during the a / h1n1 2009 global pandemic , adjuvanted influenza vaccines were widely used for the first time . along with many other countries ,
all canadians older than 6 mo were eligible to receive this vaccine , which accounted for over 96% of pandemic vaccine doses administered .
this vaccine was used in a dose - sparing formulation ( 3.75 g hemagglutinin per adult dose ) , one dose of which was adequately immunogenic in nave adults . by the end of the mass campaign in canada , over 40% of the population had received this vaccine .
it was not known at the time if adjuvant use would be associated with persistently elevated antibody responses or the rapid decay typically seen after seasonal inactivated vaccines . in studies launched
while the pandemic vaccine was initially deployed in 2009 , evidence of prior infection was already present in one - third of adults under 60 y of age .
these primed individuals developed strong booster antibody responses to h1n1pdm09 vaccination , with greater potential for persistence .
both natural infection and vaccination with adjuvant were expected to elicit greater cell - mediated immune responses than observed following standard trivalent seasonal vaccines . in 2010 the a / h1n1pdm09 virus continued to circulate with minimal antigenic change .
the world health organization recommended that the same h1n1pdm09 vaccine strain be included in the trivalent inactivated vaccine ( tiv ) for the 20102011 influenza season .
some candidates for re - vaccination with the h1n1pdm09 strain would have been strongly primed by prior infection , vaccination with adjuvant or both .
it was certainly conceivable that such individuals might have greater residual cellular immunity and/or specific antibody levels than ordinarily encountered after seasonal influenza vaccination and that this might predispose them to increased vaccine reactions .
vaccination in the presence of specific cellular immunity has been linked to increased injection site reactions after childhood booster doses of adjuvanted , pertussis - containing vaccines while vaccination in the presence of high titers of antitoxin increased reaction rates after repeated tetanus toxoid boosters .
it was not known at the time if residual cellular or humoral immunity to h1n1 would be sufficient to cause reactions with a first booster dose of h1n1 antigen .
the post - pandemic situation was unique in modern experience with influenza vaccines and warranted investigation , especially given the potential for unpredictable variations in tiv reactogenicity such as occurred in children in western australia in 2010 with a particular tiv formulation .
we conducted a pre - season study to rapidly assess the safety of re - vaccination with h1n1pdm09 antigen .
eligible adults had received the adjuvanted pandemic vaccine 910 mo previously , from community providers .
adverse events were documented daily ; rates following placebo were subtracted from rates after vaccine to determine vaccine - attributable rates .
secondary objectives were to measure residual antibody titers to h1n1pdm09 virus in previously vaccinated adults and the magnitude of the booster response to re - vaccination with this antigen , in comparison to a / h3n2 responses .
enrollment totaled 326 subjects , who were predominantly female and caucasian / white ( table 1 ) .
the rate of prior seasonal influenza vaccination was high as most subjects were employed by or affiliated with the participating health care institutions .
centers commenced enrollment on the same day ( 9 august 2010 ) and completed it within 5 d. compliance with the protocol was high ( fig . 
opposite assignment applied to second ( crossover ) injections ; * * refers to any ongoing medical condition , not just those posing increased risk with influenza .
comment : one subject withdrew before the initial injection , a second withdrew shortly after the initial injection ( placebo , with code break ) and a third withdrew prior to the second injection .
as table 2 shows , tiv vaccination was associated with higher than background ( post - placebo ) rates for myalgia , tiredness , headache , malaise , arthralgia , sleep disturbance and diarrhea during the week after vaccination .
only myalgia , tiredness and headache occurred at vaccine - attributable rates ( var ) > 5.0% , with myalgia leading at 18.6% .
most subjects who reported these symptoms rated them as mild / moderate ( table 2 ) .
the peak var s for myalgia and headache were reached later during the day of vaccination , at 16.9% and 4.9% , respectively , while reports of tiredness peaked next day , with a var of 5.8% .
reported rates of myalgia and headache then declined to match those following placebo from day 3 onwards , with tiredness rates normalizing one day later .
a similar pattern was seen with reports of malaise , arthralgia , sleep disturbance and diarrhea .
background rates during the week after placebo were > 5% for headache , tiredness , myalgia and malaise ( table 2 ) , illustrating the value of controlled observations for this age group . at the injection site ,
pain was reported by 63.2% of participants ( 204/323 ) during the week after tiv vaccine and by 8.0% ( 26/323 ) after placebo .
