During the late BA.4/BA.5 period, unvaccinated persons had higher COVID-19 mortality and infection rates than persons receiving bivalent doses (mortality RR = 14.1 and infection RR = 2.8) and to a lesser extent persons vaccinated with only monovalent doses (mortality RR = 5.4 and infection RR = 2.5). Among older adults, mortality rates among unvaccinated persons were significantly higher than among those who had received a bivalent booster (65–79 years; RR = 23.7 and ≥80 years; 10.3) or a monovalent booster (65–79 years; 8.3 and ≥80 years; 4.2). The authors of this CDC MMWR conclude that, “For the best protection against severe COVID-19, all persons should stay up to date with recommended COVID-19 vaccination, including receipt of a bivalent booster by eligible persons.”

https://www.cdc.gov/mmwr/volumes/72/wr/mm7206a3.htm?s_cid=mm7206a3_w