you do know you can research this on the web just like I did
anyway here is what happened
Bioverativ (BIVV) was a subsidiary of Biogen (BIIB). in 2/17 BIIB spun off BIVV tax-free - 1 share of BIVV for every 2 shares of BIIB. Since it was tax-fee the holding period for BIVV is the same as for BIIB. the basis allocation was that of your original cost for BIIB 92.17% remains and 7.83% is allocated to BIVV.
then in 1/18 Sanofi (SNY) acquired all of BIVV stock for $105 share - fully taxable to the extent proceeds exceed BIVV basis.
