It's after three young patients were admitted to hospital and one died.
Experts say two women in their 30s and a man in his 40s suffered clots in their large arteries, leading to ischaemic stroke, though they stressed the chances were small.
The team said the NHS must look out for patients with ischaemic stroke (where blood clots block arteries) within about one month of vaccination, saying they should be “urgently evaluated” for a very rare syndrome called vaccine-induced thrombosis and thrombocytopenia (VITT).
This condition needs to be rapidly diagnosed and managed by a team with a range of expertise, who have quick access to a range of drugs, they said.
The experts stressed that the cases of stroke are very rare and that stroke is more common in people who catch Covid-19.
There have been 309 cases of major thrombosis with low platelet count suggesting VITT from more than 30 million doses of the Oxford/AstraZeneca vaccine administered, they said.
The number of people who experience blood clots from VITT after a Covid-19 vaccine is therefore extremely low at about one per 100,000 doses.
Lead author of the report, David Werring, professor of clinical neurology at UCL, said: “Although cerebral venous thrombosis – an uncommon stroke type in clinical practice – is now recognised as being the most frequent presentation of VITT, our study shows that the much more common ischaemic stroke, due to arterial thrombosis blocking blood flow to part of the brain, may also be a presenting feature of vaccine-induced thrombosis.
“Of course, both types of thrombosis remain extremely rare, but doctors need to be vigilant if patients present with typical stroke symptoms (eg face, arm or leg weakness, or impaired speech) due to a blocked artery any time between days four and 28 post-vaccination.”
The first patient, a 35-year-old Asian woman who later died, experienced an intermittent headache on the right side and around her eyes six days after having her vaccine.
Five days later, she awoke feeling drowsy and with weakness to her face, arm and leg. She underwent brain surgery to reduce pressure in her skull alongside other treatments, but these could not save her life.
The second patient, a white woman aged 37, suffered headache, confusion, weakness in her left arm and loss of vision on the left side 12 days after her vaccine. She had several treatments and survived.
The third patient, an Asian man aged 43, was admitted to hospital three weeks after receiving his vaccination with problems speaking and understanding language. He received a platelet and plasma transfusion plus other treatment and remains stable.
In a linked commentary, Professor Hugh Markus, from the department of clinical neurosciences, at the University of Cambridge, said: “During the current period of Covid vaccination, a high index of suspicion is required to identify thrombotic episodes following vaccination.
“However, it is important to remember that these side-effects are rare, and much less common than both cerebral venous thrombosis and ischaemic stroke associated with Covid-19 infection itself.”
Other experts working on the report were from Sandwell and West Birmingham Hospitals NHS Trust and UCL’s Queen Square Institute of Neurology.
Professor Beverley Hunt, medical director of Thrombosis UK and from King’s College London, said VITT occurs rarely after Covid-19 vaccination.
“This case report gives details of three patients who presented with stroke due to arterial occlusion,” he said.
“It is well recognised within the UK that VITT, while it more commonly presents with large vein thromboses, can present in this way and the current guidance documents from the UK Expert Haematology Panel and from the College of Emergency Medicine recognise this too.”