MATTHIAS BLAMONT, GWLADYS FOUCHE and ESSI LOHTO  SEVERAL European countries are considering mixing up COVID-19 vaccines for citizens who received a first dose of AstraZeneca's shot, an unprecedented move that highlights challenges for governments struggling to tame fresh rises in infections. Vaccination programmes have been upset after a small number of reports that recipients of the AstraZeneca inoculation have suffered extremely rare blood clots, leading some countries worldwide to suspend its use out of caution. A senior official for the European Medicines Agency (EMA) said in an interview published on Tuesday there was a link between the vaccine and rare…