The weekly update from the MHRA reports more cases of TTS (Thrombotic thrombocytopenic syndrome) following the Oxford-AstraZeneca vaccine, as expected. There is now a very high level of awareness of this syndrome amongst the public and clinicians throughout the country and cases are being reported reliably and quickly but there are also cases that occurred previously now being recognised and reported as well. MHRA are using a very clearly defined case definition to ensure consistency. In other words they are making sure that the cases being reported are really this syndrome and not something else.  This can take some time and while there is uncertainty, unconfirmed cases are designated possible or probable. I would expect the true number of cases per million doses of vaccine to become clear fairly soon as these reports stabilise but it is already clear that it is going to remain a very rare event.  There remains an urgent need to understand the mechanism underlying these cases, to monitor and optimise rapid diagnosis and treatment and, if possible, to identify risk factors all of which might improve outcomes and enable cases to be prevented.