the naval health research center ( nhrc ) began conducting tri - service surveillance for febrile respiratory illness at military training centers in 1996 ; by 1999 , this surveillance network had expanded to include 8 of the largest military basic training centers in the united states ( 3 ) . this surveillance includes the systematic collection of throat swab specimens and clinical data ( including but not limited to gender , date of birth , symptoms , influenza vaccination status , type of vaccine received , and date of vaccination ) from consenting us military trainees meeting the case definition for febrile respiratory illness ( oral temperature 100.5f [ 38.0c ] and a cough or sore throat ) . samples are stored locally at each site at 70c until they are forwarded to the naval respiratory disease laboratory at nhrc for viral culture and molecular diagnostic processing . research personnel at participating surveillance sites report the weekly number of trainees who sought care for febrile respiratory illness and total trainee populations for their respective sites , and rates for such illnesses are calculated . during the 200304 influenza season , we recognized the opportunity of using data from this ongoing active surveillance to estimate influenza vaccine effectiveness in protecting against both laboratory - confirmed influenza and febrile respiratory illness of any cause among us military basic trainees . despite concerns that vaccine effectiveness during the 200304 season would be low because of the poor match between the components of the vaccine and the circulating strain ( 4 ) , the vaccine provided good protection ( 94.4% ) against laboratory - confirmed influenza that season ( 5 ) . annual vaccine effectiveness calculations are important as we heighten our preparedness for pandemic influenza strains ; therefore , we performed similar calculations for the 200405 and 200506 seasons . during the late fall and winter seasons , all active - duty military forces are required to receive the influenza vaccine , and this policy is strictly enforced in training camps . upon arrival , all incoming trainees receive mandatory influenza vaccination , either the trivalent inactivated influenza vaccine by injection ( fluzone , sanofi pasteur , lyon , france ) or intranasal cold - adapted , live , attenuated influenza vaccine ( ca - laiv ) spray ( flumist , medimmune , gaithersburg , md , usa ) . for this analysis , unvaccinated  at any given time , assuming immunity takes 14 days to develop . likewise , 33% of trainees in a 6-week training program were considered unprotected by the vaccine at any time . these assumptions allow estimates of denominator data for  vaccinated  and  unvaccinated  person - weeks in calculations of vaccine effectiveness .. from january through march 2006 all new trainees arriving for basic training received the influenza vaccine ; all recruits already present had been vaccinated . however , 2 sites , naval service training command , great lakes , and marine corps recruit depot , san diego , had completed vaccination by december 2005 . total person - weeks in recruit training during the observation period were obtained directly from the participating training centers . vaccine effectiveness was calculated for both laboratory - confirmed influenza and any cause of febrile respiratory illness as follows : 100  ( 1  relative risk = 1  [ rate in vaccinated group]/[rate in unvaccinated group ] ) . during the observation period , 6 of 8 surveillance sites had influenza activity and were included in this analysis . in 479,181 person - weeks of observation , 4,052 cases of febrile respiratory illness were reported from these 6 sites , and 722 patients were enrolled into the surveillance study ( includes throat swab specimen , case data , and consent ) . seventy ( 9.7% ) specimens tested positive for influenza , by either culture or molecular techniques . rates of laboratory - confirmed influenza were higher among unvaccinated trainees at all sites except fort benning , georgia , which had only 3 cases ( figure ) . overall , influenza vaccine effectiveness among us military trainees was 92% ( confidence interval [ ci ] 85.495.6% ) during the 200506 season ( table ) . vaccine effectiveness against laboratory - confirmed influenza was high ( range 86%94% ) in each of the past 3 seasons . vaccine effectiveness against non  laboratory - confirmed febrile respiratory illness was lower , ranging from 10% in 200506 to 52% in 200405 . afb , air force base ; nstc , naval service training command ; mcrd , marine corps recruit depot . * ci , confidence interval ; sc , south carolina ; mo , missouri ; ga , georgia ; afb , air force base ; tx , texas ; nstc , naval service training command ; il , illinois ; mcrd , marine corps recruit depot ; ca , california .  assuming 14 d before vaccine is protective . this analysis suggests that the 200506 influenza vaccine was highly effective in protecting us military basic trainees against laboratory - confirmed influenza . furthermore , these data suggest that both the trivalent inactivated vaccine injection and the ca - laiv intranasal spray were equally effective , because the marine corps recruit depot in san diego vaccinated its trainees with ca - laiv almost exclusively , and vaccine effectiveness at that site was 95% ( vaccine effectiveness at all other sites combined = 90% ) . these estimates of effectiveness were supported by results of additional analyses that would be expected to bias the outcome toward the null hypothesis . for example , a 7-day lag period before immune response was considered in an alternative analysis , and it yielded similar results : the calculated vaccine effectiveness changed only slightly , from 92% to 90% . we also analyzed vaccine effectiveness , assuming that 10% fewer trainees were vaccinated at any given point , yet the calculated vaccine effectiveness was only reduced to 87% . in contrast to the consistently high effectiveness of the vaccines against laboratory - confirmed influenza , the effectiveness against febrile respiratory illness of any cause was much lower and varied with each season ( 13.9% in 200304 , 52.1% in 200405 , and 10% in 200506 ) . this lower effectiveness in 200506 is most likely due to the generally high proportion of adenovirus infection seen in this population ( 6 ) , and the lesser effectiveness is further exacerbated by the tendency for adenoviral infections to occur beyond the second week of training . the lower vaccine effectiveness seen against febrile respiratory illness of any cause gives credence to the estimates of high vaccine effectiveness against laboratory - confirmed influenza . if a measurement bias existed , both estimates would be affected . as a highly vaccinated population , military personnel , and basic trainees in particular , can provide critical information regarding the effectiveness of each year s influenza vaccine formulations . because of the annual variations of both the vaccine formulations and the circulating strains , influenza vaccine effectiveness should be evaluated annually . with the ever - rising concerns of an imminent influenza pandemic , our existing surveillance network will allow us to repeat the methods used in this analysis each year , thus providing valuable estimates of influenza vaccine effectiveness to the public health community .
virtually all us military basic trainees receive seasonal influenza vaccine . 
 surveillance data collected from december 2005 through march 2006 were evaluated to estimate effectiveness of the influenza vaccine at 6 us military basic training centers . 
 vaccine effectiveness against laboratory - confirmed influenza was 92% ( 95% confidence interval 85%96% ) .