In a CEPR discussion paper, Christoph Trebesch and Jeromin Zettelmeyer argue that

ECB bond buying had a large impact on the price of short and medium maturity bonds … However, the effects were limited to those sovereign bonds actually bought. We find little evidence for positive effects on market quality, or spillovers to close substitute bonds, CDS markets, or corporate bonds.

A multiple equilibria view of the crisis would probably suggest otherwise.