Boosters are necessary because waning immunity and new variants have eroded the spectacular effectiveness seen in 2020’s vaccine trials: Six months-plus after vaccination, two mRNA doses mitigate hospitalizations due to Omicron by only 57 percent across all ages. To minimize hospitalizations in addition to deaths, Simonsen says the percent vaccinated over age 40 is crucial too, because the median age of COVID hospitalization is lower than that of death. Yet vaccination rates in nursing homes are not any better than in the overall elderly population: 87 percent of residents are fully vaccinated, and only 69 percent are boosted.