multiple surveillance systems are used in the united states each year to characterize seasonal influenza epidemics and to detect unusual events such as infections with novel viruses or those with pandemic potential .
these systems track a variety of outcomes , including laboratoryconfirmed influenza hospitalizations , outpatient visits for influenzalike illness ( ili ) , pneumonia and influenzacoded deaths for all ages , pediatric laboratoryconfirmed deaths , and positive laboratory samples . 
  despite the utility of these existing systems , additional data to estimate disease severity and track illness at the state level were needed during the 2009 h1n1 pandemic , as timely and representative information describing 2009 pandemic influenza a ( h1n1 ) ( ph1n1 ) activity
the centers for disease control and prevention ( cdc ) and the council of state and territorial epidemiologists ( cste ) established the aggregate hospitalization and death reporting activity ( ahdra ) as part of an overall national surveillance strategy implemented to collect timely and representative data describing ph1n1 infections in the united states .
the ahdra was designed to ( i ) track severe disease within states and territories to better characterize the focal nature of the pandemic , ( ii ) track disease trends over brief periods of time to facilitate rapid public health responses to changes in ph1n1 epidemiology , and ( iii ) accommodate variation in local resources by providing a simple , flexible method that allowed reliable reporting by all states and territories without overwhelming health departments during the pandemic response . in this report
, we describe the methods and implementation of ahdra and provide preliminary results from this new surveillance activity .
from august 30 , 2009 , through april 6 , 2010 , cdc requested weekly reporting of influenzaassociated hospitalizations and deaths from all 50 us states , the district of columbia , new york city , and six us territories .
states and territories were asked to identify hospitalizations and deaths in their jurisdictions according to either a laboratoryconfirmed or syndromic surveillance definition and could use either definition to report hospitalizations or deaths .
 polymerase chain reaction ( rtpcr ) testing , direct fluorescent antigen testing ( dfa ) , immunofluorescent antigen testing , or viral culture ; identification of influenza type or subtype was not required .
syndromic reports included cases of pneumonia and influenza based on clinical syndrome , admission or discharge data , or a combination of data elements that could include diagnostic laboratory test results .
prior to the first reporting period , 33 jurisdictions indicated they intended to submit laboratoryconfirmed hospitalizations , and 20 indicated they would submit syndromic hospitalizations .
thirtysix jurisdictions intended to submit laboratoryconfirmed death reports , and 17 indicated they would submit syndromic deaths ; information describing method of reporting was unavailable for one state and four territories .
jurisdictions were instructed to submit aggregate counts each week by age group ( 04 , 518 , 1924 , 2549 , 5064 , and 65  years ) .
aggregate counts were used to calculate agespecific weekly and cumulative rates per 100  000 according to 2008 postcensal us population estimates . laboratoryconfirmed and
syndromic data were analyzed for relative increase or decrease by state each week , and laboratoryconfirmed cumulative rates were used to describe the age distribution of ph1n1 influenzaassociated hospitalizations and deaths . owing to differences between laboratoryconfirmed and syndromic reporting definitions , we calculated two national incidence estimates of ph1n1 influenzaassociated hospitalizations and deaths : one extrapolating reports from laboratoryconfirmed jurisdictions to the entire country and one extrapolating reports from syndromic jurisdictions to the entire country .
calculation of rates involving laboratoryconfirmed influenzaassociated hospitalizations and deaths used the populations of states reporting laboratoryconfirmed cases as a denominator ; calculations involving syndromic influenzaassociated hospitalizations and deaths used the populations of states reporting syndromic cases as a denominator .
laboratoryconfirmed reports from ahdra were used to estimate weekly , age groupspecific national influenzaassociated deathtohospitalization ratios .
these ratios were incorporated into a model used to estimate the national illness burden of influenzaassociated cases , hospitalizations and deaths during the pandemic , accounting for variation in medical careseeking , laboratory practice and detection capability , and underreporting of confirmed cases . 

