This is definitely true for policy (short term) interest rates. But monetary policy, as we have seen in the recession that started in 2008 and went on — for Europe — until 2013, can also attempt to affect long term interest rates. Central banks would do (did) so by buying government/corporate long term bonds. Now, looking at the experience with this sort of ‘quantitative’ monetary policy in Europe the European Central Bank dealt with it through two programs: the Covered Bonds Purchase program (from 2009) and the Securities Market Program (2010). Looking at the second one — whose official goal was to ‘restore the transmission mechanism in some countries of the Union, but that in fact amounted to a selective easing policy — here was an instrument in the hand of the ECB that effectively created a differential monetary policy stance by purchasing government bonds of a country instead of the other. If similar operations are ‘sterilised’ to avoid inflationary effect in the euro area, nothing would prevent this measure to be used regularly.