israel s center for disease control seasonal influenza surveillance system operated throughout our 5-year study .
the system is based primarily on 1 ) anonymous patient visits for influenza - like illnesses ( ili ) to maccabi community clinics , israel s second largest health maintenance organization , insuring 1 of every 4 israelis ; and 2 ) nasopharyngeal swabs from sample ili patients at designated sentinel clinics countrywide .
ili was defined as fever ( > 37.8c ) with > 1 of the following : cough , coryza , sore throat , or myalgia .
swab samples were tested for influenza viruses at the health ministry s central virology laboratory ( 1 ) by using multiplex real - time reverse transcription pcr ( rt - pcr ) ( taqman chemistry quantitative rt - pcr ) ( 2 ) .
ili rates constituted 3 escalating waves of infection , all at times atypical for seasonal influenza ( figure 1 ) .
israel s schools close july / august , but children stay together in summer frameworks during july .
wave 2 peaked mid - september ( week 38 ) , 2 weeks into the school year , declining when schools closed for holidays until the end of week 41 . during week 42 , the third , largest wave began , peaking mid - november ( week 46 ) .
rates of weekly visits to community health clinics for influenza - like illness , by age group , june 2009april 2010 , compared with the 20052009 average ( maccabi health services ) , with school holidays indicated , israel .
the cumulative incidence ( cases/10,000 population ) of ili in children 018 years of age during the pandemic ( week 25 , 2009 to week 7 , 2010 ) was 369.3 ( 95% confidence interval [ ci ] 365.7373.1 ) , far higher than average rates documented in earlier influenza seasons ( 143.4 , 95% ci 140.7146.2 ) .
incidence was 295.8 ( 95% ci 285.7306.1 ) for children < 2 years of age , 347.1 ( 95% ci 338.4355.9 ) for children 24 years of age , and 389.4 ( 95% ci 382.5391.4 ) for children 518 years of age , compared with 107.8 ( 95% ci 100.9115.1 ) , 179.5 ( 95% ci 172.5186.7 ) , and 140.2 ( 95% ci 140.7146.2 ) , respectively , for each age group in earlier seasons .
israel identified its first pediatric pandemic ( h1n1 ) 2009 cases in june 2009 ( week 24 ) and recorded local transmission the following week ( figure 2 ) . during weeks 2843 ,
the weekly percentage of positive influenza samples among children was 40%60% , peaking at 70%80% during weeks 4449 ( late october to early december ) .
positive influenza samples among total samples collected by the sentinel network , children 018 years of age , israel , june 2009april 2010 . pandemic ( h1n1 ) 2009 in israel
was present for 36 weeks ; the overall percentage of positive influenza samples among children was 50% .
earlier influenza seasons were shorter ( 1322 weeks ) but with similar overall positive percentages ( 52%57% ) of influenza a / b .
most ( 98% ) of the influenza cases during 2009 were the pandemic strain ; in previous years a mixture of various influenza a / h1n1 , h3n2 , and b strains occurred .
we compared hospitalization of children with laboratory - confirmed influenza infection during the pandemic with the previous 4 influenza seasons in the pediatric departments of hadassah s 2 hospitals in jerusalem .
these departments provide primary medical care for 250,000 children ( 1 of every 10 children in israel ) , as well as tertiary care for chronic diseases .
we performed our study at these hospitals because respiratory specimens were routinely taken year - round for laboratory confirmation from all children with suspected influenza or respiratory virus infection during the 5-year study .
direct immunofluorescence assay was used at hadassah in previous years for detection of influenza and other respiratory viruses and multiplex real - time pcr ( taqman chemistry quantitative rt - pcr ) for detection of influenza viruses during the pandemic .
findings from pandemic ( h1n1 ) 2009 were retrospectively compared with those from previous influenza seasons .
two - sample t - tests and the mann - whitney nonparametric tests compared continuous variables .
all tests applied were 2-tailed ; a p value < 0.05 was considered statistically significant . during june 29 , 2009january 25 , 2010 ,
a total of 127 children were admitted to hadassah hospitals with documented pandemic ( h1n1 ) 2009 infection ; most hospitalizations ( 77 of the total 890 children hospitalized during that period ) occurred during the third peak , october 
table 1 summarizes the major clinical manifestations for the 127 children ; 33/124 ( 26.6% ) had leukocyte counts < 5,000 cells / mm .