tiv - related pain was rated as mild ( 186 subjects , 57.6% ) or moderate ( 18 subjects , 5.6% ) with no instances of severe pain .
local redness followed tiv in 45 participants ( 13.9% ) , with 38 ( 11.8% ) having mild ( < 25 mm ) and 7 ( 2.2% ) having moderate ( 2699 mm ) redness .
local swelling followed tiv in 31 participants ( 9.6% ) , with a single instance ( 0.3% ) of severe swelling (  100 mm ) .
injection site symptoms were short - lived : all resolved by day 6 after vaccination , well before second injections ( data not shown ) .
a strong association was evident on day 1 after vaccination between reported rates of injection site pain and myalgia . among 145 subjects with pain on that day , 41 ( 28.3% ) also had generalized myalgia whereas among 180 subjects without pain , only 10 ( 5.6% ) reported myalgia ( p < 0.0001 , chi- square test ) . during the first 24 h after tiv vaccination 32 subjects ( 9.8% ) reported 47 new - onset respiratory symptoms .
these participants more often reported concurrent myalgia than those without new respiratory symptoms ( 34.4% vs. 13.6% , p < 0.01 , odds ratio 3.3 , 95% confidence interval 1.5 , 7.4 ) . no significant association was detected with other general symptoms ( data not shown ) .
twenty - one vaccinated subjects reported a single respiratory symptom ( sore throat-8 , cough-6 , hoarseness-3 , wheezing-2 , red eyes-2 ) , all of which were rated mild . eleven vaccinated subjects ( 3.4% ) had multiple , new - onset respiratory symptoms ( table 3 ) , more consistent with oculorespiratory syndrome ( ors ) .
all symptoms were rated mild or moderate and none of those affected sought medical attention . during the first 24 h after placebo injection ,
11 subjects ( 3.4% ) reported 19 new - onset respiratory symptoms , mainly sore throat ( 6 ) , hoarseness ( 3 ) , cough ( 3 ) , chest tightness ( 3 ) and ocular redness ( 1 ) .
the frequency of respiratory symptoms was significantly lower after placebo than after tiv ( 3.4% vs. 9.8% , p < 0.001 ) but the spectrum of symptoms was similar in both settings .
only 3 subjects had multiple respiratory symptoms after placebo ( table 3 ) , yielding a vaccine - attributable rate of ors - like symptoms of 2.2% ( p < 0.01 ) .
other notable adverse events included minor allergy - like reactions ( generalized pruritus , rash , facial swelling ) in 4 vaccinees , none serious or early - onset .
two vaccinees developed neurologic symptoms , with pain or paresthesia radiating down the injected arm . both instances began within hours after vaccination , lasted several days ,
other adverse events after placebo injection included a vasovagal reaction ( 1 ) , cardiac palpitations ( 1 ) and hyperventilation syndrome ( 1 ) , all beginning within minutes after the injection .
only one subject was hospitalized during the study , after a motor vehicle accident . despite treatment blinding , 75% of subjects correctly guessed when they had received tiv , based on day 7 interviews . a preliminary report of blinded safety data after first injections was presented to the dsmb encompassing all day 1 and most day 7 observations
a detailed safety report was presented to the public health agency of canada 32 d after study commencement ( september 10 , 2010 ) and was widely shared with provincial and territorial immunization program administrators before public programs began . at study entry ,
hai antibody to h1n1pdm09 was detectable in most subjects ( table 4 ) with half having titers  40 .
age influenced the frequency of residual titers  40 , which ranged from 73.4% ( 95% ci 60.9,83.7 ) among 2029 y olds to 42.5% ( 95% ci 31.5,57.5 ) among 4049 y olds ( p = 0.0004 , cochran - armitage trend test , 2-sided ) .
following vaccination ( table 4 ) , almost all subjects developed titers  40 and the gmt increased 4.4 fold .
a subset of 52 subjects had participated in a trial of as03-adjuvanted vaccine in 2009 .
one month after the pandemic vaccine , 51 ( 98.1% ) had titers  40 , with a gmt of 285.7 ( 95% ci 208 , 391 ) .
