So far there has not been a case of Covid transmitting from breast milk
A recent study looked at mom-baby units of up to 19 months post partum and were providing many samples of their mild.
They tested before and after symptoms of SARS CoV-2 the Covid virus, as well as some stored samples.
The benefits of breastfeeding are more than proven. There is likely to be more research on this topic, and moms should consult their personal physicians for more guidance if they get sick.