this single - center prospective observational study took place between 15 july 2014 and 29 february 2016 , following approval from the geneva ethics commission ( ccer 14096 ) . eligible participants were consenting pregnant women delivering before gw 37 0/7 at the university hospitals of geneva , switzerland . they had previously been vaccinated during one of their routine follow - up visits with tetanus - diphtheria- acellular pertussis ( tdap ) according to the official swiss recommendations ( any time after gw 13 0/7 ) . all women received boostrix ( glaxosmithkline ) , which contains 8 g of pertussis toxoid , 8 g of filamentous hemagglutinin , and 2.5 g of pertactin . gestational age , determined by early ultrasound , and vaccination date were obtained from the women s medical records . the sample size was based upon the expected number of preterm births in the study hospital within a continuous 19-month period to avoid influence of a potential change in pertussis prevalence over time . we collected cord blood samples of preterm neonates after maternal tdap immunization performed between gw 13 0/7 and 25 6/7 ( second trimester ) or after gw 25 6/7 ( third trimester ) . filamentous hemagglutinin ( fha ) antibodies by enzyme - linked immunosorbent assay and calculated geometric mean concentrations ( gmcs ) with their 95% confidence intervals ( cis ) . seronegativity was defined as anti - pt 5 eu / ml . characteristics of women in second- and third - trimester groups were described and compared using student t test , fisher exact test , or  test where appropriate . student t test with unequal variances was employed for comparisons of antibody gmcs and fisher exact test for seronegativity rates . we also performed a multivariable linear regression analysis to compare gmc ratios using potential confounding factors identified in our large study of term neonates   and clinical considerations ; all unbalanced epidemiological characteristics between the second- and third - trimester term groups ( maternal age , parity , socioeconomic status )   and gestational age at birth   were introduced in the model . antibody titers were transformed with the base-10 logarithmic function ; the regression coefficients were back - transformed and reported as gmc ratios . similarly , we determined the association of predefined time intervals between vaccination and delivery and birth antibody titers by analyzing the ratio of gmcs with 95% cis . stata software version 13.0 was used for statistical analysis and graphpad prism version 7 for graphs . between august 2014 and february 2016 , 544 women delivered before term at the university hospitals of geneva ( 2429 gw , 67 [ 11% ] ; 3033 gw , 116 [ 20% ] ; 3437 gw , 361 [ 63% ] ) , with a distribution of the gestational ages following the general epidemiology of preterm births in switzerland . preterm newborn pairs were included : 68 ( 80% ) were born between gw 34 0/7 and 36 6/7 , and 17 ( 20% ) between gw 30 0/7 and 33 6/7 . among 85 mothers , 37 had been immunized during the second trimester , and 48 during the third trimester . the mean intervals between vaccination and delivery were 97.1 ( standard deviation [ sd ] , 25.5 ) days for the second trimester and 29.6 ( sd , 21.9 ) days for third - trimester immunization . there were no statistically significant differences between the baseline clinical characteristics in the early ( second trimester ) and late pregnancy ( third trimester ) vaccination group ( supplementary table 1 ) . birth antibody gmcs were significantly higher after second- compared to third - trimester immunization for both anti - pt ( 41.3 [ 95% ci , 29.657.5 ] eu / ml vs 22.1 [ 95% ci 14.334.2 ] eu / ml ; p = .024 ) and anti - fha antibodies ( 201.1 [ 95% ci , 149.7270.1 ] eu / ml vs 120.2 [ 95% ci , 80.6179.2 ] eu / ml ; p = .040 ) ( figure 1 ) . the ratio of second- to third - trimester anti - pt antibodies was significantly higher ( 1.87 [ 95% ci , 1.063.29 ] ; p = .032 ) , even after adjustment for maternal age , gestational age at birth , parity , and socioeconomic status ( 2.04 [ 95% ci , 1.153.61 ] ; p = .016 ) . for anti - fha antibodies , the gmc ratio was 1.67 ( 95% ci , 1.002.81 ; p = .051 ) , with an adjusted ratio of 1.57 ( 95% ci , .932.67 ; p = .092 ) ( supplementary table 2 ) . dividing the population into early ( gw 30 0/733 6/7 ) and late ( gw 34 0/736 6/7 ) preterm neonates , even in those few born between gw 30 and 33 , a second - trimester maternal immunization seemed to elicit higher birth anti - pt and anti - fha antibody titers ( figure 1 ) . anti  pertussis toxin ( pt ) and anti filamentous hemagglutinin ( fha ) cord blood antibody concentrations by trimester of maternal immunization and gestational age at birth . individual birth anti - pt and anti - fha antibody concentrations after maternal tetanus - diphtheria - acellular pertussis immunization during the second trimester ( gestational week [ gw ] 13 0/725 6/7 ) or third trimester ( after gw 25 6/7 ) for all preterm neonates ( left ) or divided into early ( gw 30 0/733 6/7 , middle ) and late ( gw 34 0/736 6/7 , right ) preterm neonates ; each point corresponds to 1 patient . none of the 37 preterm neonates