Especially when they start out not-so-hot to begin with.
CDC, Aug 18, 2021:
Two doses of mRNA vaccines were 74.7% effective against infection among nursing home residents early in the vaccination program (March–May 2021). During June–July 2021, when B.1.617.2 (Delta) variant circulation predominated, effectiveness declined significantly to 53.1%. ... This study could not differentiate the independent impact of the Delta variant from other factors, such as potential waning of vaccine-induced immunity. ... nursing home residents were among the earliest groups vaccinated in the United States; thus, if vaccine-induced immunity does wane over time, this decrease in VE might first be observed among nursing home residents.