nine months later , 37 ( 71.2% ) retained titers  40 , the gmt having declined 82% to 52.6 ( 95% ci 39.3 , 70.3 ) .
after tiv , all but one had titers  40 and gmt was 180.4 ( 95% ci 139 , 235 ) , a 3.4-fold increase .
an association was detected between baseline titers of antibody to a / h1n1pdm09 and post - immunization myalgia .
subjects with baseline titers  20 were less likely to experience myalgia ( 25/137 , 18% ) than those with titers  80 ( 31/106 , 29% , p < 0.05 , odds ratio 0.54 , 95%ci 0.298 , 0.996 ) .
baseline titers of antibody to h3n2 and b viruses did not show a significant association with myalgia ( data not shown ) .
no associations were detected between baseline h1n1 antibody titers and other general symptoms , perhaps because of their infrequency , or with injection site pain or erythema ( data not shown ) .
as table 2 shows , tiv vaccination was associated with higher than background ( post - placebo ) rates for myalgia , tiredness , headache , malaise , arthralgia , sleep disturbance and diarrhea during the week after vaccination .
only myalgia , tiredness and headache occurred at vaccine - attributable rates ( var ) > 5.0% , with myalgia leading at 18.6% .
most subjects who reported these symptoms rated them as mild / moderate ( table 2 ) .
the peak var s for myalgia and headache were reached later during the day of vaccination , at 16.9% and 4.9% , respectively , while reports of tiredness peaked next day , with a var of 5.8% .
reported rates of myalgia and headache then declined to match those following placebo from day 3 onwards , with tiredness rates normalizing one day later .
a similar pattern was seen with reports of malaise , arthralgia , sleep disturbance and diarrhea .
background rates during the week after placebo were > 5% for headache , tiredness , myalgia and malaise ( table 2 ) , illustrating the value of controlled observations for this age group . at the injection site ,
pain was reported by 63.2% of participants ( 204/323 ) during the week after tiv vaccine and by 8.0% ( 26/323 ) after placebo .
tiv - related pain was rated as mild ( 186 subjects , 57.6% ) or moderate ( 18 subjects , 5.6% ) with no instances of severe pain .
local redness followed tiv in 45 participants ( 13.9% ) , with 38 ( 11.8% ) having mild ( < 25 mm ) and 7 ( 2.2% ) having moderate ( 2699 mm ) redness .
local swelling followed tiv in 31 participants ( 9.6% ) , with a single instance ( 0.3% ) of severe swelling (  100 mm ) .
injection site symptoms were short - lived : all resolved by day 6 after vaccination , well before second injections ( data not shown ) .
a strong association was evident on day 1 after vaccination between reported rates of injection site pain and myalgia . among 145 subjects with pain on that day , 41 ( 28.3% ) also had generalized myalgia whereas among 180 subjects without pain , only 10 ( 5.6% ) reported myalgia ( p < 0.0001 , chi- square test ) . during the first 24 h after tiv vaccination 32 subjects ( 9.8% ) reported 47 new - onset respiratory symptoms .
these participants more often reported concurrent myalgia than those without new respiratory symptoms ( 34.4% vs. 13.6% , p < 0.01 , odds ratio 3.3 , 95% confidence interval 1.5 , 7.4 ) . no significant association was detected with other general symptoms ( data not shown ) .
twenty - one vaccinated subjects reported a single respiratory symptom ( sore throat-8 , cough-6 , hoarseness-3 , wheezing-2 , red eyes-2 ) , all of which were rated mild . eleven vaccinated subjects ( 3.4% ) had multiple , new - onset respiratory symptoms ( table 3 ) , more consistent with oculorespiratory syndrome ( ors ) .
all symptoms were rated mild or moderate and none of those affected sought medical attention . during the first 24 h after placebo injection , 11 subjects ( 3.4% ) reported 19 new - onset respiratory symptoms , mainly sore throat ( 6 ) , hoarseness ( 3 ) , cough ( 3 ) , chest tightness ( 3 ) and ocular redness ( 1 ) .
the frequency of respiratory symptoms was significantly lower after placebo than after tiv ( 3.4% vs. 9.8% , p < 0.001 ) but the spectrum of symptoms was similar in both settings .
only 3 subjects had multiple respiratory symptoms after placebo ( table 3 ) , yielding a vaccine - attributable rate of ors - like symptoms of 2.2% ( p < 0.01 ) .
