A total of 318 healthy full-term infants (161 males and 157 females) born between 26 July 2006 and 8 March 2007 were enrolled in the study and randomized into groups (). Maternal and cord blood samples were collected from 307 subjects, cord blood only was collected from 10, and maternal blood only was collected from 1. Seventeen infants were withdrawn from the study before the first injection: 10 parents changed their minds, 5 were ill on the day of vaccination, and 2 moved out of the area. As a result, 301 infants comprised the study group, with 100 in the Vi-rEPA group, 101 in the Hib-TT group, and 100 in the EPI group (). Another 60 were withdrawn or lost to follow-up, including 7 after the first injection, 11 after the second, 35 after the third, and 7 after the fourth: 26 parents withdrew because of concerns about the child's well-being (43.3%), 16 withdrew due to illness on the vaccination day (26.6%), 10 refused blood sampling (16.6%), 3 moved out of the area (5.0%), 3 gave no reasons (5.0%), and 2 withdrew due to the child's death (3.3%).
Infants' gestational ages ranged from 37 to 44 weeks, and birth weights were ≥2,500 g (median, 3,000 g). All had a normal neonatal course. Mothers' ages ranged from 17 to 38 years (median, 24 years). The median maternal ages, gestational ages, birth weights, and maternal ages were similar among the three groups.
Three hundred one infants received the first injection, 294 the second, 283 the third, and 167 the fourth of either Vi-rEPA or Hib-TT. The numbers of infants who received the first, second, and third injections were similar among the three groups, and the numbers that received the fourth injection were similar between the Vi-rEPA and Hib-TT groups (). The mean age at each injection for each of the three groups was 76 to 77 days for the first injection (range, 61 to 92 days), 137 days for the second (range, 120 to 154 days), 197 to 198 days for the third (range, 181 to 212 days), and 381 to 382 days for the fourth (range, 365 to 420 days).
There were no vaccine-related serious adverse events (). Only the highest temperature and maximal local reaction for each individual were used for analyses. No differences were found among the three groups. The most common minor adverse reaction was mild fever (temperature of 38.0 to 38.4°C) on the day of first injection, which subsided the next day, in 14 to 18% of vaccinees of each vaccine group.
Adverse reactions after vaccinationa
After the first injection, 63 (20.9%) infants had a temperature of ≥38.0°C (maximum, 39.8°C): 22 in the Vi-rEPA group, 24 in the Hib-TT group, and 17 in the EPI group. This fever lasted for <24 h in 59 vaccinees and for 24 to 48 h in 4 vaccinees. One infant in the Vi-rEPA group had a temperature of 39.8°C 6 h after injection which had returned to normal at 24 h. After the second injection, 20 infants had a temperature of ≥38.0°C (maximum, 39.2°C): 7 each in the Vi-rEPA and Hib-TT groups and 6 in the EPI group. This fever lasted for <24 h in 18 vaccinees and for 24 to 48 h in 2 vaccinees. After the third injection, 6 infants had a temperature between 38.0°C and 39.5°C: 4 in the Vi-rEPA group, 2 in the Hib-TT group, and none in the EPI group. All fevers lasted for <24 h. After the fourth injection, 2 infants in the Vi-rEPA group had a temperature of 38.0°C, and none of the infants in the Hib-TT group had a fever. No fever was associated with convulsions or intercurrent infections.
(ii) Local reactions.
Induration of ≥2.5 cm was observed at 14 injection sites (10 after the first injection and 4 after the second), with a maximal size of 4 cm. All but one occurred at the DTP site, and none lasted for >48 h. Erythema of ≥5 cm was observed at 10 injection sites (8 at the DTP site and 1 each at the Vi-rEPA and Hib-TT sites); 8 incidences were 5 to 7 cm, and 1 each at the DTP and Hib-TT sites was 15 cm. All subsided within 2 days.
(iii) Systemic reactions.
Inconsolable crying was reported for 17 infants after the first injection, lasting from 15 min to 6 h: seven cases lasted for <1 h, six for 1 to 2 h, one for 2.5 h, two for 5 h, and one for 6 h. Two deaths, not related to vaccination, were reported: a 5-month-old who received 1 injection of Vi-rEPA at 2.5 months of age died of septicemia following an operation for “nonfunctioning” kidneys, and a 4-month-old died of pneumonia 19 days after the second injection of Hib-TT.
Serum IgG anti-Vi.
