The third phase of clinical trials for the so-called 'Oxford vaccine' against Covid-19, developed by AstraZeneca and Oxford University, have been suspended while an independent committee investigates a suspected adverse reaction in one of the participants.
One volunteer in the trial in the UK has suffered an inflammation of the spine - transverse myelitis - which is often caused by viral infections. The investigation, which could take days, will try to ascertain whether this is a reaction caused by the vaccine.
Pharmaceutical company AstraZeneca, which will produce the vaccine when it is licensed by drug regulators, said in a statement on Tuesday that the stoppage was "routine".
This news could be a blow to the Spanish Ministry of Health which had hoped to have three million doses available by December, as revealed on Monday by Minister of Health Salvador Illa.
Illa said that he hoped the Health Service could start vaccinating "the most vulnerable groups" by the end of the year.