other notable adverse events included minor allergy - like reactions ( generalized pruritus , rash , facial swelling ) in 4 vaccinees , none serious or early - onset . two vaccinees developed neurologic symptoms , with pain or paresthesia radiating down the injected arm .
both instances began within hours after vaccination , lasted several days , were rated severe but resolved fully .
other adverse events after placebo injection included a vasovagal reaction ( 1 ) , cardiac palpitations ( 1 ) and hyperventilation syndrome ( 1 ) , all beginning within minutes after the injection .
only one subject was hospitalized during the study , after a motor vehicle accident . despite treatment blinding , 75% of subjects correctly guessed when they had received tiv , based on day 7 interviews . a preliminary report of blinded safety data after first injections was presented to the dsmb encompassing all day 1 and most day 7 observations .
a detailed safety report was presented to the public health agency of canada 32 d after study commencement ( september 10 , 2010 ) and was widely shared with provincial and territorial immunization program administrators before public programs began .
at study entry , hai antibody to h1n1pdm09 was detectable in most subjects ( table 4 ) with half having titers  40 .
age influenced the frequency of residual titers  40 , which ranged from 73.4% ( 95% ci 60.9,83.7 ) among 2029 y olds to 42.5% ( 95% ci 31.5,57.5 ) among 4049 y olds ( p = 0.0004 , cochran - armitage trend test , 2-sided ) .
following vaccination ( table 4 ) , almost all subjects developed titers  40 and the gmt increased 4.4 fold .
a subset of 52 subjects had participated in a trial of as03-adjuvanted vaccine in 2009 .
one month after the pandemic vaccine , 51 ( 98.1% ) had titers  40 , with a gmt of 285.7 ( 95% ci 208 , 391 ) .
nine months later , 37 ( 71.2% ) retained titers  40 , the gmt having declined 82% to 52.6 ( 95% ci 39.3 , 70.3 ) .
after tiv , all but one had titers  40 and gmt was 180.4 ( 95% ci 139 , 235 ) , a 3.4-fold increase .
an association was detected between baseline titers of antibody to a / h1n1pdm09 and post - immunization myalgia .
subjects with baseline titers  20 were less likely to experience myalgia ( 25/137 , 18% ) than those with titers  80 ( 31/106 , 29% , p < 0.05 , odds ratio 0.54 , 95%ci 0.298 , 0.996 ) .
baseline titers of antibody to h3n2 and b viruses did not show a significant association with myalgia ( data not shown ) .
no associations were detected between baseline h1n1 antibody titers and other general symptoms , perhaps because of their infrequency , or with injection site pain or erythema ( data not shown ) .
this study provided timely assurance that re - exposure of canadian adults to h1n1pdm09 antigen contained in tiv for 201011 would be safe , following prior vaccination with a novel , adjuvanted pandemic vaccine .
the observed rates and severity of common adverse effects were within the ranges previously described for similar tiv vaccines .
however , myalgia was reported more frequently in this study than others assessing the same product , including a similar placebo - controlled , crossover study in 20012002 in which the vaccine - attributable rate of myalgia was 2.6% ( 95% ci 0.4 , 4.8 ) compared with 18.6% ( 95% ci 13.2 , 24.1 ) in the present study .
peak rates of myalgia in the current study were as high as 28% in subjects with injection site pain and 29% in those with baseline h1n1pdm09 hai titers  80 .
however , most instances were mild / moderate and short - lived . to our knowledge , such a titer threshold effect for an adverse event following influenza vaccination has not previously been reported .
the lack of significant association with baseline titers of h3n2 and b antibodies raises the possibility that the myalgia reported by our subjects was triggered by elements of the immune response specific to the h1n1pdm09 antigen itself , the adjuvant or the combination .
the strong association between myalgia and injection site pain suggests that whatever immune mechanisms were involved in the local inflammatory process , they generated cytokines with this adverse effect , such as interferon . however , confirmation of this hypothesis would have required inclusion of a control group of adults not previously exposed to the adjuvanted vaccine . in a study of uk children
similarly revaccinated with tiv in 2010 after receiving 2 doses of as03-adjuvanted or whole virion h1n1pdm09 vaccine a year earlier , injection site redness and severe local reactions were more frequent in children < 5 y of age given the adjuvanted vaccine , supporting the possibility of unique effects after the latter vaccine .