 
all data were maintained in a database on a secure server at cdc , and all analyses were performed using microsoft excel and sas v 9.1 ( sas institute , cary , nc , usa ) .
this activity was determined by cdc to be part of routine public health practice and was not subject to institutional review board approval for human research protections .
the median number of jurisdictions reporting laboratoryconfirmed hospitalizations each week was 36 ( range 2938 ) , and the median number of jurisdictions reporting syndromic hospitalizations each week was 18 ( range 1219 ) . the median number of jurisdictions reporting laboratoryconfirmed deaths each week was 39 ( range 3040 ) , and the median number of jurisdictions reporting syndromic deaths each week was 14 ( range 816 ) . with the exception of 12  weeks at the beginning and end of the surveillance period ,
only two jurisdictions changed their surveillance definition during the reporting period ( one from laboratory confirmed to syndromic and one from syndromic to laboratory confirmed ) , and only two jurisdictions failed to report for more than 1  week during the reporting period . 
number of jurisdictions reporting to the aggregate hospitalization and death reporting activity by surveillance definition and by week  august 30 , 2009 to april 6 , 2010 . in 27 of 36 jurisdictions reporting laboratoryconfirmed hospitalizations for
which information was available , the median proportion of hospitals under surveillance was 100% of all hospitals within the jurisdiction ( range 18100% ) . for 16 of 18 jurisdictions using a syndromic hospitalization definition ,
the median proportion of hospitals under surveillance was 45% of all hospitals within the jurisdiction ( range 9100% ) .
information regarding the type of diagnostic test used to identify cases was available for 24 jurisdictions reporting laboratoryconfirmed hospitalizations and 22 jurisdictions reporting laboratoryconfirmed deaths from september 8 to october 6 , 2009 .
sixteen of 24 ( 67% ) jurisdictions employed rtpcr , viral culture , or dfa testing to identify the majority of reported hospitalizations ( at least 75% of reported cases in each jurisdiction ) , and 18 of 22 ( 82% ) jurisdictions used one of these methods to identify the majority of reported deaths .
laboratoryconfirmed cases not identified by rtpcr , dfa , or viral culture were identified using rapid antigen testing or an unspecified diagnostic test .
a total of 41  689 laboratoryconfirmed hospitalizations and 2096 laboratoryconfirmed deaths were reported from august 30 , 2009 , through april 6 , 2010 .
weekly laboratoryconfirmed hospitalizations peaked at > 5000 during the last week of october 2009 and declined from that date to < 200 by the end of march 2010 ( figure  2 ) .
weekly laboratoryconfirmed deaths peaked at nearly 200 during the same week as the laboratoryconfirmed hospitalization peak , before declining to < 20 per week by the end of march 2010 ( figure  2 ) .
the highest laboratoryconfirmed hospitalization rate was observed in the 0 to 4yearold age group , which had a rate 2 to 3fold higher than those observed in the other age groups ( figure  3 ) .
the majority of laboratoryconfirmed hospitalizations ( > 70% ) reported to ahdra were in patients < 50  years of age , and fewer than 10% were in patients 65  years of age or older .
the ahdra weekly laboratoryconfirmed death rate peaked in october 2009 at 0078 and fell to < 0001 per 100  000 persons by march 2010 .
the highest laboratoryconfirmed death rate was seen in the 5064  year old age group , and 69% of laboratoryconfirmed deaths occurred in patients between 25 and 64  years of age ( figure  3 ) . 
weekly laboratoryconfirmed and syndromic ph1n1 hospitalizations and deaths reported to the aggregate hospitalization and death reporting activity  august 30 , 2009 to april 6 , 2010 .  estimated rates per 100  000 persons of laboratoryconfirmed and syndromic ph1n1 hospitalizations and deaths reported to the aggregate hospitalization and death reporting activity , by age group , august 30 , 2009 to april 6 , 2010 .
a total of 134  441 syndromic hospitalizations and 13  983 syndromic deaths were reported to ahdra .
weekly syndromic hospitalizations peaked at nearly 7000 during the last week of october 2009 and were distributed in a pattern similar to the weekly laboratoryconfirmed hospitalization curve .
weekly syndromic deaths peaked at 605 approximately 1  month later but did not show a pattern resembling the weekly laboratoryconfirmed death curve ( figure  2 ) .
the highest rates of syndromic hospitalizations were reported in patients 65  years of age ( 399 per 100  000 ) , and in patients 04  years of age ( 255 per 100  000 ) .
greater than 80% of all syndromic deaths reported were in patients 65  years of age , and fewer than 2% were in patients < 25  years of age ( figure  3 ) .
extrapolating ahdra reports to the entire country yielded cumulative counts of hospitalizations and deaths that estimate what may have been observed had all jurisdictions reported using either a laboratoryconfirmed or syndromic surveillance definition ( table  1 ) .  observed and extrapolated * estimates of ph1n1associated hospitalizations and deaths in the united states reported to the aggregate hospitalization and death reporting activity from august 30 , 2009 to april 6 , 2010 * extrapolated counts were calculated using the direct method of standardization and represent the number of hospitalizations and deaths that would have occurred in the united states if all states had used either a laboratoryconfirmed or a syndromic surveillance definition .
laboratoryconfirmed hospitalization and death rates were calculated by dividing the number of cases by the sum of the state populations for states using a laboratoryconfirmed definition ( 207  654  216 for hospitalizations ; 245  351  708 for deaths ) .
syndromic hospitalization and death rates were calculated by dividing the number of cases by the sum of the state populations for states using a syndromic definition ( 96  405  508 for hospitalizations ; 58  708  016 for deaths ) .
both laboratoryconfirmed and syndromic hospitalization and death rates were then applied to the standard population ( u.s .
census , july 2008 ; 304  059  724 ) to derive extrapolated counts .
although the weekly laboratoryconfirmed deathtohospitalization ratio demonstrated considerable variability especially during the latter part of the surveillance period ( 3081376% ) , the cumulative ratio quickly stabilized near its mean of 502% in october 2009 and remained within 1% of this value throughout the remainder of the surveillance period ( figure  4 ) .
the cumulative age groupspecific laboratoryconfirmed deathtohospitalization ratio was substantially lower for 0 to 18yearolds compared to older age groups and the overall ratio for all age groups ( figure  4 ) . 
weekly deathtohospitalization ratio from laboratoryconfirmed reports submitted to the aggregate hospitalization and death reporting activity  august 30 , 2009 to april 3 , 2010 .
laboratoryconfirmed data collected by ahdra helped characterize the epidemiology of ph1n1associated influenza hospitalizations and deaths in the united states , revealing a time course and illness distribution for ph1n1 that were substantially different from those seen in seasonal influenza epidemics .
aggregate hospitalization and death reporting activity laboratoryconfirmed peak hospitalizations and deaths occurred much earlier than the typical peak for seasonal influenza activity , which most often occurs during january or february each year . 