born after second - trimester maternal immunization were seronegative , compared with 11 of 48 ( 22.9% ; p = .002 ) in the third - trimester group . following third - trimester immunization , the proportion of seronegative preterm neonates was high in both age groups ( gw 30 0/733 6/7 , 38% ; gw 34 0/736 6/7 , 20% ) . we finally assessed the time interval between vaccination and delivery required to maximize maternofetal antibody transfer . an interval of 15 days was sufficient to observe significantly higher cord antibody titers in this preterm population ( supplementary table 4 ) . a growing number of seronegative neonates was observed as gestational age increased ( supplementary figure 1 ) . this report is the first to show that preterm neonates benefit from second- rather than third - trimester tdap maternal immunization . this effect is already seen for preterm births between gw 30 and 33 when antibody transfer was thought to be inefficient . these results are unexpected , as the greater efficacy of placental transfer during the third trimester is well established . this transfer is mediated through the neonatal fc receptor ( fcrn ) , expressed in the syncytiotrophoblast as of gw 13 , when antibody transfer begins . fcrn - mediated transport efficacy , which is both active and saturable , slowly increases with time as the cytotrophoblast becomes discontinuous . consequently , the proportion of fetal to maternal total immunoglobulin g ( igg ) antibody concentrations at birth increases steadily from 10% ( gw 1722 ) to 50% ( gw 2832 ) to 100% of maternal concentrations at term . following early pregnancy vaccination , maternal antibodies are thus likely transferred to the fetus with lower efficiency . we postulate this reduced daily transfer is compensated by the longer total transfer time , resulting in antibody accumulation in the fetal circulation . this longer transfer time may be the key element of the higher antibody levels following earlier maternal immunization . nevertheless , the half - life of anti - pt antibodies is short , such that antibodies elicited by maternal immunization early during pregnancy could wane prior to delivery and counterbalance the benefit of a prolonged in utero transfer . this suggests an additional mechanism protecting igg antibodies from degradation better in the fetal than the maternal compartments . the catabolism vs recycling of serum igg occurs in endothelial cells : only fcrn - bound igg antibodies are protected from lysosomal degradation and transported back into the serum . as fcrn - mediated transport is saturable , the catabolism of igg is faster at higher igg serum concentrations . we hypothesize that the low concentration of fetal total igg does not saturate the fcrn receptors , such that most maternal igg antibodies successfully bind to fcrn and are largely protected from degradation . the relative contribution of antibody transfer duration and protection against degradation may not be directly tested : whether this is valid for most or all antigens will come from impending studies comparing early vs late maternal immunization against other pathogens such as respiratory syncytial virus and group b streptococcus . first , its observational rather than randomized design does not rule out confounding factors influencing antibody transfer . such factors were unidentified in our larger study including term infants , but nevertheless may exist . information on prior tdap vaccination is limited to the last 5 years , and may not exclude occult exposure to pertussis . second , it does not include data for preterm neonates born before gw 30 0/7 and lacks power to detect an effect for preterm births between gw 30 0/7 and 33 6/7 . further studies are warranted to assess the benefits from early maternal immunization for very early preterm neonates . recruitment was based on recruitment capacity and was interrupted after 19 months so as not to increase heterogeneity in the baseline risks of maternal exposure to pertussis . no increase in the rate of pertussis cases the effect of trimester on antibody titers remained statistically significant in the multivariable analysis , supporting the validity of our observations . we only measured antibody levels as surrogates for infant pertussis protection ; demonstrating the effectiveness will require large ecological studies . in conclusion , the transplacental transfer of maternal antibodies is effective sufficiently early in gestation so that the majority of preterm neonates born between 30 0/7 and 36 6/7 gw benefit from a maternal tdap immunization performed during the second trimester of pregnancy . future recommendations on the optimal timing of maternal tdap immunization should give consideration to these findings to hopefully increase protection against pertussis in both term and preterm neonates .
abstractpreterm infants are most vulnerable to pertussis . 
 whether they might benefit from maternal immunization is unknown . extending our previous results in term neonates 
 , this observational study demonstrates that second- rather than third - trimester maternal vaccination results in higher birth anti 
 pertussis toxin titers in preterm neonates .