Of Vaccine Efficacy and COVID-19 Variants: 4 Questions

Terry Brenneman, MD

In the early days of December, 2022, the XBB1.5 variant of the SARS-CoV-2 virus was responsible for 4% of new infections; 4 weeks later, that rose to 44%. Experts were concerned that the bivalent vaccine in use at the time (based on previous Omicron variant BA.5) might not be as effective as needed.

Work began on a new vaccine based on this concerning new variant and was ready for use in 9 months.

In September 2023, when the updated monovalent vaccines were available, a new COVID-19 variant was detected. Labeled JN.1, the strain expressed approximately 30 mutations on the spike protein, more than had been seen on any of the variants known to that point. By early January 2024, JN.1 was responsible for about two-thirds of new COVID-19 cases.

The antigenic drift caused understandable concern. Would the monovalent shot based on XBB1.5 now be less effective? In vitro antibody testing suggested that the vaccine would still work against this new variant but there were no real-world data yet available. Would the Centers for Disease Control and Prevention (CDC) and vaccine companies be engaged in a never ending “whack-a-mole” contest as new strains overtook those we had vaccines to protect against?

The February 1, 2024, issue of the CDC's MMWR (Medical and Morbidity Weekly Report) presented the first “real world” data on the vaccine efficacy (VE) of the new monovalent vaccine against the JN.1 strain. While critics called out study limitations (retrospective, based on pharmacy COVID testing with follow-up phone calls, no data on disease severity, among other issues) it does provide some needed information on VE against symptomatic disease.

Try these 4 questions on what the research found.