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Monovalent mRNA COVID-19 vaccines provide durable protection against critical outcomes


Studies have shown that monovalent mRNA COVID-19 vaccines prevented critical outcomes, including COVID-19-related hospitalization, IMV, and death, during predominance eras of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) early variants of concerns (VOCs), Alpha, Delta, and early Omicron variants.

Since the beginning of the pandemic, over 1.13 million COVID-19–related deaths have occurred in the United States, with most among patients aged ≥65 years. Authorities recommended a bivalent mRNA booster for all individuals who had completed a primary COVID-19 vaccination series from September 1, 2022, onward and were at a higher risk of progression to severe disease. However, bivalent vaccination uptake remained low among adults aged ≥18 years, i.e., only 20%, and 42% among adults aged ≥65 years. The remaining adults only received monovalent mRNA vaccines.

Thus, data on the durability of protection conferred by monovalent mRNA vaccines against critical COVID-19 outcomes is vital. However, there is limited data on vaccine effectiveness (VE) of monovalent mRNA vaccines beyond the Omicron BA.1 lineage period, i.e., between December 26, 2021, and March 26, 2022. VE against critical outcomes is crucial to inform revaccination intervals in future COVID-19 vaccination policies.

About the study

In the present case-control analysis, researchers recruited immunocompetent adults aged ≥18 between February 1, 2022, and January 31, 2023, using the Investigating Respiratory Viruses in the Acutely Ill (IVY) network data to measure the VE of mRNA-based monovalent COVID-19 vaccines against COVID-19–related in-hospital death and IMV.

They collected demographic and clinical patient data through electronic health record (EHR) review, or patient/proxy interview, including IMV receipt and in-hospital death within 28 days of hospital admission.

The researchers used logistic regression to compute VE against IMV and in-hospital mortality as (1 − adjusted odds ratio [aOR]) x 100%; additionally, they compared the odds of monovalent mRNA vaccine against unvaccinated COVID-19 cases and control patients. The study model accounted for the U.S. Department of Health and Human Services region, age, gender, calendar time, and self-reported ethnicity/race.

The team stratified study results by age group, time elapsed since the last vaccine dose, and number of vaccine doses taken. They considered varying VE estimates with 95% Confidence Intervals (CIs) statistically significant. This study adhered to the CDC policy and applicable federal laws.

Study findings

The number of enrollees with IMV or in-hospital mortality in the IVY Network between February 1, 2022, and January 31, 2023, was 6,354; however, the final analysis sample set had only 70% of 6354, i.e., 4,421 individuals, of which 362 were cases, and 4,059 were controls. The average age of all included patients was 64 years. Notably, ~91% of COVID-19 patients had one or more persistent health issues, and 20% had experienced COVID-19 at least once.

Moreover, 146, 216, 293, and 156 of 362 COVID-19 case-patients who received IMV or suffered in-hospital death were unvaccinated, monovalent-vaccinated, received IMV, and died within 28 days of hospital admission, respectively. Of 4,059 controls, 979 (24%) and 3080 (76%) were unvaccinated and monovalent-vaccinated, respectively. Among monovalent vaccine recipients, the average time lapsed from vaccine receipt to illness onset was 248 days.

VE of two to four doses of monovalent mRNA vaccine against IMV and in-hospital mortality was 62% among adults aged ≥18 years and 57% and 69% among patients aged 18–64 years and ≥65 years, respectively. Stratifying by time since the last dose yielded VE of 76%, 54%, and 56% at seven to 179 days, 180 to 364 days, and ≥365 days, respectively.

Overall, monovalent mRNA vaccines showed a 76% efficacy in preventing COVID-19–related IMV and death up to six months after the last dose and up to 56% efficacy between one to two years. Thus, the authors recommended that all eligible adults take COVID-19 vaccines to prevent critical outcomes of COVID-19, such as IMV.


The current analysis reported VE for monovalent mRNA COVID-19 vaccine against IMV and in-hospital death for 12 months during the Omicron VOC era. Its results suggested that protection against IMV and death after six months from receipt of the last dose waned substantially but still remained clinically significant and offered durable immune protection for over 12 months.

In stratified subanalyses of this study, VE correlated more with time elapsed since the last dose than the number of vaccine doses received. Overall, these findings reinstate the importance of staying up to date with COVID-19 vaccination to prevent critical outcomes of COVID-19, including additional bivalent mRNA booster shots for individuals at the highest risk of progression to severe disease.

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