Bivalent COVID-19 boosters offer stronger protection for seniors, Australian study reveals

By Hugo Francisco de SouzaReviewed by Benedette Cuffari, M.Sc.Nov 27 2023

In a recent study published in the journal Eurosurveillance, researchers investigate the efficacy of bivalent booster vaccine doses against Omicron, the most widespread and deadly severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant circulating in the world today.

Study: Effectiveness of bivalent COVID-19 boosters against COVID-19 mortality in people aged 65 years and older, Australia, November 2022 to May 2023. Image Credit: Aleksandra Suzi / Shutterstock.com

Monovalent vs. bivalent vaccines

Since the onset of the coronavirus disease 2019 (COVID-19) pandemic in late 2019, SARS-CoV-2 has infected over 772 million individuals and claimed almost seven million lives. While no specific cure exists, research has revealed that vaccination against SARS-CoV-2 can significantly reduce the risk of infection, morbidity, and mortality.

Unfortunately, SARS-CoV-2 is a rapidly evolving virus, with thousands of strains already identified. The surface protein compositions of these new strains may differ significantly from those of the ancestral variant, thus resulting in the reduced efficacy of conventional vaccines against novel viral variants. This, combined with the low observed half-life of vaccine-primed immune cells, necessitates administering booster doses even to previously vaccinated or infected individuals.

Conventional monovalent messenger ribonucleic acid (mRNA) vaccines were developed against the ancestral COVID-19 variants. However, recent research has focused on the development of bivalent mRNA vaccines, which, unlike their monovalent counterparts, contain both antigens against ancestral SARS-CoV-2, as well as variant-specific antigens against the prevalent viral variants present in the intended geographical region of vaccine administration. Despite the advantages of bivalent vaccines, there remains a lack of research into the efficacy of these interventions.

Elucidating differences in efficacy between conventional and bivalent vaccines will help inform researchers, policymakers, clinicians, and the general public, thereby allowing them to make optimal choices when choosing which vaccine to administer or receive.

About the study

The present study aimed to compare the relative vaccine effectiveness (rVE) of bivalent mRNA vaccines against the rVE of conventional monovalent COVID-19 vaccines. The study cohort comprised all Australian adults aged 65 or older, with data collated from the Australian Immunization Register.

The study was conducted between November 1, 2022, and May 31, 2023, during which 4,081,257 Australians were monitored for COVID-19-related morbidity and mortality. Data collection comprised vaccination status, which included the date and type of vaccine received, age, sex, residential jurisdiction, income, and medical records of comorbidities and clinician visits. Demographic data were used during analyses to adjust hazard ratios (HRs) to reveal population differences in rVE.

The study cohort consisted of 53.6% women with a mean age of 74.8 years. About 52% of the study cohort suffered from four or more COVID-19-associated comorbidities, and 77.8% received a COVID-19 vaccine in 2022.

Following initial vaccination, 66% of the study cohort received two booster doses, with 21.7% receiving one booster dose and 0.9% receiving three booster doses. The most commonly used booster vaccines were conventional monovalent mRNA vaccines containing ancestral SARS-CoV-2 antigens at 56.6%, whereas 16.8% bivalent vaccines included ancestral/BA.1 or ancestral/BA.4-5 antigens.

Study findings

Bivalent vaccines were associated with a rVE of 66% against COVID-19-associated mortality as compared to the 44.7% rVE of monovalent ancestral vaccines. Sensitivity analyses revealed that these findings were consistent, irrespective of HR adjustments for demographic variables or the number of boosters taken, provided the booster was obtained within the last 180 days before analyses.

Both the bivalent ancestral/BA.1 or ancestral/BA.4-5 booster provide significant protection against severe illness from COVID-19. However, there are more limited data on whether the protection provided is greater than that from monovalent ancestral boosters.”

Conclusions

The study findings reveal that the bivalent booster dose had a rVE of more than 20% higher than monovalent COVID-19 vaccines. These results emphasize the importance of variant-specific vaccine boosters, especially in regions like Australia, where multiple Omicron variants are currently in circulation.

A fundamental limitation of this study is the data source, as using data from the Australian Immunization Register and mortality records prevents linked data on COVID-19 infection, especially repeat infections. A growing body of literature suggests that prior COVID-19 infections can alter an individual's response to vaccination; however, the present study could not evaluate these factors.