, 
  furthermore , the age distribution of laboratoryconfirmed hospitalizations reported to ahdra was markedly different from typical influenza seasons when hospitalizations are more common among persons over 65  years of age . 
 , 
 , 
  other recent studies corroborate this finding , showing that nearly half of all patients in the united states hospitalized with ph1n1 influenza infections were under the age of 25  years , and < 10% were over the age of 65 . 
 , 
  overall , the age distribution of laboratoryconfirmed death rates determined from ahdra data was also markedly different from that seen in typical influenza seasons .
in contrast to typical influenza seasons , when 90% of deaths occur in the elderly , 
 , 
  86% of laboratoryconfirmed deaths reported to ahdra were in persons < 65  years of age , with the highest rate found in persons aged 5064  years .
laboratoryconfirmed ahdra data were also useful in monitoring trends in the distribution of illness and age groups over time in specific jurisdictions . aggregate hospitalization and death reporting activity laboratoryconfirmed data helped define the beginning and end of the 20092010 influenza season and accurately depicted the second wave of ph1n1 illness seen in the fall of 2009 ; 
  similar doublewave patterns have been seen in previous pandemics . 
 , 
 , 
  ahdra was also instrumental in the detection of and response to a minor third wave of ph1n1 activity in the southeast united states in early 2010 . 
 
