double-blind, placebo-controlled, phase 2 trial
This is the first randomised controlled trial for assessment of the immunogenicity and safety of a candidate non-replicating adenovirus type-5 (Ad5)-vectored COVID-19 vaccine, aiming to determine an appropriate dose of the candidate vaccine for an efficacy study.
This randomised, double-blind, placebo-controlled, phase 2 trial of the Ad5-vectored COVID-19 vaccine was done in a single centre in Wuhan, China. Healthy adults aged 18 years or older, who were HIV-negative and previous severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection-free, were eligible to participate and were randomly assigned to receive the vaccine at a dose of 1 × 1011 viral particles per mL or 5 × 1010 viral particles per mL, or placebo. Investigators allocated participants at a ratio of 2:1:1 to receive a single injection intramuscularly in the arm. The randomisation list (block size 4) was generated by an independent statistician. Participants, investigators, and staff undertaking laboratory analyses were masked to group allocation. The primary endpoints for immunogenicity were the geometric mean titres (GMTs) of specific ELISA antibody responses to the receptor binding domain (RBD) and neutralising antibody responses at day 28. The primary endpoint for safety evaluation was the incidence of adverse reactions within 14 days. All recruited participants who received at least one dose were included in the primary and safety analyses.
603 volunteers were recruited and screened for eligibility between April 11 and 16, 2020. 508 eligible participants (50% male; mean age 39·7 years, SD 12·5) consented to participate in the trial and were randomly assigned to receive the vaccine (1 × 1011 viral particles n=253; 5 × 1010 viral particles n=129) or placebo (n=126). In the 1 × 1011 and 5 × 1010 viral particles dose groups, the RBD-specific ELISA antibodies peaked at 656·5 (95% CI 575·2–749·2) and 571·0 (467·6–697·3), with seroconversion rates at 96% (95% CI 93–98) and 97% (92–99), respectively, at day 28. Both doses of the vaccine induced significant neutralising antibody responses to live SARS-CoV-2, with GMTs of 19·5 (95% CI 16·8–22·7) and 18·3 (14·4–23·3) in participants receiving 1 × 1011 and 5 × 1010 viral particles, respectively. Specific interferon γ enzyme-linked immunospot assay responses post vaccination were observed in 227 (90%, 95% CI 85–93) of 253 and 113 (88%, 81–92) of 129 participants in the 1 × 1011 and 5 × 1010 viral particles dose groups, respectively. Solicited adverse reactions were reported by 183 (72%) of 253 and 96 (74%) of 129 participants in the 1 × 1011 and 5 × 1010 viral particles dose groups, respectively. Severe adverse reactions were reported by 24 (9%) participants in the 1 × 1011 viral particles dose group and one (1%) participant in the 5 × 1010 viral particles dose group. No serious adverse reactions were documented.
The Ad5-vectored COVID-19 vaccine at 5 × 1010 viral particles is safe, and induced significant immune responses in the majority of recipients after a single immunisation.
This study is the first randomised controlled trial for evaluation of the immunogenicity and safety of a candidate non-replicating Ad5-vectored COVID-19 vaccine. The phase 2 trial is a necessary and crucial step to turn an early-stage experimental vaccine into a promising vaccine candidate in an efficacy trial in a large population. In this study, a single injection of the Ad5-vectored COVID-19 vaccine at 1 × 1011 viral particles and 5 × 1010 viral particles induced comparable specific immune responses to the spike glycoprotein at day 28, with no significant differences noted between the two groups. The vaccine induced seroconversion of the neutralising antibodies in 59% and 47% of participants, and seroconversion of binding antibody in 96% and 97% of participants, in the 1 × 1011 and 5 × 1010 viral particles dose groups, respectively. Positive specific T-cell responses measured by IFNγ-ELISpot were found in 90% and 88% of participants receiving the vaccine at 1 × 1011 and 5 × 1010 viral particles, respectively. 95% of participants in the 1 × 1011 viral particles dose group and 91% of the recipients in the 5 × 1010 viral particles dose group showed either cellular or humoral immune responses at day 28 post vaccination. Pre-existing immunity to the Ad5 vector and increasing age could partially hamper the specific immune responses to vaccination, particularly for the humoral immune responses.
