Summary Key Points

Summary Key Points

CDC Influenza Division Key PointsFebruary 22, 2013previous week. This translates to a rate of 34.2 influenza-associated hospitalizationsper 100,000 people in the United States. Overall, hospitalization rates are leveling offfor the season, but remain high among people 65 and older, who account for morethan half of all reported hospitalizations.oHospitalization data are collected from 15 states and represent approximately9% of the total U.S. population. The number of hospitalizations reported doesnot reflect the actual total number of influenza-associated hospitalizations inthe United States.The proportion of deaths attributed to pneumonia and influenza (P&I) based on the122 Cities Mortality Reporting System decreased this week, but remains above theepidemic threshold.Fourteen influenza-related pediatric deaths were reported during the week ofFebruary 10-16, 2013. Three of the deaths were associated with influenza A (H3)viruses, and two were associated with influenza A viruses for which the subtype wasnot determined. Six deaths were associated with influenza B viruses, and one deathwas associated with both and influenza A and influenza B virus. Two deaths wereassociated with 2009 H1N1 viruses. These are the first reports of pediatric deathsassociated with 2009 H1N1 viruses this season. This brings the total number ofinfluenza-associated pediatric deaths reported to CDC for 2012-2013 to 78.Additional information regarding pediatric deaths is available through FluViewInteractive.Nationally, the percentage of respiratory specimens testing positive for influenza inthe United States during the week of February 10-16, 2013 continued to decrease.Influenza A (H3N2), 2009 influenza A (H1N1), and influenza B viruses have all beenidentified in the U.S. this season. During the week of February 10-16, 752 of the1,371 influenza-positive tests reported to CDC were influenza A and 619 wereinfluenza B viruses. Of the 360 influenza A viruses that were subtyped,approximately 91% were H3 viruses and 9% were 2009 H1N1 viruses.Since October 1, 2012, CDC has antigenically characterized 1,185 influenza viruses,including 86 2009 influenza A (H1N1) viruses, 744 influenza A (H3N2) viruses and355 influenza B viruses.oooAll 86 of the 2009 influenza A (H1N1) viruses were characterized asA/California/7/2009-like. This is the influenza A (H1N1) component of theNorthern Hemisphere vaccine for the 2012-2013 season.Of the 744 influenza A (H3N2) viruses, 740 (99.5%) were characterized asA/Victoria/361/2011-like. This is the influenza A (H3N2) component of theNorthern Hemisphere influenza vaccine for the 2012-2013 season.Of the 355 influenza B viruses characterized, 70.7% belonged to theB/Yamagata lineage of viruses, and were characterized asB/Wisconsin/1/2010-like, the influenza B component for the 2012-2013Northern Hemisphere influenza vaccine. The remaining 29.3% of the testedinfluenza B viruses belonged to the B/Victoria lineage of viruses.3

CDC Influenza Division Key PointsFebruary 22, 2013 Since October 1, 2012, CDC has tested 274 2009 influenza A (H1N1), 1,193influenza A (H3N2), and 419 influenza B virus samples for resistance toneuraminidase inhibitors. While the majority of the tested virus samples showedsusceptibility to the antiviral drugs oseltamivir and zanamivir, two 2009 H1N1viruses (reported during week 3 and week 6) showed a mutation indicative ofresistance to oseltamivir. High levels of resistance to the adamantanes (amantadineand rimantadine) persist among 2009 influenza A (H1N1) and A (H3N2) viruses.Adamantanes are not effective against influenza B viruses.FluView is available – and past issues are archived – on the CDC website.Note: Delays in reporting may mean that data changes over time. The most up to datedata for all weeks during the 2012-2013 season can be found on the current FluViewwebpage at Vaccine EffectivenessHow well the flu vaccine works can vary by year, virus type/subtype, age, season,host immunity, and the outcome being measured. Recent studies show vaccine can reduce the risk of flu illness by about 60% amongthe overall population during seasons when most characterized circulating influenzaviruses are like the viruses included in the vaccine. On January 11, 2013, CDC published interim early estimates of the 2012-2013influenza vaccine’s effectiveness at preventing medical visits due to laboratoryconfirmedinfluenza. CDC reported an overall VE of 62% with a 95% confidence interval of 51% to 71%.These estimates were adjusted for site, but were not adjusted for age or otherpotential confounders.At that time, CDC committed to publish later updated estimates that were adjustedfor other factors, including age.On February 21, 2013, CDC published these updated and adjusted estimates in theMorbidity and Mortality Weekly Report (MMWR) entitled: “Interim Adjusted Estimatesof Seasonal Influenza Vaccine Effectiveness—United Sates, February 2013”. (Thereport is available at Overall, the VE estimate was 56% (95% confidence interval [CI]: 47% to 63%),after adjustment for age, study site, race/ethnicity, self-rated health, and days fromillness onset to enrollment.This finding is very similar to the earlier interim estimate and falls well within the95% confidence interval for the original estimate.The one exception to this was the VE among people 65 and older against influenza A(H3N2) viruses.The adjusted VE against outpatient medical visits due to laboratory-confirmedinfluenza A (H3N2) in adults aged 65 and older was 9% (95% CI:-84% to 55%).4

