XBB.1.5 updated COVID vaccine booster offers 54% increased protection against symptomatic infection

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In a recently published study, Dr Morbidity and Mortality Weekly ReportResearchers estimate the efficacy of the updated coronavirus disease 2019 (COVID-19) vaccine by 2023-24.

The Advisory Committee on Immunization Practices of the United States (US) Centers for Disease Control and Prevention (CDC) recommended on September 12, 2023 that all persons six months of age and older receive the 2023-24 updated stand alone COVID-19 vaccine. The updated vaccine contains an element of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron XBB.1.5 lineage and lacks elements of the ancestral strain.

In September 2023, the JN.1 lineage was identified in the United States, harboring 30 additional mutations in the spike (S) protein compared to XBB.1.5. Real-time reverse-transcription polymerase chain reaction (RT-PCR) results can help distinguish some SARS-CoV-2 lineages. S-gene target failure (SGTF) was detected in JN.1 and other BA.2.86 lineages, whereas S-gene target presence (SGTP) was detected in most lineages from 2023, including XBB.

Study: Updated 2023–2024 Preliminary Projections (Monovalent 2023–January 2024. Image Credit: New Africa / ShutterstockStudy: Updated preliminary estimates for 2023-2024 (Monovalent January 2024). Image credit: New Africa/Shutterstock

About the study

In the current study, the researchers estimated the effectiveness of the 2023-24 updated COVID-19 vaccine against symptomatic infections. They included tests conducted between Sept. 21, 2023, and Jan. 14, 2024, in adults with at least one symptom reported. Cases were individuals with a positive nucleic acid amplification test (NAAT).

Controls were individuals with negative NAAT. The team excluded Janssen vaccine recipients, Novavax recipients, immunocompromised individuals, those who received the most recent dose of vaccine within a week before testing, and those who tested positive for COVID-19 within the past 90 days.

Vaccine efficacy (VE) against symptomatic COVID-19 was estimated by comparing the odds of receiving versus not receiving the updated vaccine among cases and controls. Multivariable logistic regression was used to estimate odds ratios. VE was calculated by SGTP and SGTF status separately. Test-positive samples with low or zero S-gene amplification were considered an SGTF, whereas those without SGTF were considered SGTP.


Of more than 9,200 NAAT results for people with symptoms of a COVID-19-like illness, 3,295 tested SARS-CoV-2-positive. About 1,125 people received the updated COVID-19 vaccine ≥ seven days ago. More controls than cases received the updated vaccine. The median time from last dose among updated vaccine recipients was 60 days for cases and 51 days for controls.

About 8,100 individuals did not receive the updated vaccine. Of these, 30% were unvaccinated and 70% were vaccinated. Among those vaccinated, the median time since vaccination was 378 days for cases and 363 days for controls. The estimated VE was 57% in the 18–49 age group and 46% in those ≥ 50 years of age. VE was estimated to be 58% and 49% among people tested 7–59 and 60–119 days after receiving the updated vaccine, respectively.

679 tests with S-gene target results were available. Of these, 258 showed SGTF and 421 showed SGTP. VE was not specific for testing with SGTF between 7–59 days after receipt of updated vaccine given the advent of JN.1 in the United States. VE between 60–119 days post-receipt of updated vaccine was 60% for trials with SGTP and 49% for SGTF.


The study provided preliminary efficacy estimates of the updated monovalent XBB.1.5 vaccine against symptomatic infections and the first VE estimates against the JN.1 lineage. These VE estimates include data up to 119 days after vaccination. However, VE will likely decrease with time since vaccination, as observed after the original monovalent or bivalent COVID-19 vaccine. History of prior infection, medical status, and immunization status were self-reported, thereby subject to recall bias.

In addition, VE estimates were calculated for a population that chose to be tested for COVID-19; Therefore, the estimate is subject to selection bias. Furthermore, updated vaccine coverage is low in adults and varies by age. Taken together, the updated monovalent vaccine provides 54% protection against symptomatic illness. A decrease in VE over time is expected, especially against less severe outcomes.

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