizations and deaths compared with seasons that were not influenza A(H3N2)-predominant, especially among children aged <5 years and adults aged ≥65 years (3,4).The majority of influenza viruses antigenically characterized at CDC were similar to the reference viruses representing the recommended components for the 2016-17 vaccine.A small subset of antigenically distinct influenza B/Victoria viruses was detected.No antiviral resistance to oseltamivir, zanamivir, or peramivir was identified among tested influenza viruses from the 2016-17 season.
Final vaccine effectiveness estimates of 34% (95% CI = 24%-42%) against illness caused by influenza A(H3N2) viruses and 56% (95% CI = 47%-64%) against illness caused by influenza B viruses were similar to previous seasons when recommended vaccine viruses have been well matched to (i.e.,"like") circulating viruses, including the lower effectiveness observed against well-matched A(H3N2) viruses.Evidence of reduced protection against A(H3N2) viruses, even when vaccine viruses and circulating viruses are well matched, has been observed since the 2011-12 season.In general, vaccination with inactivated influenza vaccine has offered better protection against influenza A(H1N1) and influenza B viruses (5).Even during seasons when vaccine effectiveness is reduced, vaccination can offer substantial benefit and reduce the likelihood of severe outcomes such as hospitalization and death (6,7).During the 2012-13 season with estimated vaccination effectiveness against A(H3N2)-related illness of 39% (95% CI = 29%-47%), vaccination averted an estimated 5.6 million illnesses, 2.7 million medical visits, 61,500 hospitalizations, and 1,800 deaths (8).Estimates of the number of influenza illnesses, medical visits, and hospitalizations averted by influenza vaccination during the 2016-17 season are scheduled to be published in December 2017.
Influenza antiviral medications are an important adjunct to vaccination in the treatment and prevention of influenza.Treatment with influenza antiviral medications as close to the onset of illness as possible is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for influenza complications.Antiviral treatment should not be withheld from patients who are at high risk for complications or who are severely ill with suspected influenza infection, even if rapid antigen-detection influenza diagnostic test results are negative (9).
Although summer influenza activity in the United States typically is low, influenza cases and outbreaks have occurred during summer months and clinicians should remain vigilant in considering influenza in the differential diagnosis of summer respiratory illnesses.Testing for seasonal influenza viruses and monitoring for novel influenza A virus infections should continue year round.Health care providers also are reminded to consider novel influenza virus infections in persons with influenza-like illness and swine or poultry exposure, or with severe acute respiratory infection after travel to areas where avian influenza viruses have been detected.Providers should alert the local public health department if novel influenza virus infection is suspected.Clinical laboratories using a commercially available influenza diagnostic assay that includes influenza A virus subtype determination should contact their state public health laboratory to facilitate transport and additional testing of any specimen that is positive for influenza A, but for which the subtype cannot be determined.Public health laboratories should immediately send influenza A virus specimens that they cannot subtype using standard methods to CDC and submit all specimens that are otherwise unusual as soon as possible after identification.
Influenza surveillance reports for the United States are posted online weekly (https://www.cdc.gov/flu/weekly).Additional information regarding influenza viruses, influenza surveillance, influenza vaccine, influenza antiviral medications, and novel influenza A infections in humans is available online (https:// www.cdc.gov/flu).

FIGURE 1 .
FIGURE 1. Number* and percentage of respiratory specimens testing positive for influenza reported by clinical laboratories, by influenza virus type and surveillance week -United States, October 2, 2016-May 20, 2017 †

FIGURE 3 .
FIGURE 3. Percentage of visits for influenza-like illness (ILI)* reported to CDC, by surveillance week -Outpatient Influenza-Like Illness Surveillance Network, United States, 2016-17 influenza season and selected previous influenza seasons †

FIGURE 2. Number* of respiratory specimens testing positive for influenza reported by public health laboratories, by influenza virus type, subtype/lineage, and surveillance week -United States, October 2, 2016-May 20, 2017 †
* Specimens from 121,223 (14.0%) of 865,168 persons tested positive during October 2, 2016-May 20, 2017.†As of June 9, 2017.¶ Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant influenza viruses when isolated from humans.Seasonal influenza viruses that circulate worldwide in the human population have important antigenic and genetic differences from influenza viruses circulating in swine.† As of June 9, 2017.
The number of jurisdictions experiencing elevated ILI activity peaked during the week ending February 11, 2017 (week 6) when 31 states experienced high ILI activity.Thirty-seven jurisdictions (36 states and Puerto Rico) experienced high ILI activity during at least 1 week this season.The peak number of jurisdictions experiencing high ILI activity during a single week from 2011 to 2016 has ranged from four during the 2011-12 season to 45 during the 2014-15 season.