The IgG anti-Vi GM for maternal and cord sera were similar among the 3 groups (). Among mothers who were ≥30 years of age, 20% had anti-Vi levels of ≥3.5 EU (estimated protective level), compared to 6% of those who were <30 years old (P = 0.001).
Serum IgG anti-Vi levels of vaccinees
At 7 months, the IgG anti-Vi GM for the Vi-rEPA group increased from the cord level of 0.66 to 17.42 EU (P < 0.001), with 90% of infants having levels of ≥3.5 EU. At 12 months, it dropped to 4.76 EU (P < 0.001), with 62% of infants having levels of ≥3.5 EU. At 13 months, 1 month after the fourth injection, the GM rose to 50.07 EU (P < 0.001), with 95% of infants having levels of ≥3.5 EU. The serum IgG anti-Vi GM for the Hib-TT and EPI groups, in contrast, declined from cord levels of 0.55 and 0.52 EU to 0.16 and 0.05 EU, respectively, at 7 months and remained at about these levels through 13 months of age. Two infants in the Hib-TT group and another two in the EPI group had elevated anti-Vi levels at 7 months. Anti-Vi levels of these 4 infants declined at 12 months; in 2 of the infants, anti-Vi levels were ≥3.5 EU. These two had no blood samples at 13 months. We were unable to verify whether these four infants received Vi-rEPA vaccine inadvertently or their serum samples were mislabeled.
Effect of cord IgG anti-Vi level on infant antibody response.
The estimated protective level (3.5 EU) was used to define high and low cord anti-Vi levels. Of the 90 infants in the Vi-rEPA group whose serum samples were collected at 7 months (1 month after the third injection), 81 were in the low group (IgG anti-Vi GM, 0.57 EU) and 9 were in the high group (IgG anti-Vi GM, 6.83 EU) (). For infants in the low group, the IgG anti-Vi GM increased from the cord level of 0.57 EU to 20.3 EU at 7 months, declined to 5.32 EU at 12 months, and rose to 60.5 EU at 13 months. For infants in the high group, however, an increase of IgG anti-Vi GM, from 1.87 to 9.13 EU, was not observed until 13 months. Nine infants in the EPI group and 6 in the Hib-TT group had high cord anti-Vi levels; their IgG anti-Vi GM declined from 11.23 EU at birth to 0.11 EU at 7 months.
Vaccinee responses to Vi-rEPA according to cord IgG anti-Vi levels
IgG anti-Vi GM after injections were significantly higher at all times for infants of the low cord group than for those of the high cord group. The percentage of infants that achieved a level of ≥3.5 EU was higher for infants of the low cord group than for those of the high cord group: 93.8% versus 55.6% (P = 0.001) at 7 months, 65.8% versus 33.3% (P = 0.006) at 12 months, and 97.2% versus 75.0% (P = 0.048) at 13 months. Multiple regression analysis was used to assess whether the cord and 12-month levels independently affected the response at 13 months. The results showed that cord and 12-month levels were independently associated with the response at 13 months (P < 0.001) and that high cord anti-Vi levels continued to partially suppress the response at 13 months.
Antibody responses to tetanus and diphtheria toxoids, pertussis toxin, and Hib CP.
There were no differences in IgG GM for antibodies to DT, PT, and Hib CP between the Vi-rEPA and EPI groups at all time points (). Recipients of Hib-TT had similar levels of DT and PT antibodies to those of the Vi-rEPA and EPI groups. At 7 months, all groups had similar levels of anti-TT. The levels declined at 12 months for all groups; infants of the Hib-TT group, however, had a higher anti-TT GM than those for the Vi-rEPA and EPI groups (0.87 versus 0.46 [P < 0.002] and 0.87 versus 0.53 [P = 0.001], respectively). At 13 months (1 month after the 4th injection), recipients of Hib-TT had higher levels of anti-TT than recipients of Vi-rEPA (5.15 versus 0.39 [P < 0.001]), likely due to the additional dose of Hib-TT. The 3 groups had similar cord anti-Hib IgG GM. There was a significant rise of anti-Hib 1 month after 3 injections of Hib-TT (8.35 μg/ml) that declined to 1.79 μg/ml at 12 months and rose to 24.6 μg/ml after the 4th injection. Similar to anti-Vi IgG responses, anti-Hib IgG responses were inversely correlated with the cord level (R2 = 0.92) (not shown).
Serum IgG antibodies to TT, DT, PT, and Hib CP of vaccinees