all patients received oseltamivir treatment ; 1 child in whom resistance to oseltamivir developed was given zanamivir .
children > 2 years of age had a significantly higher rate of underlying illness compared with children < 2 years of age ( 49/79 [ 62.0% ] vs. 17/48 [ 35.4% ] ; p = 0.006 ) . all survived , including 2 children who were mechanically ventilated ( 11 and 25 days , respectively ) and another 6 who required intensive care .
nationwide , 9 children ( median age 12.5 years ) died from pandemic ( h1n1 ) 2009 , a mortality rate of 3.69/1,000,000 .
no deaths were reported during previous influenza seasons , including among infants , for which reporting is mandatory . in
previous shorter influenza a / b seasons , fewer children were hospitalized ; none were treated with antiviral agents , and statistically significant differences included age , underlying chronic diseases , underlying chronic lung disease , and neonatal fever as the initial symptom ( table 2 ) .
no significant differences were found regarding history of prematurity ( <33 weeks ) , weight percentile , pediatric intensive care unit admission , evidence of pneumonia , oxygen saturation < 90% , and leukopenia . in previous seasons , 6 nosocomial influenza infections and 2 co - infections with respiratory syncytial virus were reported ; none were seen for pandemic ( h1n1 ) 2009 . *
children , mainly those 510 years of age , were affected by pandemic ( h1n1 ) 2009 markedly more so than by seasonal influenza , similar to results reported from the united states , spain , and switzerland ( 36 ) . during the 1918 spanish influenza pandemic ,
the highest incidence rates were among older children ( 7 ) . in our study ,
hospitalized children infected with pandemic ( h1n1 ) 2009 were older and findings were compatible with reports from several other countries ( 8,9 ) , but findings were unlike those from argentina , where 60% were infants ( 9 ) .
the age of children who died in israel also underlines the impact on older children , as reported elsewhere ( 10,11 ) . although pandemic ( h1n1 ) 2009 virus may cause severe , life - threatening disease in previously healthy children of all ages ( 12 ) , the children we studied had significantly more underlying chronic diseases than did with children hospitalized for seasonal influenza ( 13 ) .
we , like others ( 3 ) , found no increase in pneumonia or pediatric intensive care unit admissions caused by pandemic ( h1n1 ) 2009 .
however , this finding could be because antiviral therapy was administered during the pandemic but not in previous years ; 98/127 ( 77.2% ) of children hospitalized for pandemic ( h1n1 ) 2009 received oseltamivir ( table 2 ) .
the nationwide pandemic ( h1n1 ) 2009 influenza mortality rate in israel is similar to that reported for the united kingdom ( 14 ) but can not be compared with previous years because laboratory data are lacking and there was no requirement to report the death of children > 12 months of age .
parents were advised not to attend the clinic for mild disease , although anxiety may have increased visits . there may have been differences between diagnoses of ili among different maccabi physicians .
the 2 hospitals studied , which represented 10% of hospitalized children , were selected not as nationally representative but because of the feasibility of viral diagnosis since 2005 .
influenza detection during the pandemic in patients hospitalized at hadassah was based on pcr ; immunofluorescent antibody assay was used for previous seasons . awareness that pandemic influenza may have unique clinical characteristics , risk factors , and increased incidence , mainly among children 518 years of age , is advocated . because school opening in late summer 2009 triggered the wave of pandemic ( h1n1 ) 2009 influenza ( 15 ) , closing or delaying opening schools until vaccine is available
should be considered among mitigation strategies in future influenza pandemics , especially for more virulent viruses .
 during the pandemic ( h1n1 ) 2009 outbreak in israel , incidence rates among children were 2 higher than that of the previous 4 influenza seasons ; hospitalization rates were 5 higher . 
 children hospitalized for pandemic ( h1n1 ) 2009 were older and had more underlying chronic diseases than those hospitalized for seasonal influenza . 
The Study
Conclusions