ahdra reporting by state and local health departments allowed tracking of trends in severe disease with greater geographic representativeness than would have been possible with existing systems alone and informed decisionmaking at the state and national levels .
for example , although the emerging infections program ( eip ) has conducted populationbased surveillance for laboratoryconfirmed influenzaassociated hospitalizations in the united states since 1995 , the eip network of hospitals conducts surveillance in certain counties in only 16 states . 
 
laboratoryconfirmed surveillance via ahdra during the pandemic was implemented in more than twice the number of states as in the eip network .
aggregate hospitalization and death reporting activity laboratoryconfirmed hospitalization and death surveillance was also consistent with data from existing influenza surveillance systems .
emerging infections program hospitalizations peaked at approximately the same time in october 2009 as did ahdra laboratoryconfirmed reports , and the age distribution of ph1n1 hospitalizations described by the two systems was similar , with each identifying the highest rate in the 0 to 4yearold age group and a similar distribution of rates in other age groups . 
 
aggregate hospitalization and death reporting activity data also accurately reflected outpatient influenza illness during the pandemic .
outpatient influenzalike illness surveillance network ( ilinet ) collects data from over 3000 healthcare providers each week on the proportion of patient visits for ili . *
the ilinet weekly percentage peak for the 20092010 season ( 77% of all patient visits ) occurred 1  week prior to the ahdra laboratoryconfirmed and syndromic hospitalization peaks in october 2009 . 
 
this approximate 1week lag between ili onset and severe outcome ( hospitalization or death ) has been noted consistently during both annual influenza seasons and during pandemics . 

  despite its usefulness during the pandemic , ahdra
first , jurisdictions were permitted to report according to different surveillance criteria ( e.g. , use of a laboratoryconfirmed or syndromic case definition , multiple diagnostic testing methods ) and results therefore did not measure identical outcomes .
this disparity is evident in the age distributions of laboratoryconfirmed and syndromic hospitalization rates  although both reporting methods show similar peaks in the hospitalization rate for the youngest age group , laboratoryconfirmed rates thereafter generally decline with increasing age , while syndromic rates initially decline but then show a dramatic increase for the 65yearold age group . presumably , the difference is largely because of the relatively low specificity of a syndromic compared to a laboratoryconfirmed definition of influenza infection .
syndromic reporting likely captures many hospitalizations associated with noninfluenza respiratory illness , which often occur with greater frequency in young children and the elderly , 
 , 
 , 
 , 
  while laboratoryconfirmed reporting is much more likely to identify only cases of influenza illness .
thus , it is inappropriate to make comparisons between reporting jurisdictions in ahdra without adjusting for differences in reporting methods and practices . instead ,
ahdra s best use may have been to track the progression of the epidemic within each state , a goal consistent with the original intent of the system .
a more important consequence of the decision to allow two surveillance definitions in ahdra is the inherent limitation of using a syndromic definition to conduct national surveillance for influenzaassociated infections .
although ahdra syndromic data were useful to track trends of disease within those jurisdictions submitting syndromic reports , they may not otherwise accurately reflect the burden or severity of influenzaassociated hospitalizations and deaths in the united states .
because the proportion of syndromic respiratory hospitalizations and deaths attributable to influenza is small , 
  it is unclear whether syndromic findings in ahdra represent influenza infections or hospitalizations and deaths caused by other respiratory illnesses .
interpretation of syndromic data was further complicated by the limited number of hospitals included in syndromic reports ( a median of 45% of all hospitals within each reporting jurisdiction ) .
a further limitation of ahdra is that the system does not collect several potentially useful data elements , such as population denominator information and influenza type or subtype ( although > 99% of circulating influenza viruses during the surveillance period were ph1n1 ) .
also , the additional effort required of reporting jurisdictions to conduct ahdra surveillance may be high and may exhaust state and local health department resources during a pandemic . finally , extrapolated burden estimates derived from ahdra data do not account for variations in medical careseeking , laboratory practice and detection capability , degree of underreporting of confirmed cases , and other population differences across jurisdictions .
however , ahdra laboratoryconfirmed data were important components of modelbased estimates that do account for these sources of underestimation . 


