Safety and immunogenicity of an oral, replicating adenovirus serotype 4 vector vaccine for H5N1 influenza: a randomised, double-blind, placebo-controlled, phase 1 study

Abstract

Background: Replication-competent virus vector vaccines might have advantages compared with non-replicating vector vaccines. We tested the safety and immunogenicity of an oral adenovirus serotype 4 vector vaccine candidate (Ad4-H5-Vtn) expressing the haemagglutinin from an avian influenza A H5N1 virus.

Methods: We did this phase 1 study at four sites in the USA. We used a computer-generated randomisation list (block size eight, stratified by site) to assign healthy volunteers aged 18-40 years to receive one of five doses of Ad4-H5-Vtn (10(7) viral particles [VP], 10(8) VP, 10(9) VP, 10(10) VP, 10(11) VP) or placebo (3:1). Vaccine or placebo was given on three occasions, about 56 days apart. Participants, investigators, and study-site personnel were masked to assignment throughout the study. Subsequently, volunteers received a boost dose with 90 μg of an inactivated parenteral H5N1 vaccine. Primary immunogenicity endpoints were seroconversion by haemagglutination-inhibition (HAI), defined as a four-times rise compared with baseline titre, and HAI geometric mean titre (GMT). We solicited symptoms of reactogenicity daily for 7 days after each vaccination and recorded symptoms that persisted beyond 7 days as adverse events. Primary analysis was per protocol. This trial is registered with ClinicalTrials.gov, number NCT01006798.

Findings: We enrolled 166 participants (125 vaccine; 41 placebo) between Oct 19, 2009, and Sept 9, 2010. HAI responses were low: 13 of 123 vaccinees (11%, 95% CI 6-17) and three of 41 placebo recipients (7%, 2-20) seroconverted. HAI GMT was 6 (95% CI 5-7) for vaccinees, and 5 (5-6) for placebo recipients. However, when inactivated H5N1 vaccine became available, one H5N1 boost was offered to all participants. In this substudy, HAI seroconversion occurred in 19 of 19 participants in the 10(11) VP cohort (100%; 95% CI 82-100) and eight of 22 placebo recipients (36%; 17-59); 17 of 19 participants in the 10(11) VP cohort (89%; 67-99) achieved seroprotection compared with four of 22 placebo recipients (18%; 5-40); GMT was 135 (89-205) with 10(11) VP, compared with 13 (7-21) with placebo. The cumulative frequency of abdominal pain, diarrhoea, and nasal congestion after all three vaccinations was significantly higher in vaccinees than placebo recipients (21 [16·8%] of 125 vs one [2·4%] of 41, p=0·017; 24 [19·2%] of 125 vs two [4·9%] of 41, p=0·027; 41 [32·8%] of 125 vs six [14·6%] of 41, p=0·028; respectively). No serious treatment-related adverse events occurred.

Interpretation: Oral Ad4 vector priming might enhance the efficacy of poorly immunogenic vaccines such as H5N1.

Funding: Wellcome Trust Foundation, PaxVax.

Figure 1

Trial profile Vaccination refers to oral administration of Ad4-H5-Vtn or oral placebo; boost vaccination refers to parenteral administration of subvirion inactivated H5N1 vaccine. Discontinuation means that a participant either dropped out of the study, or stopped vaccinations but continued follow-up. *Some participants who discontinued or missed vaccinations later received the boost vaccination. †Early entrants to the study were required to sign an additional consent form to receive the third vaccination but some declined to do so.

Figure 2

Vaccine take and cellular immune response by vaccination and by median number of ELISPOT forming cells Cumulative vaccine take (A) defined as Ad4 seroconversion or a PCR-positive rectal swab at 7 days or 14 days after vaccination, or both; cumulative percentage of participants with at least one take after one, two, and three Ad4-H5-Vtn vaccinations. Vaccine take components (B) partitioned to show proportion of participants who had PCR-positive rectal swab, Ad4 seroconversion, or both after any vaccination; data not available for placebo recipients, who were not assessed for shedding of vaccine virus. Cumulative percentage of participants with an ELISPOT interferon-γ response (C), defined as 80 or more spot forming cells (SFC) and four-times greater than baseline SFC, after one, two, and three vaccinations. Median number of ELISPOT SFC/10⁶ cells at baseline and after each vaccination (D); error bars show IQR. ELISPOT=enzyme-linked immunosorbent spot.

Figure 3

Effect of pre-existing Ad4 immunity on occurrence of take, cellular immune response before boost, and seroconversion by HAI and H5 haemagglutininin microneutralisation after boost For vaccine take (A), defined as Ad4 seroconversion or a PCR-positive rectal swab at 7 or 14 days after vaccination, or both, bar height represents the cumulative percentage of participants with a take at any time before study completion or receipt of boost vaccination. Analogous percentages for ELISPOT interferon-γ response before boost, defined as 80 or more spot forming cells (SFC) and four-times greater than baseline SFC, are shown (B). HAI seroconversion after boost (C) and H5 haemagglutinin microneutralisation (D) required a four-times rise compared with the last result before boost. The number of responders and the number of participants with evaluable samples are given above each bar. Pre-existing Ad4 immunity was defined as an Ad4 microneutralisation titre greater than 6 before initial vaccination.