Safety and immunogenicity of a live-attenuated influenza virus vector-based intranasal SARS-CoV-2 vaccine in adults: randomised, double-blind, placebo-controlled, phase 1 and 2 trials

Abstract

Background: All currently available SARS-CoV-2 vaccines are administered by intramuscular injection. We aimed to evaluate the safety and immunogenicity of a live-attenuated influenza virus vector-based SARS-CoV-2 vaccine (dNS1-RBD) administered by intranasal spray in healthy adults.

Methods: We did double-blind, randomised, placebo-controlled phase 1 and 2 trials, followed by a phase 2 extension trial, at a single centre in Jiangsu, China. Healthy adults (≥18 years) who had negative serum or fingertip blood total antibody tests for SARS-CoV-2 (in phases 1 and 2), with no prevalent SARS-CoV-2 infection or history of infection and no SARS-CoV-2 vaccination history (in all three trials reported here), were enrolled. Participants were randomly allocated (4:1 in phase 1, 2:1 in phase 2, and 1:1 in the extension trial) to receive two intranasal doses of the dNS1-RBD vaccine or placebo on days 0 and 14 or, for half of the participants in phase 2, on days 0 and 21. To avoid cross-contamination during administration, vaccine and placebo recipients were vaccinated in separate rooms in the extension trial. The phase 1 primary outcome was safety (adverse events recorded on days 0-44; serious adverse events recorded from day 0 until 12 months after the second dose). In the phase 2 and extension trials, the primary immunogenicity outcomes were SARS-CoV-2-specific T-cell response in peripheral blood (measured by IFN-γ ELISpot), proportion of participants with positive conversion for SARS-CoV-2 receptor-binding domain (RBD)-specific IgG and secretory IgA (s-IgA) antibodies, and concentration of SARS-CoV-2 RBD IgG in serum and SARS-CoV-2 RBD s-IgA in the nasopharynx (measured by ELISA) at 1 month after the second dose in the per-protocol set for immunogenicity. χ² test and Fisher's exact test were used to analyse categorical data, and t test and Wilcoxon rank sum test to compare the measurement data between groups. These trials were registered with the Chinese Clinical Trial Registry (ChiCTR2000037782, ChiCTR2000039715, and ChiCTR2100048316).

Findings: Between Sept 1, 2020, and July 4, 2021, 63, 724, and 297 participants without a history of SARS-CoV-2 vaccination were enrolled in the phase 1, phase 2, and extension trials, respectively. At least one adverse reaction after vaccination was reported in 133 (19%) of 684 participants in the vaccine groups. Most adverse reactions were mild. No vaccine-related serious adverse event was noted. Specific T-cell immune responses were observed in 211 (46% [95% CI 42-51]) of 455 vaccine recipients in the phase 2 trial, and in 48 (40% [31-49]) of 120 vaccine recipients compared with one (1% [0-5]) of 111 placebo recipients (p<0·0001) in the extension trial. Seroconversion for RBD-specific IgG was observed in 48 (10% [95% CI 8-13]) of 466 vaccine recipients in the phase 2 trial (geometric mean titre [GMT] 3·8 [95% CI 3·4-4·3] in responders), and in 31 (22% [15-29]) of 143 vaccine recipients (GMT 4·4 [3·3-5·8]) and zero (0% [0-2]) of 147 placebo recipients (p<0·0001) in the extension trial. 57 (12% [95% CI 9-16]) of 466 vaccine recipients had positive conversion for RBD-specific s-IgA (GMT 3·8 [95% CI 3·5-4·1] in responders) in the phase 2 trial, as did 18 (13% [8-19]) of 143 vaccine recipients (GMT 5·2 [4·0-6·8]) and zero (0% [0-2]) of 147 placebo recipients (p<0·0001) in the extension trial.

Interpretation: dNS1-RBD was well tolerated in adults. Weak T-cell immunity in peripheral blood, as well as weak humoral and mucosal immune responses against SARS-CoV-2, were detected in vaccine recipients. Further studies are warranted to verify the safety and efficacy of intranasal vaccines as a potential supplement to current intramuscular SARS-CoV-2 vaccine pools. Steps should be taken in future studies to reduce the potential for cross-contamination caused by the vaccine strain aerosol during administration.

