It said there was a very rare chance of something called TTS. TTS is thrombosis and thrombocytopenia syndrome. Thrombosis means blood clots, thrombocytopenia means a reduction in the level of platelets. Platelets prevent bleeding so people who have low platelets bleed at the slightest provocation and this bleeding leads to internal clotting. A dengue virus infection can also cause thrombocytopenia. So they have admitted to a very rare event.