In this study, most reactions reported post vaccination were mild or moderate. Although the proportions of participants who had adverse reactions such as fever, fatigue, and injection site pain were significantly higher in vaccine recipients than those in placebo recipients, adverse reactions within 28 days were generally not severe, and resolved within a short period of time (no more than 48 h). Among recipients of either dose of the Ad5-vectored COVID-19 vaccines, all grade 3 adverse reactions were reported from the dose group of 1 × 1011 viral particles, with the exception of one from the 5 × 1010 viral particles dose group. Although this study was powered only to capture common adverse events after immunisation, the results suggest that the experimental Ad5-vectored COVID-19 vaccine has a good safety profile, which was in line with the results of our phase 1 trial in healthy adults. We initiated this phase 2 trial before the full analysis of the data from the phase 1 study was available. The vaccine doses chosen were mainly based on the safety data from the dose-escalating phase 1 trial: a higher proportion of participants reported grade 3 adverse reactions in the high-dose group (1·5 × 1011 viral particles) compared with low-dose (5 × 1010 viral particles) or middle-dose (1 × 1011 viral particles) groups (17% vs 6% and 6%, respectively).Therefore, we assumed that the vaccine doses at 1 × 1011 and 5 × 1010 viral particles might have similar safety profiles. In addition, an increasing antigen dose is often associated with increasing immunogenicity; thus, we expected the vaccine dose at 1 × 1011 viral particles to be the better of the two doses. Therefore, we designed the randomisation of this study in a ratio of 2:1:1 for the dose groups of 1 × 1011 and 5 × 1010 viral particles, and placebo group, respectively, giving more weight to the first group. We found, in contrast to our expectations, that the vaccine at 5 × 1010 viral particles had a better safety profile than, and comparable immunogenicity to, the vaccine at 1 × 1011 viral particles.
Age and pre-existing anti-Ad5 immunity of the participants could have affected the candidate vaccine's safety and immunogenicity. We noted that the occurrence of fever was associated with decreasing age and low pre-existing immunity to the vaccine vector Ad5 virus. 19 (90%) of the 21 participants who experienced grade 3 fever had no pre-existing immunity to Ad5, with a neutralising antibody titre below detection. Increasing age and high pre-existing anti-Ad5 immunity were found to be able to significantly reduce the immune responses to the vaccine. In some participants with high pre-existing anti-Ad5 immunity, one injection of the vaccine might be inadequate to induce a high level of humoral immune responses, particularly for people aged 55 years or older. These results align with the finding that older people are more likely to have exposure history to Ad5, with higher baseline neutralising antibody to Ad5, which indicates that this population might be more tolerant of higher dose or a booster dose regimen of the Ad5-vectored COVID-19 vaccine than people who are young and naive to Ad5. Pre-existing anti-Ad5 immunity is considered to be the biggest obstacle for the candidate Ad5-vectored COVID-19 vaccine to overcome. A flexible additional dose (between months 3 and 6) might be a potential solution to provide enhancement of immune responses, according to our previous experience with an Ad5 vector-based Ebola vaccine in a homologous prime-boost immunisation study.More evidence about the immunogenicity and feasibility of additional dose immunisation in the older population will be evaluated in a phase 2b trial. However, the vaccine recipients in this study showed increased anti-Ad5 neutralising antibodies, by 5·0-times and 3·8-times in the 1 × 1011 viral particles and 5 × 1010 viral particles dose groups, respectively, at day 28 post vaccination. The high level of anti-Ad5 immunity could affect the boosting effect of the vaccine; therefore, we planned to follow the dynamic change of the Ad5-specific antibodies in participants until month 6 to determine the timing of booster administration.
We determined the participants' serostatus before and after immunisation by ELISA, neutralising tests to live SARS-CoV-2 or pseudovirus, and ELISpot, providing evidence of humoral and cell-mediated immunity for the candidate vaccine. Because the live virus neutralising antibody test needs to be done in biosafety level three laboratories, a pseudovirus neutralising antibody test was developed to serve as an alternative.However, we found the magnitudes of neutralising antibody responses to pseudovirus were greater than those to the live virus, which might be associated with the different methodological principles of the two tests. In the pseudovirus neutralising test, when the specific antibody in serum binds to the pseudovirus, it inhibits the pseudovirus from entering the cells, reducing the expression of luciferase on the cell surface. Thus, we can calculate the amount of neutralising antibodies to pseudovirus by detecting the total fluorescence. The neutralising antibody is detected by measuring the cytopathic effect after the viral infection. Although, generally, the output values of the two methods are correlated, the two methods have different detection sensitivities and the detection values do not always have a one-to-one corresponding relationship.
