The 39-year-old woman had received an AstraZeneca jab early in May, officials said.
“The woman may also have been on medication for another condition and it was unknown also what role that might have played,” they added.
The incident is being investigated and the all data gathered will be sent to the European Medicines Agency.
The Oxford–AstraZeneca COVID‑19 vaccine is used to provide protection against infection by the SARS-CoV-2 virus in order to prevent COVID-19 in adults aged 18 years and older.
The vaccine can lead to blood clots in combination with low levels of blood platelets. The UK MHRA as of 14 April 2021 reports an overall case incidence of thromboembolic events with concurrent low platelets of 7.9 per million doses, less than 1 in 100,000 people.