serum samples were obtained during a nationwide influenza vaccine serologic study in taiwan that started in 2006 .
children ( < 5 years of age ) , adults ( 2049 years of age ) , older adults ( 5074 years of age ) , and elderly adults ( > 75 years of age ) were recruited .
serum samples were obtained immediately before and 3 weeks after intramuscular injection with 1 dose of nonadjuvanted , trivalent , inactivated influenza vaccine formulated for the 200809 northern hemisphere winter season ( samples were obtained from some participants > 75 years of age before and after receiving 1 dose of the vaccine formulated for the 200708 winter season ) .
microneutralization ( mn ) and hemagglutination inhibition ( hi ) assays were performed according to the world health organization manual on animal influenza diagnosis and surveillance ( 4 ) . using these assays with 0.75% guinea pig erythrocytes , we assayed samples for antibodies against a / california/07/2009 ( h1n1 ) virus .
the seroprotection rate was defined as the percentage of serum titers > 40 by hi or titers > 160 by mn .
the seroconversion rate was defined as the percentage of vaccine recipients whose serum hi titers or mn titers increased by at least 4-fold after vaccination .
stata software version 8.2 ( statacorp lp , college station , tx , usa ) was used for analysis .
a total of 176 participants ( 40 children , 36 adults , 50 older adults , and 50 elderly adults ) were enrolled ( table ) .
few or no preexisting cross - reactive antibodies against pandemic ( h1n1 ) 2009 virus were detected by hi assay in samples from children ( prevaccination seroprotection rate 0% ) .
after vaccination , seroprotection rates and geometric mean titers measured by hi assay were essentially unchanged but increased significantly in the 3 adult groups when measured by mn assay ( p<0.05 ) .
seroprotection rates determined by hemagglutination inhibition ( hi ) assay ( white bars ) or microneutralization ( mn ) assay ( gray bars ) and geometric mean titer ( gmt ) of antibodies against pandemic ( h1n1 ) 2009 virus in each 10-year age cohort , taiwan , 20072008 .
we log - transformed mn and hi titers , and used multiple regression , controlling for age groups to analyze the correlation between age and titer .
doubling of hi titers corresponded to an estimated 75% ( p<0.01 ) increment in mn titers adjusted by age .
when adjusted for hi titers , mn titers in older adults and elderly adults were 1.74 ( p<0.01 ) and 2 ( p<0.01 ) , respectively , those in adults .
older adults and elderly adults with the same hi titers were more likely to have higher mn titers than adults ( p<0.05 , by ordinal logistic regression analysis ) .
we found that children in taiwan had few or no cross - reactive antibodies against pandemic ( h1n1 ) 2009 virus .
a major finding was that 18 ( 36% ) of 50 elderly adults in taiwan born before 1935 had protective antibodies against currently circulating pandemic ( h1n1 ) 2009 virus .
the mn assay showed that seasonal influenza vaccines generated large increases in geometric mean titers in vaccinees in all age groups .
we suggest that seasonal influenza vaccines are likely to elicit a certain degree of cross - reactive antibodies against pandemic ( h1n1 ) 2009 virus and may provide some level of protection . in persons who had no preexisting seroprotective titers against pandemic ( h1n1 )
2009 virus , the cross - reactivity produced was not sufficient to prevent disease ; however , it may protect against the severe forms of the disease .
hancock et al . ( 5 ) reported that only 4% of persons in the united state born after 1980 had preexisting cross - reactive antibodies against pandemic ( h1n1 ) 2009 virus , and that 34% of persons born before 1950 had neutralizing titers > 80 .
however , itoh et al . ( 6 ) reported that blood donors from japan who were born after 1920 had almost no appreciable neutralizing antibodies against this virus . because the hemagglutinin gene of pandemic ( h1n1 ) 2009 virus is similar to that of viruses that circulated in humans during 19181957 ( 7 ) , itoh et al .
suggested that pandemic ( h1n1 ) 2009 virus is antigenically divergent from human influenza viruses ( h1n1 ) that circulated during the 1920s1950s .
( 5 ) , who suggested that human influenza virus ( h1n1 ) circulating in taiwan after 1920 resembled the 1918 pandemic virus ( h1n1 ) and pandemic ( h1n1 ) 2009 virus and could lead to cross - protection against the current virus . furthermore , unlike the situation in the united states , there was no program for vaccination against the 1976 swine influenza virus ( a / nj/76 ) in taiwan .
the results of our study also explain why only 7% of patients hospitalized for pandemic ( h1n1 ) 2009 virus in taiwan were > 65 years of age ( 8) .
similar epidemiologic observations have been reported in the united states ( 3 ) and new zealand ( 9 ) . whether elderly persons still have cross - reactivity several decades after exposure to 1918 ( h1n1 ) virus is unknown .
original antigenic sin has been described in relation to influenza virus , dengue virus , hiv , and several other viruses ( 1012 ) . for persons > 65 years of age
, the 1918 ( h1n1 ) virus is likely the first influenza virus to which they were exposed , and their antibody response should have increased in subsequent years .
regression analysis showed that older persons with high hi titers were more likely than younger adults to have higher mn titers .
because the hi assay detects only antibodies against hemagglutinin , the mn assay provides more information about the level of protective antibodies against influenza viruses . as a person ages , production of antibodies against hemagglutinin or other components of influenza virus should protect against potential infections .
although a titer of 40 by hi is accepted as a cutoff value for seroprotection , a consensus for protective titers by mn is lacking ( 5,13 ) .
we suggest that mn is probably more sensitive than hi for evaluating neutralizing antibodies against pandemic ( h1n1 ) 2009 virus .
 we studied preexisting immunity to pandemic ( h1n1 ) 2009 virus in persons in taiwan . 
 a total of 18 ( 36% ) of 50 elderly adults in taiwan born before 1935 had protective antibodies against currently circulating pandemic ( h1n1 ) 2009 virus . 
 seasonal influenza vaccines induced antibodies that did not protect against pandemic ( h1n1 ) 2009 virus . 
The Study
Conclusions