If they do indeed outlive conventionally transfused cells, as the researchers hope and preclinical studies suggest they will, then recipients will not need such frequent transfusions.
The preclinical data for the COVID-19 mRNA vaccine showed no impact on fertility in animals that would lead researchers to suspect there would be issues with humans.
The goal of preclinical tests is to see if the vaccine truly works on the pathogen -- in this case, the virus that causes COVID-19 -- and also to test the safety of a vaccine.