CDC Influenza Division Key PointsFebruary 22, 2013One possible explanation for these findings is that some older adults did not mountan effective immune response to the A (H3N2) component of the 2012-2013influenza vaccine.In general, VE estimates for adults aged 65 and older have been lower than VEestimates in younger adults, but estimates have varied between years, studies andby virus types.While some studies have failed to demonstrate a statistically significant benefit ofvaccination in people 65 and older in some years or against specific viruses, otherstudies have demonstrated a significant benefit.Most current studies of effectiveness haven’t had enough elderly persons enrolled toderive precise estimates.Some older people, including those with certain chronic illnesses, are at greater riskof serious flu complications, but also may respond less well to vaccination.Vaccine effectiveness (VE) in people 65 and older appears to be low this seasonagainst influenza A (H3N2) viruses.However, during other seasons, other studies using the same methodology as thestudies above have measured significant benefits among people 65 and older interms of preventing infection.In addition, there is significant evidence to support the benefits of vaccinating peoplein this – and other – age groups each year, including evidence that vaccination canavert more serious outcomes like hospitalizations and deaths.Based on the substantial burden of influenza in the United States, and on the factthat many studies point to some vaccination benefits, CDC concludes that annualinfluenza vaccination remains the first and most important step in protecting againstinfluenza and its complications.While improved influenza vaccines are needed, the current vaccine can still offersignificant protection and important public health benefits.Recent study results do highlight, however, that some people who got vaccinated willget influenza; therefore, antiviral medications should be used as recommended fortreatment in patients, regardless of vaccination status. (See section on CDC AntiviralTreatment Recommendations.)Influenza‐Related Pediatric Deaths• Fourteen pediatric deaths were reported during the week of February 10-16.• A total of 78 influenza-associated pediatric deaths have been reported during the2012-2013 season from Chicago [1], New York City [3] and 30 states (Arkansas [2],Arizona [2], California [1], Colorado [5], Florida [6], Hawaii [1], Illinois [1], Indiana[3], Kansas [2], Louisiana [1], Maine [1], Maryland [1], Massachusetts [1], Michigan[5], Minnesota [3], Mississippi [1], Nebraska [1], New Hampshire [1], New Jersey[4], New Mexico [2], New York [5], Ohio [4], Pennsylvania [1], South Carolina [1],South Dakota [1], Tennessee [1], Texas [13], Utah [1], Washington [1], andWisconsin [2]). A pediatric death is a death in a person younger than 18 who diedfrom an illness related to infection with an influenza virus.5

CDC Influenza Division Key PointsFebruary 22, 2013• Last season, 34 influenza-associated pediatric deaths were reported to CDC. Thiswas the lowest number of pediatric deaths reported to CDC since the 2005-2006influenza season, during which 46 pediatric deaths were reported.• Since 2004, when pediatric deaths associated with influenza infection became anationally notifiable condition, the number of deaths reported to CDC each year hasranged from 34 (2011-2012 season) to 282 deaths (2009-2010 season). (During the2009 H1N1 pandemic—April 15, 2009 to October 2, 2010—348 pediatric deaths werereported to CDC.)• These deaths are a somber reminder of the danger flu poses to children.• In the past, about 80 percent of reported flu-related pediatric deaths have been inchildren who were not vaccinated against influenza. Preliminary data for 2012-2013are consistent with this.• The single best way to protect against seasonal flu and its potential severeconsequences is to have children receive a seasonal flu vaccine each year.• Vaccination is especially important for children younger than 5 years of age andchildren of any age with an underlying medical condition like asthma, a neurologicaland neurodevelopmental disease, or immune suppression. These children are athigher risk of serious complications if they get the flu.• Information about the pediatric deaths, including basic demographics, underlyingconditions and time and place of death, is collected through the Influenza-AssociatedPediatric Mortality Surveillance System. Information for the 2012-2013 season isnow available through the Influenza Associated Pediatric Mortality application ofFluView Interactive at• Yearly vaccination also is especially important for people in contact with high riskchildren in order to protect the child (or children) from the flu.• Even previously healthy children can become seriously ill if they get the flu. Thelatest laboratory-confirmed influenza hospitalization data reported in this week’sFluView indicate that approximately 44% of children hospitalized with the flu had noidentified underlying medical conditions.• Flu-related deaths in children younger than 18 years old should be reported throughthe Influenza-Associated Pediatric Mortality Surveillance System. The number of fluassociateddeaths among children reported during the 2012-2013 flu season will beupdated each week and can be found at Antiviral Treatment Recommendations• Clinical benefit is greatest when antiviral treatment is administered early. Whenindicated, antiviral treatment should be started as soon as possible after illnessonset, ideally within 48 hours of symptom onset. However, antiviral treatment mightstill be beneficial in patients with severe, complicated or progressive illness and inhospitalized patients when started after 48 hours of illness onset, as indicated byobservational studies.6

CDC Influenza Division Key PointsFebruary 22, 2013• Antiviral treatment is recommended as early as possible for any patient withconfirmed or suspected influenza whoooois hospitalized;has severe, complicated, or progressive illness; oris at higher risk for influenza complications.• Treatment of persons with suspected influenza should not wait for laboratoryconfirmation of influenza. While influenza vaccination is the first and best way toprevent influenza, a history of influenza vaccination does not rule out the possibilityof influenza virus infection in an ill patient with clinical signs and symptomscompatible with influenza.• Antiviral treatment also can be considered for any previously healthy, symptomaticoutpatient not at high risk with confirmed or suspected influenza on the basis ofclinical judgment, if treatment can be initiated within 48 hours of illness onset.• More information is available at

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