two studies of canadian children re - exposed to h1n1pdm09 antigen in 2010 after receiving as03-adjuvanted vaccine in 2009 reported no unusual increase in adverse effects after vaccination but also lacked a comparison group not primed with adjuvanted vaccine .
our screening for possible unusual adverse effects of re - vaccination with h1n1/2009 antigen included monitoring for symptoms of oculorespiratory syndrome ( ors ) .
ors was described in 2000 as an adverse effect of a canadian - manufactured tiv ( fluviral , shire biologics ) for the 20002001 season .
the syndrome was defined as onset within 224 h after vaccination of any of bilateral red eyes , facial swelling , cough , hoarseness , sore throat , difficulty swallowing , wheezing , chest tightness or difficulty breathing , not obviously associated with an allergic reaction or respiratory infection .
the overall ors rate in 2000 was estimated at 3.4% of vaccinated adults , ranging as high as 16% in women 4059 y old .
between 1024% of individuals with ors consulted a health care provider . a re - formulated product for 20012002 ( using a second virus splitting detergent )
was shown to cause mild ors at a rate of 2.9% among vaccinated adults . in the present study we applied a more stringent case definition than used for surveillance of case reports ,
given the high rate at which vaccinees and controls described new - onset , single , mild respiratory symptoms . with a requirement for two or more eligible symptoms , the observed rate of ors - like symptoms after vaccination was 3.4% , with a vaccine - attributable rate of 2.5% .
all instances were mild to moderate , with none requiring medical attention . on this basis
it seemed unlikely that ors would occur with increased frequency or severity in public programs .
while we detected some infrequent events such as generalized pruritus and limb pain / paresthesia it was not possible to predict whether they were unique to the study population or would be seen as infrequent events in the mass programs . because of their timing and evolution , the neurological events were likely related to the  act of vaccination  rather than the vaccine components themselves , as has been previously reported in other immunization programs .
ferguson et al . recently reported that 83% of adults 1865 y of age given one dose of as03-adjuvanted h1n1pdm09 vaccine in a prospective study had hai titers  40 six months after the vaccination . in the present study , 910 mo after such vaccination in field conditions , 54% of adults 1959 y old still had titers  40 .
both studies showed greater titer declines in older than younger adults , as was also described after administration of two doses of mf59-adjuvanted pandemic vaccine .
this tiv - like decay rate likely reflected the low dose of antigen in the as03-adjuvanted vaccine , intended as a dose - sparing strategy .
rapid loss of protection after this dosage could have been problematic had the pandemic continued so the optimal balance between dose minimization and duration of protection warrants further study .
effective memory responses were evident after the low dose of antigen as all subjects boosted well with tiv 910 mo later , with almost 100% developing titers  40 . however , the post - vaccination gmt of h1n1pdm09 antibodies was just 2.7-fold higher than the gmt of h3n2 antibodies , while the gm fold - rise and seroconversion rates were nearly identical with the two influenza a strains , speaking against any long - lived benefit in terms of hai responses after exposure to adjuvanted vaccine .
a strength of this study was the randomized , blinded , placebo - controlled design .
our experiences with anxiety - related events after placebo injections and with closely - scrutinized respiratory symptoms underscore the value of placebo - controlled observations when assessing vaccine safety .
the crossover trial design provides an efficient means of obtaining placebo - controlled observations in perfectly matched groups .
unfortunately , reactogenicity of the vaccine and placebo differed sufficiently to enable 75% of participants to correctly guess when they had been given tiv , potentially diminishing their objectivity . recruiting mainly healthcare workers may have increased symptom ascertainment bias but facilitated high protocol compliance .
participants had a high rate of past tiv vaccinations , residual effects of which may have narrowed the differences between h1n1pdm09 and h3n2 responses .
other limitations included our inability to include participants known to have had natural h1n1pdm09 infection , to determine if they responded differently to the subsequent seasonal vaccination .
as only single pandemic and seasonal influenza vaccines were assessed , our observations may not reflect antibody decline after other h1n1pdm09 vaccines or the effects of re - exposure to other seasonal influenza vaccines for 20102011 .
