Zelensky said on Monday that Ukraine, one of the poorest countries in Europe, would begin the first phase of the vaccination campaign later this month.
Several European diplomats in Brussels said the commission president's widely reported tendency to rely on a small number of trusted aides had let her down in such a complex crisis.
Brussels says "security cooperation can be suspended in case of violations by the UK of its commitment for continued adherence to the European Convention of Human Rights".
"The next days are going to be decisive," the president of the European Commission told the European Parliament, as intense talks on a post-Brexit trade deal continued nearby in Brussels.