"Think of a patient which is out of a deep crisis but still on two crutches," Lagarde said.
"You don't want to remove either crutch, the fiscal or the monetary, until the patient can actually walk fine, and to do that means support well into the recovery."
The central bank ramped up buying of sovereign bonds after a surge in U.S. Treasury yields lifted borrowing costs in the bloc and threatened Europe's tentative recovery.
Credit insurer Euler Hermes estimates that the EU is now seven weeks adrift of its target to have 70 percent of the population vaccinated by the end of the summer, compared with five weeks in February.
"The near-term economic outlook is subject to uncertainty, relating in particular to the dynamics of the pandemic and the speed of vaccination campaigns," Lagarde said in a blog post on the ECB's website.
We therefore stand ready to adjust all of our instruments, as appropriate, to ensure that inflation moves towards our aim in a sustained manner, in line with our commitment to symmetry.
The European Central Bank may need some time before the recently agreed acceleration in the pace of money printing, ECB President Christine Lagarde said on Thursday.
Ten-year US Treasury yields rose to over 1.63%, their highest level since February 2020 and their German equivalents, the benchmark for the region, rose as high as -0.293%.
Italian 10-year yields, which outperformed the market on Thursday as the country is among the top beneficiaries of ECB bond buying, were up 5 basis points at 0.63%.
A series of one-off effects is expected to see consumer prices climb further this year and perhaps even overshoot the target, complicating the ECB's job.
ECB President Christine Lagarde said on Monday the central bank was "closely monitoring" rising borrowing costs - a comment that knocked Germany's 10-year Bund yield off its eight-month high.
The ECB's strategy "is not to burn too much cash trying to lean against the wind - rather to intervene verbally", he said, citing lower levels of ECB bond-buying last week.
Risk-free overnight indexed swap rates and sovereign yields are particularly important, because they are good early indicators of what happens at downstream stages of monetary policy transmission.
"Accordingly, the ECB is closely monitoring the evolution of longer-term nominal bond yields," she said.
European Central Bank President Christine Lagarde declared this month that bitcoin was "not a currency" and was a "highly speculative" asset requiring global regulation.
Lagarde also called on European Union leaders to kick start the bloc's 750 billion euro Next Generation EU spending package as some economic restrictions could remain in place until the second half of the year.
She forecast that 2021 would be a year of "recovery" but acknowledged that the eurozone would not return to pre-pandemic levels of activity "before mid-2022".
More significantly, ECB policymakers are also debating what role climate considerations should play in the institution's multi-trillion euro bond-buying programme.