understanding the impact of ph1n1 influenza hospitalizations and deaths was important to guide the pandemic response and will be important to inform preparedness and response plans for future public health crises .
the ahdra was an important component of us influenza surveillance efforts during the pandemic and provided a level of geographic representativeness and timeliness for reporting of severe influenzaassociated outcomes that was not available from existing national surveillance systems .
laboratoryconfirmed reporting in ahdra supplied valuable information to public health practitioners during the pandemic and should inform refinements to seasonal surveillance activities in the coming seasons , as well as revisions of pandemic surveillance plans . although useful in monitoring trends within jurisdictions , ahdra syndromic reports , as a measure of influenzaassociated hospitalizations and deaths , were difficult to interpret .
these data were complicated by limited representativeness and a low specificity for detecting influenzaattributable hospitalizations and deaths among events associated with respiratory illness . using only syndromic surveillance data to monitor epidemic or
pandemic influenza is thus not recommended , particularly in a setting like the 2009 h1n1 pandemic when elderly persons were largely immune because of prior exposures to antigenically related influenza viruses . 
  because ahdra was implemented within a few weeks time
, the system may prove particularly useful as a prototype for a pandemic or epidemic respiratory infection surveillance system that needs to be implemented quickly and efficiently on a national scale .
the findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the cdc .
 
 please cite this paper as : jhung et  al . 
 ( 2011 ) preliminary results of 2009 pandemic influenza surveillance in the united states using the aggregate hospitalization and death reporting activity . influenza and other respiratory viruses 5(5 ) , 321327 . 
 
 background   to augment established influenza surveillance systems in the united states , the centers for disease control and prevention and the council of state and territorial epidemiologists implemented the aggregate hospitalization and death reporting activity ( ahdra ) in august 2009 . 
 the ahdra was designed to meet increased demands for timely and detailed information describing illness severity during the 2009 h1n1 influenza a ( ph1n1 ) pandemic response . 
 
 objectives   we describe the implementation of ahdra and provide preliminary results from this new surveillance activity . 
 
 methods   all 50 us states were asked to report influenzaassociated hospitalizations and deaths to ahdra each week using either a laboratoryconfirmed or syndromic surveillance definition . 
 aggregate counts were used to calculate agespecific weekly and cumulative rates per 100  000 , and laboratoryconfirmed reports were used to estimate the age distribution of ph1n1 influenzaassociated hospitalizations and deaths . 
 
 results   from august 30 , 2009 , through april 6 , 2010 , ahdra identified 41  689 laboratoryconfirmed influenzaassociated hospitalizations and 2096 laboratoryconfirmed influenzaassociated deaths . 
 aggregate hospitalization and death reporting activity rates peaked earlier than hospitalization and death rates seen in previous influenza seasons with other surveillance systems , and the age distribution of cases revealed a tendency for hospitalizations and deaths to occur in persons < 65  years for age . 
 
 conclusions   aggregate hospitalization and death reporting activity laboratoryconfirmed reports provided important information during the 2009 pandemic response . 
 aggregate hospitalization and death reporting activity syndromic reports were marked by low representativeness and specificity and were therefore less useful . 
 the ahdra was implemented quickly and may be a useful surveillance system to monitor severe illness during future influenza pandemics . 