In response to the ongoing COVID-19 pandemic and the emergence of highly transmissible and vaccine-resistant variants, many countries have adopted booster vaccination strategies to bolster immunity and combat the evolving virus. One such approach gaining attention is the use of bivalent mRNA vaccines as a fourth dose (second booster) to enhance protection against severe and fatal COVID-19 outcomes.
In the Nordic countries, a groundbreaking study was conducted to investigate the comparative effectiveness of bivalent mRNA booster vaccines and their potential to improve immunity in target populations, particularly among adults aged 50 years and above. The study’s findings, which were implemented in autumn 2022 and subsequently included in booster vaccination programs in the Nordic countries, shed light on the efficacy of these vaccines against omicron variants and their implications for global vaccination strategies.Â
According to BMJ, the bivalent BA.4-5 and BA.1 mRNA booster vaccines yielded mixed results concerning their ability to increase neutralizing antibody responses against the omicron variants compared to the original monovalent mRNA vaccines. Some studies suggested a substantial improvement, while others did not consistently show the same effect. As a result, the question of the effectiveness of bivalent boosters in preventing severe COVID-19 outcomes remained unanswered, underscoring the need for comprehensive research.Â
The results of the Nordic study were promising, indicating that the bivalent BA.4-5 or BA.1 mRNA booster vaccines were associated with significantly lower rates of COVID-19 related hospital admission and death among individuals aged ≥50 years who had previously been vaccinated with three doses of monovalent vaccines. The comparative vaccine effectiveness was ≥65%, suggesting a robust protective effect. Importantly, no significant difference in protection was observed between the bivalent BA.4-5 and BA.1 mRNA booster vaccines when compared directly.Â
While the comparative vaccine effectiveness did not differ by sex or age, the risk difference was greater among people aged ≥70 years compared to younger individuals. This finding emphasized the potential benefits of administering the fourth dose to the elderly population, who are at higher risk of severe COVID-19 outcomes. Furthermore, the study extended the follow-up to day 180, demonstrating sustained protection against severe COVID-19 outcomes. The comparative vaccine effectiveness against hospital admission with COVID-19 was 54.9% for the BA.4-5 booster and 63.5% for the BA.1 booster during this extended period.Â
The findings of the Nordic study have significant implications for booster vaccination strategies worldwide. As more countries confront the challenge of emerging variants, these results provide valuable insights into the potential benefits of administering bivalent mRNA vaccines as a fourth dose to improve protection against severe and fatal COVID-19 outcomes, especially among vulnerable populations. The effectiveness of bivalent boosters against the omicron variant indicates their relevance in regions with a high prevalence of this variant, potentially reducing the burden of severe COVID-19 cases and deaths.Â
In conclusion, the Nordic study sheds light on the promising protective potential of bivalent mRNA booster vaccines as a fourth dose. These vaccines have shown efficacy in improving immunity against severe COVID-19 outcomes, including hospital admission and death. The findings underscore the importance of ongoing research to optimize vaccination strategies and adapt to the evolving pandemic. As countries continue to navigate the complexities of the COVID-19 pandemic, the knowledge gained from this study can inform evidence-based decisions and help safeguard public health worldwide.