in dutch adults given two doses of mf59-adjuvanted h1n1pdm09 vaccine , antibody decline was remarkably similar to our observations , suggesting that the shared components ( squalene and h1n1 antigen ) determined the pattern of decline .
second doses of the mf59-adjuvanted vaccine increased titers significantly only in persons over age 50 so the dosing regimens were not too dissimilar between that study and the present one .
in this prospective , randomized , evaluator - blinded , crossover trial , each subject received vaccine and placebo , in random sequence .
the study was conducted at 5 academic centers across canada belonging to the phac / cihr influenza research network ( pcirn ) , during august and september , 2010 .
participating centers were located in quebec city , montreal , ottawa , calgary and vancouver .
a number of strategies were employed to accelerate enrollment and the safety evaluation , which have been described separately .
subjects had to have received as03-adjuvanted a / h1n1pdm09 influenza vaccine ( arepanrix ) in late 2009 .
exclusion criteria included : allergy to egg or other vaccine constituent ; bleeding disorder ; pregnancy ; immune compromise from medication or illness ; recent blood product infusion ; or prior receipt of a 20102011 influenza vaccine .
the study was conducted in accordance with the code of ethics of the world medical association ( declaration of helsinki ) and was approved by the research ethics board of each participating institution .
the clinicaltrials.gov registry identifier was nct01140009 . a web - based central randomization method ( daciforms )
was used to assign subjects ( 1:1 ) to receive influenza vaccine or placebo at the first visit .
randomization assignments were computer - generated ( proc plan in sas , sas institute ) in balanced blocks of four and stratified by sex and age ( 2039 and 4059 y ) .
participants were given the opposite assignment at visit 2 , ten days later , with masking maintained .
the study vaccine ( fluviral ) was an egg - derived , formalin - inactivated , detergent - split preparation , containing thimerosal as preservative , formulated for the 201011 northern hemisphere winter season .
it contained 15 g of hemagglutinin from each of a / california/7/2009 ( h1n1v)-like , a / perth/16/2009 ( h3n2)-like and b / brisbane/60/2008-like viruses , per 0.5 ml dose . a single lot was used ( # aflla574ab ) , obtained prior to commercial sale authorization with approval of the biologic and genetic therapies directorate of health canada .
injectables were prepared out of sight of subjects ; vaccine and placebo were identical in appearance .
both were stored at 28c and injected into the deltoid muscle using 25 gauge needles , one inch ( 25 mm ) long .
only the nurse who administered injections had access to the treatment information ; all other study staff remained blinded , including safety interviewers .
they were given instructions about using a daily symptom diary ( supplied ) to record any solicited or unsolicited local , respiratory or general symptoms , including fever , for 6 d after each injection .
an electronic thermometer was supplied to measure oral temperature , as was a device to measure injection site redness and swelling .
subjects were contacted by telephone one and seven days after each injection to review and record any symptoms .
day 7 interviews included a question about which injection ( vaccine or placebo ) subjects thought that they had received , with blinding maintained .
health care utilization and symptom resolution were reviewed during each clinic visit and throughout the period of study participation .
safety information was entered into a secure , web - based reporting system ( daciforms ) within 24 h after each encounter and reviewed centrally for completeness .
a study statistician tallied severe adverse event numbers daily and cumulatively , without breaking the assignment code , to monitor for any increased vaccine reactogenicity .
a data safety monitoring board ( dsmb ) reviewed severe adverse event rates after the first round of injections , before the second round was undertaken .
blood samples ( 810 ml ) were obtained from subjects at study entry and 2130 d after the tiv vaccination . to determine the correct sampling date it was necessary to un - blind the group assignment after safety assessments were completed 7 d after the second injection .
blood samples were processed promptly and sera were stored at -20c or colder pending testing at the national reference laboratory .
serum pairs were tested concurrently , in duplicate , using hemagglutination inhibition ( hai ) assays for each vaccine strain per who methods .
live viruses were used for influenza a assays and ether - treated viruses for influenza b assays .
results of duplicate tests on samples were expressed as the geometric mean titer ( gmt ) .
the cumulative rate of individual symptoms after placebo was subtracted from the rate after vaccination to determine the vaccine - attributable rate .
symptom severity ratings were provided by subjects according to criteria printed on the diary forms .
