Coronavirus disease 2019 (COVID-19) is potentially a deadly disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that targets the lung mainly, resulting in respiratory tract infections in humans. It has developed into a pandemic with serious global public health problems.

Recent research has shown that the new SARS-CoV-2 variants reduces the efficacy of the vaccinations and are predominantly more transmissible or infective. A few countries namely Bahrain, United Arab Emirates, and Turkey have recently started introducing a booster dose following primary two doses of the COVID-19 immunization series.

This study aims to identify which booster dose is more effective; taking a booster dose from the same vaccine initially taken or a booster dose from a different vaccine than initially taken.

According to the World Health Organization COVID-19 Dashboard, the coronavirus disease 2019 pandemic, has caused over 181 million infections and more than 3 million deaths worldwide as of July 1, 2021. COVID-19 is potentially a deadly disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that targets the lung mainly resulting in respiratory tract infections in humans. This has become a serious concern for public health.

Among the currently approved COVID-19 vaccines in the Kingdom of Bahrain, BBIBP-CorV (inactivated virus) vaccine and BNT162b2 (mRNA vaccine) is being administered to the population.

Inactivated vaccines have been extensively studied. In a phase 1/2 trial, the BBIBP-CorV vaccine has shown to be generally safe against COVID-19 and induce antibody responses. However, WHO's Strategic Advisory Group of Experts (SAGE) experts have summarized information from clinical trials in Bahrain, United Arab Emirates, Egypt, Jordan, and China indicating that individuals with comorbidities and older adults (≥60 years) who received 2 doses of BBIBP-CorV have low confidence in the efficacy of preventing COVID-19.

Current clinical trials have played a key role in the approval of different COVID vaccines based on their efficacy data, however, there is still uncertainty regarding the duration of protection from these vaccines towards the COVID -19 virus. Recent evidence has shown that the new SARS-CoV-2 variants reduces the efficacy of the vaccinations and are predominantly more transmissible or infective.

A few countries namely Bahrain, United Arab Emirates, and Turkey have recently started introducing a booster dose following primary two doses of the COVID-19 immunization series. The enhanced humoral response has been seen in homologous vaccination. Heterologous vaccination has shown to significantly induce more immunogenicity than homologous vector boost, and higher or comparable to the homologous mRNA regimens. Strong humoral and immune response has also been induced by heterologous vector-mRNA boosting with an acceptable reactogenicity profile.

To our knowledge, there has been no research conducted to date on the reactogenic and immunogenetic response of a COVID-19 booster dose after completing the primary two doses of the COVID-19 immunization series. This study will compare the reactogenic and immunogenetic response of heterologous BNT162b2 booster dose after completing two doses of BBIBP-CorV vaccination versus homologous BBIBP-CorV booster after completing two doses of BBIBP-CorV vaccination.

Inclusion Criteria:

• Adults aged ≥21yo.
• Asymptomatic 24h before the administration of booster dose.
• Has no active or previous RT-PCR lab-confirmed COVID-19 diagnosis.
• Completed three months to six months after the second dose of BBIBP-CorV.
• Have at least one Antibody test done before receiving the BBIBP-CorV booster dose OR can be done if the participant is yet to receive the BNT162b2 booster dose.
• Tested negative using Rapid Antigen Detection Test on the day of receiving the booster (positive results will confirm with RT-PCR).
• Study participants must have the ability to give informed consent.

Exclusion Criteria:

• Children aged <21yo.
• Symptomatic within 24h before the administration of booster dose.
• Has active or previous RT-PCR lab-confirmed COVID-19 diagnosis.
• Did not complete three months to six months after the second dose of BBIBP-CorV.
• Does not have at least one Antibody test done before receiving the BBIBP-CorV booster dose
• Tested positive using Rapid Antigen Detection Test on the day of receiving the booster (positive results will be confirmed with PCR).
• Patients unable to give informed consent.

Adults aged ≥21yo. Asymptomatic 24h before the administration of booster dose. Has no active or previous RT-PCR lab-confirmed COVID-19 diagnosis. Tested negative using Rapid Antigen Detection Test on the day of receiving the booster

Monitoring, audits, and Research Ethics Committee review will be permitted and provide direct access to source data and documents. The Lead PI and the researchers assigned by him will have access to the stored data/specimens. Only the Lead PI and the researchers assigned working on this study will be eligible to obtain the data/specimens from the participants during data collection.

Dr Manaf will act as the data custodian and is responsible for the storage, handling and quality of the study data.

Data will be collected in the case report form to allow for cross referencing to check validity.

Study documents (paper and electronic) will be retained in a secure (kept locked when not in use) location during and after the trial has finished. All essential documents including source documents will be retained for a period of 5 years after study completion (last patient, last study point). A label stating the date after which the documents can be destroyed will be placed on the inside front cover of the case notes of trial participants.

Study documents (paper and electronic) will be retained in a secure (kept locked when not in use) location during and after the trial has finished.