Top 5 Takeaways

1. Effectiveness of Bivalent mRNA Vaccine: The bivalent mRNA vaccine, which includes a component against the Omicron BA.4/BA.5 sublineages, was found to be effective in preventing COVID-19–related medical encounters and hospitalizations among immunocompetent adults.
2. Comparative Vaccine Effectiveness (VE): VE of the bivalent booster, after 2, 3, or 4 monovalent doses, was significant against emergency department/urgent care encounters and hospitalizations when compared with no vaccination or only monovalent vaccination.
3. Duration of Protection: The relative protection offered by the bivalent vaccine increased with time elapsed since the last monovalent dose, suggesting waning immunity from the monovalent vaccine.
4. Study Period and Context: The study was conducted during the prevalence of the Omicron BA.5 sublineage and other Omicron sublineages in the U.S., from September to November 2022.
5. Limitations and Future Research: The study acknowledges limitations like not accounting for previous SARS-CoV-2 infections and potential residual confounding factors. Further research is needed to determine the long-term effectiveness of the bivalent vaccine and its optimal timing.

Original Article Author and Citation

Corresponding Author

Mark W. Tenforde

Suggested Citation

Tenforde MW, Weber ZA, Natarajan K, et al. Early Estimates of Bivalent mRNA Vaccine Effectiveness in Preventing COVID-19–Associated Emergency Department or Urgent Care Encounters and Hospitalizations Among Immunocompetent Adults — VISION Network, Nine States, September–November 2022. MMWR Morb Mortal Wkly Rep 2023;71:1637–1646.



The MMWR article reports on a study evaluating the effectiveness of the bivalent mRNA COVID-19 vaccine in preventing COVID-19–associated medical encounters among immunocompetent adults. Conducted by the VISION Network in nine states during a period where Omicron BA.5 was predominant, the study found that the bivalent booster dose was effective in reducing the occurrence of emergency department/urgent care encounters and hospitalizations for COVID-19–like illnesses. The bivalent booster showed increased effectiveness when compared to no vaccination and additional protection compared to monovalent vaccination only.


The study utilized data from seven health systems in nine states. Participants included adults who received a molecular test for SARS-CoV-2 during specific encounters. They were classified based on their vaccination status: unvaccinated, vaccinated with monovalent-only mRNA vaccines, or vaccinated with monovalent doses plus a bivalent booster. The vaccine effectiveness was estimated using a test-negative case-control design and calculated using multivariable logistic regression, adjusting for various factors including age, race, and local SARS-CoV-2 circulation.


The analysis showed that the bivalent booster offered significant protection against medically attended COVID-19. However, the study also recognized several limitations, such as the non-consideration of prior SARS-CoV-2 infection and potential residual confounding factors. The need for additional research was emphasized to further understand the vaccine’s effectiveness, particularly against severe respiratory illness or death, and to evaluate the durability of protection provided by the bivalent booster.


The study concludes that the bivalent mRNA COVID-19 vaccine offers significant protection against COVID-19–related medical encounters and hospitalizations, especially among those who have already received monovalent vaccine doses. The findings support the ongoing efforts to improve coverage with bivalent vaccines and highlight the need for further research to establish optimal timing for bivalent vaccine booster doses.

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