Funding: National Key Research and Development Program of China, National Science, Fujian Provincial Science, CAMS Innovation Fund for Medical Sciences, and Beijing Wantai Biological Pharmacy Enterprise.

Figure 1

Trial profile (A) In phase 1, participants were separated into two age cohorts with a sequential design. Enrolment of cohort 2 (participants aged ≥60 years) did not begin until a 7-day safety observation following the initial vaccination in cohort 1, supervised by the data monitoring committee, had been completed. (B) In phase 2, among the 16 participants who did not receive the second dose, one developed a transient high fever (39·9°C) within 48 h after the initial vaccination from the vaccine group; four met the exclusion criteria for vaccination (one from the vaccine group, three from the placebo group); and the other participants refused to attend the second vaccination visit. (C) In the phase 2 extension trial, subgroups of volunteers with and without a history of SARS-CoV-2 vaccination were recruited. Results for the subgroup who had previously received a SARS-CoV-2 vaccination will be reported elsewhere. Three participants in the vaccine group refused to attend the second vaccination visit.

Figure 1

Trial profile (A) In phase 1, participants were separated into two age cohorts with a sequential design. Enrolment of cohort 2 (participants aged ≥60 years) did not begin until a 7-day safety observation following the initial vaccination in cohort 1, supervised by the data monitoring committee, had been completed. (B) In phase 2, among the 16 participants who did not receive the second dose, one developed a transient high fever (39·9°C) within 48 h after the initial vaccination from the vaccine group; four met the exclusion criteria for vaccination (one from the vaccine group, three from the placebo group); and the other participants refused to attend the second vaccination visit. (C) In the phase 2 extension trial, subgroups of volunteers with and without a history of SARS-CoV-2 vaccination were recruited. Results for the subgroup who had previously received a SARS-CoV-2 vaccination will be reported elsewhere. Three participants in the vaccine group refused to attend the second vaccination visit.

Figure 2

Cross-contamination caused by the vaccine virus strain during administration As an intranasal spray vaccine (a), it is difficult to ensure zero leakage of the vaccine into the environment during administration; vaccine droplets or aerosol might be released into the air when the vaccinator removes bubbles from the spray syringe or when the medical waste recycling bin is repeatedly opened, especially in a small and enclosed vaccination room (b). During administration, participants must take off their mask and hold their face upwards at an angle of roughly 30–45° for 10–20 sec following vaccination for full absorption of the vaccine in the nasopharynx (c). A block randomisation design was used, with ratios of assignment of vaccine to placebo of 4:1 in phase 1 (A), 2:1 in phase 2 (B), and 1:1 in the extension trial (C). In phases 1 and 2, participants in both groups received the allocated treatment in the same room. In phase 1 (A), vaccine or placebo administration was completed within half a day for each dose so all participants were vaccinated at a similar time. In phase 2 (B), vaccine or placebo administration was done over 5 days for each dose, with 100–200 participants vaccinated per day. In the extension trial (C), participants were randomly assigned to one of four rooms for administration, with each room used for only one type of investigational product (vaccine or placebo). According to the results of environmental detection studies, only the two rooms used for vaccine administration had detectable vaccine virus strain in the air and on surfaces.

Figure 3

SARS-CoV-2 spike protein-specific cellular immune responses following vaccination in the phase 2 extension trial (A, B) Number of IFN-γ-secreting cells per 10⁶ PBMCs overall (A) and per age group in the vaccine group (B). Each datapoint represents the mean number of spots from triplicate stimulated wells for one participant after subtraction of the unstimulated control, with values less than 1 corrected to 1. Dotted lines indicate the cutoff for positive responses, defined as those in which the number of IFN-γ-secreting cells per 10⁶ PBMCs was more than 30 (and in which the number of spots in stimulated wells increased to at least 2·1-times that in unstimulated control). (C, D) Proportion of participants with positive responses to vaccination overall (C) and by age group within the vaccine group (D). Error bars are IQRs. PBMCs=peripheral blood mononuclear cells. *Analysed by paired t test. †Analysed by Wilcoxon rank-sum test. ‡Analysed by Fisher's exact test.