Oct 21 (Reuters) - The UK drug regulator added an extremely rare nerve-damaging disorder, Guillain-Barré syndrome (GBS), as a very rare side effect of the AstraZeneca (AZN.L) COVID-19 vaccine, updates on the agency"s website showed on Thursday. Medicines and Healthcare products Regulatory Agency"s (MHRA) decision comes after the European medicines agency added GBS as a possible side-effect last month. read more