PRIBIVAC will assess a heterologous prime-boost-boost strategy in comparison with a homologous regimen in order to compare short and long-term immunogenicity of different COVID-19 vaccine combinations against the ancestral SARS-CoV-2 as well as different variants of concern (VOCs). Initial phases of the study (Phases A-C) have studied homologous versus heterologous vaccines at the first booster, later phases (Phase D) will study these as the second booster.

Hypothesis: One or more heterologous prime-boost-boost COVID-19 vaccine combinations will produce humoral and cellular immunity that is non-inferior to an homologous prime-boost-boost vaccination against wildtype SARS-CoV-2 and/or 1≥ VOC. In Phases A-C of the study the primary 2 dose mRNA vaccine series was defined as 'Prime-boost'. For phase D we will define these 2 doses as 'Prime' and the 3rd vaccine dose as 'Boost'.

The COVID-19 pandemic has spurred vaccine development and phase 1/2/3 clinical trials in record time. In Singapore, novel mRNA vaccines produced by Pfizer-BioNTech (BNT162b2) and Moderna (mRNA-1273) were the first COVID-19 vaccines to receive interim authorisation under the Pandemic Special Access Route (PSAR). Vaccination in Singapore started first for frontline and healthcare workers with BNT162b2 at the end of December 2020. Over the following months this vaccine program was extended first to older adults and then the general population, while mRNA-1273 was also introduced alongside BNT162b2.

While the pivotal phase 3 clinical trials of BNT162b2 and mRNA-1273 have reported a vaccine efficacy of >95% against symptomatic and severe disease, waning antibody levels and the emergence of variants of concern (VOCs) capable of evading protective immunity (from a wildtype SARS-CoV-2 virus vaccine) have raised the need for a long term COVID-19 immunisation strategy. The emergence of the highly transmissible Omicron VOC in November 2021 heralded a shift in booster vaccination policy in Singapore and overseas. First, recommendations for a third dose of an mRNA vaccine were strengthened and in Singapore receipt of a booster dose within 9 months of completing the primary vaccine series was mandated for maintenance of vaccinated status. Following these efforts booster vaccination uptake increased markedly, and currently 79% of the total population in Singapore has received their first booster vaccination dose. Secondly, recommendations changed to advise that individuals complete the standard vaccination regimen even if they have had prior COVID-19.

Although the original monovalent COVID-19 vaccines have proven effective at preventing death and severe disease, breakthrough infections and reinfections have become more common in the face of an evolving virus. The ever-mutating Omicron virus has resulted in several sublineages such as BA.1, BA.2, BA.4 and BA.5 that have caused surges in COVID-19 cases worldwide. Lab studies consistently suggest that antibodies triggered by vaccines (targeting the ancestral SARS-CoV-2 strain) are less effective at blocking BA.4 and BA.5 than earlier Omicron strains BA.1 and BA.2. This could leave previously infected and/or vaccinated and boosted individuals vulnerable to multiple Omicron infections. Thus, pharmaceutical companies have moved towards developing variant-based vaccines to ensure individuals maintain a high level of protection. On 31 August 2022, the US FDA authorized the bivalent formulations of the Moderna and Pfizer-BioNTech vaccines for use as a single booster dose at least two months following primary or booster vaccination. Since the first detection of the XBB Omicron subvariant in August 2022 in India, it has been detected in more than 17 countries to-date including Singapore, Australia, Bangladesh, Denmark, Japan and the US. To combat the rising COVID-19 infections and reinfections in Singapore, the bivalent Moderna/Spikevax and bivalent Pfizer-BioNTech vaccines were added to the National Vaccination Programme from October 2022.

Third-dose boosters increase humoral and cellular immunity, but the rapid waning of protection against symptomatic infection with VOCs such as Omicron has prompted countries to call for a fourth-dose vaccination program. However, the clinical need, effectiveness and timing of a fourth-dose vaccine booster remain to be assessed.

This study will assess the immunogenicity and safety of heterologous boost COVID-19 vaccine regimens compared with a homologous boost regimen. In the early phases of this study (Phase A to C) volunteers who previously received a homologous primary vaccine series with BNT162b2 (Comirnaty/Pfizer-BioNTech) or mRNA-1273 (Moderna) were enrolled. With robust data now available about the benefits of a first booster dose of the vaccine, phase D of the study will shift to investigating the second booster dose and will simplify eligibility criteria to reflect heterogeneity in vaccinations received among the Singapore population to date, and the high prevalence of COVID-19 infection during the Omicron wave.

