Immunogenicity and safety of an enterovirus 71 vaccine in children aged 36-71 months: A double-blind, randomised, similar vaccine-controlled, non-inferiority phase III trial.

Background: The enterovirus 71 (EV71) vaccine produced by Wuhan Institute of Biological Products Co., Ltd. (WIBP) (B-EV71) has been given to children aged 6-35 months, and it has shown good safety, immunogenicity and efficacy. However, the administration of EV71 vaccine in children aged 36-71 months, which is another target population, needs further exploration. Methods: We conducted a double-blind, randomised, controlled, non-inferiority phase III clinical trial in children aged 36-71 months, with a further comparison group of children aged 6-35 months in China. Children aged 6-71 months with no history of hand, foot and mouth disease or prior-vaccination of EV71 vaccine were eligible and recruited. Eligible participants aged 36-71 months were randomly assigned (1:1) to receive two doses of the B-EV71 vaccine (Older-B group) or the control EV71 vaccine (C-EV71 vaccine, produced by Institute of Medical Biology, Chinese Academy of Medical Sciences) (Older-C group), administered at a 30-day interval. Eligible participants aged 6-35 months were enrolled consecutively to receive two doses of the B-EV71 vaccine (Younger-B group) at a 30-day interval. Participants, investigators and those assessing outcomes were masked to the vaccine received. Non-inferiority analyses were conducted to compare the immunogenicity of EV71 vaccine in the Older-B group with that in the Older-C and Younger-B groups. Non-inferiority margins were 10% for seroconversion rate differences and 0.5 for geometric mean titre (GMT) ratios. The primary endpoints were the GMT level and seroconversion rate of anti-EV71 neutralising antibody 30 days after the second dose of vaccination. The primary analysis was performed in the per-protocol population. Safety analyses were conducted amongst participants receiving at least one dose of vaccine. This trial was registered at Chinadrugtrials.org.cn (#CTR20192345). Findings: Between June 3 and June 30, 2020, 1600 participants were enrolled and assigned, including 625 participants in the Older-B group, 625 participants in the Older-C group and 350 participants in the Younger-B group. The seroconversion rate of anti-EV71 neutralising antibody in the Older-B group (99.66%; 95% CI: 99.18%-100.00%) was non-inferior to that of the Older-C (99.32%; 95% CI: 98.65%-99.98%) and Younger-B groups (100.00%; 95% CI: 100.00%-100.00%). The differences in seroconversion rates in the Older-B group to those in the Older-C and Younger-B groups were 0.34% (95%CI: -2.17%-2.86%) and -0.34% (95%CI: -2.78%-2.09%). The GMT of the anti-EV71 neutralising antibody in the Older-B group (693.87) was also non-inferior to that in the Older-C (289.37) and Younger-B groups (634.80). The ratios of GMTs in the Older-B group to those in the Older-C and Younger-B groups were 2.67 (95%CI: 2.00-3.00) and 1.00 (95%CI: 0.75-1.00), respectively. The incidence of any adverse event (AE) related to vaccination was similar amongst the three groups (34/625 [5.44%] in the Older-B group, 32/623 [5.14%] in the Older-C group, and 26/349 [7.45%] in the Younger-B group), with only 2 (0.57%) participants having grade 3 AEs in the Younger-B group. Fifteen (0.94%) participants from these three groups had reported serious AEs (SAEs), all of which were unrelated to vaccines. Interpretation: EV71 vaccine produced by WIBP could extend to be administered to children aged 36-71 months against EV71 infection. However, the persistence of vaccine-induced immunities needs to be further investigated. Funding: Hubei Province's young medical talent program (20191229), Hubei Province's young talent program (2021), Hubei Province's young public health talent program (2021); and the Wuhan Institute of Biological Products Co., Ltd.