In a recent study published in the Morbidity and Mortality Weekly Report, researchers estimated the effectiveness of the 2023–24 updated vaccine for coronavirus disease 2019 (COVID-19).
The Advisory Committee on Immunization Practices of the U.S. Centers for Disease Control and Prevention (CDC) recommended on September 12, 2023 that all individuals aged six months and older receive the updated monovalent COVID-19 vaccine 2023–24. The updated vaccine contains a component from the lineage of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron.
The JN.1 lineage was identified in the United States in September 2023 and has 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) is observed in JN.1 and other BA.2.86 lineages, whereas S-Gene Target Presence (SGTP) is observed in most lineages from 2023, including XBB.
About the Study
In the present study, researchers estimated the effectiveness of the 2023–2024 updated COVID-19 vaccine against symptomatic infections. This included tests conducted on adults reporting at least one symptom between September 21, 2023, and January 14, 2024. The cases consisted of individuals with a positive Nucleic Acid Amplification Test (NAAT).
The controls were individuals with a negative NAAT. The team excluded tests from Janssen vaccine recipients, Novavax recipients, immunocompromised individuals, individuals who received the last vaccine dose within one week of the test, and individuals with a positive COVID-19 test within the last 90 days.
The vaccine’s effectiveness (VE) against symptomatic COVID-19 was estimated by comparing the likelihood of receiving the updated vaccine with the likelihood of not receiving it among cases and controls. A multivariable logistic regression was used to estimate the odds ratios. VE was calculated separately based on SGTP and SGTF status. Test-positive samples with reduced or no S gene amplification were assumed to have SGTF, while samples without SGTF were classified as SGTP.
Out of over 9,200 NAAT results for persons with symptoms of COVID-19-like illness, 3,295 tested positive for SARS-CoV-2. Around 1,125 individuals received the updated COVID-19 vaccine ≥ seven days earlier. More controls received the updated vaccine than cases. The mean interval since the last dose in recipients of the updated vaccine was 60 days for cases and 51 days for controls.
Almost 8,100 individuals did not receive the updated vaccine. Of these, 30% were unvaccinated and 70% were vaccinated. Among the vaccinated, the average 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 the ≥50 age group. VE was estimated at 58% and 49% for individuals tested 7–59 and 60–119 days after receiving the updated vaccine, respectively.
There were 679 tests with S-Gene Target results available. Of these, 258 showed SGTF and 421 showed SGTP. Due to the emergence of JN.1 in the United States, the VE for tests showing SGTF in the 7–59 days after receiving the updated vaccine was not precise. The VE was 60% for tests showing SGTP and 49% for those showing SGTF 60–119 days after receiving the updated vaccine.
The study provided initial effectiveness 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 is likely to decrease over time after vaccination, especially for less severe outcomes, as observed after the original monovalent or bivalent COVID-19 vaccination. Prior infections, medical conditions, and vaccination status were self-reported and therefore subject to recall bias.
In addition, the VE estimates were calculated for a population that had chosen to be tested for COVID-19; therefore, the estimates are subject to selection biases. Furthermore, the uptake rate of the updated vaccine among adults is low and varies by age. In summary, the updated monovalent vaccine provided 54% protection against symptomatic illness. It is expected that VE will decrease over time, especially for less severe outcomes.
- Link-Gelles R, Ciesla AA, Mak J, et al. Early estimates of updated effectiveness of the COVID-19 vaccine 2023–2024 (monovalent January 2024. MMWR Morb Mortal Wkly Rep, 2024, DOI: 10.15585/mmwr.mm7304a2, https:/ /www.cdc.gov/mmwr/volumes/73/wr/mm7304a2.htm