Corporate borrowing costs have declined sharply since the start of the year, particularly on the edges of the 19-member euro zone, with the start of ECB buys giving the markets another push.
"Note that there was no 'buying the rumor, selling the fact' syndrome: the tightening in credits since the announcement continued after they began hitting the market," said Erik Nielsen, Group Chief Economist at UniCredit.
Nielsen said interest rates, particularly for small and medium-sized firms in the euro zone's periphery, have dropped sharply, with loans in Italy for up to five years costing roughly the same as in Germany, down from a spread of 130-140 basis points two years ago.
Investment grade, non-financial euro zone corporate debt was yielding 1.113 at the end of last week, well below levels around 1.6 percent when the program was announced in March and marginally below the 1.155 percent seen just before the beginning of actual purchases.
"It seems the corporate bond market remains relatively unaffected from what's happening in other markets due to the prospects of the ECB buying," ING said, referring to big stock market falls on Friday and Monday.
The ECB was seen in the market buying a wide range of corporate debt, including from Italian insurer Generali , Spain's Telefonica and French utility Engie .
ECB sources earlier said that buying is likely to start slow and small, possibly struggling to pick up speed during the summer months as liquidity tends to fall sharply during the peak holiday periods.
But the purchases under the ECB's 80 billion euros per month asset-buying programme could then rise to around the 2 billion to 3 billion euros per month and possibly over 5 billion if the ECB succeeds in getting companies to start issuing new debt, analysts say.
The key hurdle is that the market for investment-grade euro-denominated corporate bonds is worth 500 billion to 600 billion euros and tends to be dominated by big French and Dutch companies who already enjoy easy access to credit and may not need ECB cash.
For purchase volumes to ramp up, the ECB will need to get new issuers from the periphery, like Italy or Spain to start borrowing, issuing new debt, giving the ECB to buy bigger chunks in the primary market.