When the COVID-19 booster targeting the XBB 1.5 variant was made available late last summer, the variant was the most dominant in the United States. Within weeks of the booster's introduction, variant JN.1 began to rise and replaced XBB as the dominant virus strain by the end of 2023.
A new study in Clinical Infectious Diseases looks at outcomes among more than 3 million adult Singaporeans who received the XBB booster amid JN.1 virus circulation and finds protection against both infection and serious outcomes.
The study was conducted from November 26, 2023 to January 13, 2024, and assessed COVID-19–associated emergency department (ED) visits and hospitalizations, stratified by vaccination status or prior infection. Outcomes among those vaccinated 8 to 120 days prior to infection were compared to people last boosted at least 1 year prior to their XBB dose.
In total, during Singapore's JN.1 outbreak, 28,160 SARS-CoV-2 infections were recorded, including 2,926 hospitalizations and 3,747 ED visits, and the cases were driven by increasing circulation of the JN.1 variant (more than 90% of sequenced viruses).
By the end of the study period, 20.8% (696,344 of 3,340,800) received a bivalent (two-strain) booster; while 3.2% (107,966 of 3,340,800) received an XBB 1.5 booster.
Lower risk of all negative outcomes
An XBB booster was associated with a 41% reduced risk of JN.1 infection (adjusted hazard ratio [aHR], 0.59). COVID-19–associated ED visits were reduced 50% among those with XBB boosters (aHR, 0.50), while hospitalizations were 42% lower (aHR, 0.58).
There was no statistically significant reduction of risk for JN.1 infections, ED visits, or hospitalizations in people vaccinated with a bivalent booster within 8 to 365 days after the shot.
For Singaporeans with prior COVID-19 infections, compared to adults with no history of COVID-19 infection, those infected less than 1 year prior during XBB-predominant transmission had a 35% lower risk of COVID-19–associated ED visits (aHR, 0.65) and a numerically lower risk of hospitalization that was not statistically significant.
Those with COVID-19 infections recorded more than 1 year prior had an increased risk of JN.1 infection and COVID-19–associated ED visits and hospitalizations.
Our results highlight the benefit of updated booster doses in maintaining relative protection, even during a wave of transmission driven by the emerging SARS-CoV-2 JN.1 variant
"Our results highlight the benefit of updated booster doses in maintaining relative protection, even during a wave of transmission driven by the emerging SARS-CoV-2 JN.1 variant," the authors wrote. "Regular annual boosting remains important in preserving healthcare systems' resilience during COVID-19 endemicity."