severe symptom rates were the primary safety outcome , defined as fever > 40.0c , injection site redness or swelling  100 mm diameter or any symptom that precluded normal daily activities or prompted medical attention .
serologic responses were analyzed according to standard international ( emea / chmp ) criteria for adults < 60 y old given seasonal influenza vaccine .
seroprotection was considered the primary serologic outcome measure , defined as an hai titer  40 .
geometric mean titers were calculated , with any test - negative sample assigned to value of 5 .
geometric mean fold rise was calculated as the within - subjects ratios of the post - vaccination hai titer to the baseline hai titer .
the seroconversion rate was defined as either a 4-fold or greater titer rise or conversion from a negative baseline titer to one  40
. a sample size of 300 evaluable subjects was desired to permit detection of severe or unusual adverse events at rates as low as 1% with 90% probability .
event rate differences after vaccine and placebo  6% could be detected with 80% probability (  = 0.05 ) with 300 observations after each treatment , using fisher s exact test . to allow for drop - outs , the intended enrollment was 320 subjects , 64 per center .
the data from this study showed that adults given a low - dose , aso3-adjuvanted h1n1pdm09 vaccine in 2009 had substantially reduced rates of seroprotection ( hai titers  40 ) 910 mo later .
while the low - dose formulation was intended to be dose - sparing , the short duration of protection could have been problematic had the pandemic been more prolonged .
however , individuals given a subsequent non - adjuvanted dose of tiv containing the h1n1pdm09 antigen boosted well , reflecting excellent immune memory .
the second vaccination with the h1n1pdm09 antigen was generally well tolerated and did not generate unusually high anti - h1n1 titers .
the data from this study showed that adults given a low - dose , aso3-adjuvanted h1n1pdm09 vaccine in 2009 had substantially reduced rates of seroprotection ( hai titers  40 ) 910 mo later .
while the low - dose formulation was intended to be dose - sparing , the short duration of protection could have been problematic had the pandemic been more prolonged .
however , individuals given a subsequent non - adjuvanted dose of tiv containing the h1n1pdm09 antigen boosted well , reflecting excellent immune memory .
the second vaccination with the h1n1pdm09 antigen was generally well tolerated and did not generate unusually high anti - h1n1 titers .
 many canadians received a novel as03-adjuvanted vaccine during the 2009 influenza a / h1n1 pandemic . 
 longer term implications of adjuvant use were unclear : would anti - h1n1 immune responses persist at high levels and , if so , could that result in increased or unusual adverse effects upon re - exposure to h1n1pdm09 antigen in the trivalent influenza vaccine ( tiv ) for 201011 ? to answer these questions , adults given as03-adjuvanted h1n1pdm09 vaccine ( arepanrix , gsk canada ) 910 mo earlier were enrolled in an evaluator - blinded , crossover trial to receive 20102011 non - adjuvanted tiv ( fluviral , gsk canada ) and placebo 10 d apart , in random order . 
 adverse effects were monitored for 7 d after each injection . 
 vaccine - attributable adverse event ( vaae ) rates were calculated by subtracting rates after placebo from those after vaccine . 
 blood was obtained pre - vaccination and 2130 d afterward to measure hemagglutination inhibiting antibody titers . in total , 326 participants were enrolled and 321 completed the study . 
 vaae rates were low except for myalgia ( 18.6% ) and injection site pain ( 63.2% ) . at baseline , 
 h1n1pdm09 titers  40 were present in 176/325 subjects ( 54.2% , 95% confidence interval 48.6 , 59.7 ) , with a geometric mean titer ( gmt ) of 37.4 ( 95% ci 32.8 , 42.6 ) . 
 post - immunization , 96.0% ( 95% ci 92.3 , 97.8 ) had h1n1pdm09 titers  40 , with gmt of 167.4 ( 95% ci 148.7 , 188.5 ) . 
 responses to both influenza a strains in tiv were similar , implying no lasting effect of adjuvant exposure . 
 in summary , titers  40 persisted in only half the participants 910 mo after adjuvanted pandemic vaccine but were restored in nearly all after tiv vaccination , with minimal increase in adverse effects . 
Introduction
Results
Safety observations
Immunogenicity data
Responses to A/H3N2 and B components of the vaccine are included in 
Discussion
Methods and Participants
Conclusion