Both neutralising antibody and T-cell responses were important in eliminating the virus and controlling disease development in patients with COVID-19 who were naturally infected by SARS-CoV-2.Antibodies are very likely to be effective against SARS-CoV-2, considering that convalescent serum samples have been applied with apparently good clinical results in COVID-19.But for the vaccine-induced immune responses, whether neutralising antibody alone is capable of preventing infection remains undetermined. Specific T-cell responses are essential for directly attacking and killing virus-infected cells.In addition, the CD4 T-cell responses are critical for the cytotoxic T-cell response and the maturating of neutralising antibodies.Thus, the evaluation of the cell-mediated responses, in addition to the neutralising antibodies, is important for a successful candidate vaccine.
Our trial has some limitations. First, this phase 2 trial started before the full analysis of the data from the phase 1 study was available, so we did not calculate the sample size based on study power in advance, which might lead to a lack of power to show the difference between dose groups. Second, the participants included in this study are all from Wuhan, China. The baseline anti-Ad5 immunity of the participants seemed to be representative of Chinese adults, according to the previously reported studies; however, anti-Ad5 immunity in adults varies from place to place globally, with reported proportions of adults with neutralising antibodies titres for Ad5 of more than 1:200 of about 80% in India, 78% in Kenya, 67% in Thailand, 64% in Uganda, around 60% in South Africa, 45% in Sierra Leone, and less than 30% in the USA.We might expect the candidate Ad5-vectored vaccine to have a superior immunogenicity in the population with a lower pre-existing anti-Ad5 immunity, but an inferior immunogenicity in people with a higher pre-existing anti-Ad5 immunity than observed in this phase 2 trial. Third, this trial did not include children. Although COVID-19 appears to have a more benign course in children, with almost no fatalities reported,an ideal candidate vaccine for the ongoing pandemic should cover susceptible populations in all ages. Fourth, we report only data within 28 days of vaccination, and do not include data about the durability of the vaccine-induced immunity, which was not available at the time of publication. A subset of patients infected with SARS-CoV-2 might not develop long-lasting antibodies to the virus, and S-antibodies were reported to rapidly decline for people infected with seasonal coronaviruses and who recovered from COVID-19, especially those with mild symptoms or asymptomatic infection. The ongoing phase 1 and 2 trials will enable the continued collection of safety data and assessment of antibody persistence over a 6-month period. Fifth, in this study, no participants had SARS-CoV-2 exposure after the vaccination, so we were unable to assess the efficacy of the candidate vaccine or any immunological risk associated with antibody induced by vaccination when exposed to the virus. However, the risk of COVID-19 and antibody-enhanced disease on exposure to the virus will be monitored long term. Finally, the clinical significance of these changes is difficult to assess because of the absence of an identified correlate of protective immunity, and reference standards for measuring neutralising antibodies against COVID-19. Future studies will need to establish an immune correlate of protection and a protective threshold to assess the feasibility of using the Ad5-vectored COVID-19 vaccine to provide protection for high-risk populations or for outbreak intervention.
WHO is facilitating collaboration and efforts to support vaccine development by defining the desired characteristics of promising candidate vaccines to combat COVID-19.This situation emphasises the importance of being prepared to initiate an international multicentre, randomised, double-blind, controlled phase 3 effectiveness trial as soon as possible.One immunisation of the Ad5-vectored COVID-19 vaccine at 5 × 1010 viral particles has a good safety profile (limited to common adverse reactions following immunisation) and could elicit significant specific immune responses to SARS-CoV-2, making it a potential candidate for emergency vaccination of acute protective response.
In conclusion, the results of this trial have extended our knowledge of the immunogenicity and safety of the Ad5-vectored COVID-19 vaccines. The results support testing of the Ad5-vectored COVID-19 vaccine at 5 × 1010 viral particles in a phase 3 effectiveness trial in healthy adults.
Reference & Source information: https://www.thelancet.com/
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