In Phases A-C of this study, the booster vaccine for the control group was the homologous mRNA vaccine (e.g. BNT162b2 + BNT162b2 + BNT162b2 or mRNA-1273 + mRNA-1273 + mRNA-1273), while the heterologous vaccine candidates were an alternative COVID-19 vaccine. For individuals randomised to intervention group 1 the mRNA booster vaccine administered was heterologous to the primary series (e.g. BNT162b2 + BNT162b2 + mRNA-1273 or mRNA-1273 + mRNA-1273 + BNT162b2), while for other intervention groups this included vaccines of alternate designs:

Control group: Homologous mRNA booster vaccine Intervention group 1: Heterologous mRNA booster vaccine (Phase A) Intervention group 2: COVAXIN® (Phase B) Intervention group 3: Nuvaxovid (Phase C)

Vaccine candidates in phases A-C entered the study at different time points and participants were randomised at equal probability to the available intervention arms at the time of randomisation. This will reduce the risk of bias (e.g. participant preference for a certain arm) compared with a non-randomised design. While an ideal scenario is to have all the selected vaccines approved prior to the start of recruitment, due to the unknown time of availability of vaccine candidates A, B and C, and the urgency of the current situation, recruitment to the control and intervention group 1 (Phase A) was initiated from 12 Oct 2021. Intervention group 2 (Phase B) was added in January 2022 and discontinued 15 March 2022 per the recommendation from the Data Safety Monitoring Board, who reviewed the trial's preliminary results from the first 10 COVAXIN recipients up to Day 28 post-booster. Recruitment to Intervention group 3 (Phase C) was terminated prior to initiation due to delays in the availability of Nuvaxovid which despite receiving interim authorisation by HSA in February 2022 has only become available for administration in late May 2022.

As of 18 May 2022 uptake of the first booster has increased to 75% of the population - with a sharp increase in uptake from January 2022 following governmental policy changes to the definitions of vaccinated status. A significant proportion of those who have not taken their booster are expected to have either had COVID-19 during the Omicron wave, which severely restricts the ability of the study to continue as initially designed. Related to this, focus is shifting to longer term strategies with a second COVID-19 vaccine booster program beginning for selected individuals in Singapore in March 2022. Phase D of the study will examine this immune response following a second COVID-19 vaccine booster. Study participants will be randomised to receive one of three vaccines in equal proportions: Comirnaty or Cormirnaty Bivalent (Original/Omicron BA.4/5), Moderna/Spikevax or Moderna/Spikevax Bivalent (Original/Omicron BA.1) or Nuvaxovid. For analysis the reference point for determining homologous and heterologous vaccination regimen may include the primary vaccine series or first booster.

Inclusion Criteria:

1. Willing and able to provide informed consent for participation in this study;
2. Aged ≥21years at the time of study enrolment;
3. Received the second dose of BNT162b2 or mRNA-1273 Coronavirus Disease 2019 vaccines at least 6 months prior to enrolment;
4. Willing and able to comply with all scheduled visits, vaccination plan, laboratory tests and other study procedures.

For Phase D inclusion criteria 3 will be amended to 'Received the first booster dose of BNT162b2 or mRNA-1273 Coronavirus Disease 2019 vaccine at least 5 months prior to enrolment'. Only individuals recommended to receive a second booster dose under MOH recommendations will be enrolled.

Exclusion Criteria:

1. Known history of SARS-CoV-2 or SARS-CoV-1 infection;
2. Previously received an investigational coronavirus vaccine;
3. Previously received a SARS-CoV-2 monoclonal antibody;
4. Current or planned simultaneous participation in another interventional study;
5. A history of anaphylaxis, urticaria, or other significant adverse reaction requiring medical intervention after receipt of a COVID-19 vaccine, or otherwise have a contraindication to one of the available study vaccines per the approved label;
6. Individuals who are immunocompromised (e.g. active leukaemia or lymphoma, generalised malignancy, aplastic anaemia, solid organ transplant, bone marrow transplant, current radiation therapy congenital immunodeficiency, HIV/AIDS with CD4 lymphocyte count < 200 and patients on immunosuppressant medications);
7. Received systemic immunosuppressants or immune-modifying drugs for >14 days in total within 6 months prior to Screening (for corticosteroids >/= 20 milligram per day of prednisone equivalent). Topical tacrolimus is allowed if not used within 14 days prior to Day 1;
8. Individuals who are pregnant or breast feeding;
9. Chronic illness that, in the opinion of the study team, is at a stage where it might interfere with trial conduct or completion;
10. Deprived of freedom by an administrative or court order, or in an emergency setting, or hospitalised involuntarily;
11. Current alcohol abuse or drug addiction that might interfere with the ability to comply with trial procedures in the opinion of the study team;
12. Moderate or severe acute illness/infection (according to study team's judgement) on the day of vaccination, or febrile illness (temperature ≥ 37.5°C). A prospective participant should not be included in the study until the condition has resolved or the febrile event has subsided.

For Phase D of the trial exclusion criteria 1 and 4 will no longer apply: exclusion criteria 1 is no longer relevant as it not a consideration in MOH recommendations for COVID-19 booster vaccination, and exclusion criteria 4 because of the broad recommendations from MOH for keeping up to date with COVID-19 boosters.

All study findings and documents will be regarded as confidential. The investigators and other study personnel must not disclose such information without prior written approval from the PI. Participant confidentiality will be strictly maintained to the extent possible under the law and local hospital policy. Identifiable information will be removed from any published data.

Only study participants will be blinded to the vaccine allocation. This is to reduce the risk of bias in participant-reported AEs. The study participant will be unblinded at Day-28 visit (Visit 3).