ESCALATING PAIN IN LOWER BACK; ESCALATING PAIN IN LEG; BLOOD CLOT IN LEG; VACCINE-INDUCED IMMUNE THROMBOTIC THROMBOCYTOPENIA; This spontaneous report received from a patient via a company representative (media article) concerned a 4 decade old male (man in 30s). The patient's weight, height, and medical history were not reported. The patient received covid-19 vaccine ad26.cov2.s (suspension for injection, route of admin not reported, batch number: Unknown) dose was not reported, frequency once total, administered on 08-APR-2021 for prophylactic vaccination. The batch number was not reported. The Company is unable to perform follow-up to request batch/lot numbers. No concomitant medications were reported.  On an unspecified date in APR-2021, within two weeks after receiving a dose of the J&J COVID-19 Vaccine, the patient experienced blood clot in leg. Patient began to experience escalating pain in the lower back and leg on 16-APR-2021. Patient was hospitalized on 21-APR-2021 with escalating pain in his lower back and leg, 13 days after taking the vaccine. It was reported that this was the first male patient with VITT (vaccine-induced immune thrombotic thrombocytopenia) syndrome. The patient was recuperating and making good progress and would be discharged within a few days. Patient's bloodwork showed signature low levels of platelets and fibrinogen, a blood-clotting factor made in the liver, which had been seen in other patients with vaccine-induced clots. Bloodwork showed the patient had the same syndrome as the other patients although initial imaging did not show a blood clot. Physicians later discovered a tiny clot.  The action taken with covid-19 vaccine ad26.cov2.s was not applicable. The patient was recovering from escalating pain in lower back, escalating pain in leg, blood clot in leg, and vaccine-induced immune thrombotic thrombocytopenia. This report was serious (Hospitalization Caused / Prolonged, and Other Medically Important Condition). This case, from the same reporter is linked to 20210451418.  Initial information was processed with additional information received on 28-APR-2021.; Sender's Comments: V0: This spontaneous report received from a media article concerned a man in 30s who experienced vaccine-induced thrombotic thrombocytopenia 13 days after vaccine. Medical history and concomitant medications were not reported. Eight days after vaccine, patient began to experience escalating pain in the lower back and leg. Initial imaging did not show a blood clot but physicians later discovered a "tiny clot" in the leg. Thirteen days after vaccine, patient was hospitalized with escalating pain in his lower back and leg. The patient was recuperating and making good progress and would be discharged within a few days. Patient's bloodwork showed "signature low levels of platelets and fibrinogen." Although the other risk factors (not reported) may have contributed, based on evolving knowledge of Thrombosis with Thrombocytopenia Syndrome (TTS, per definition from Brighton Collaboration - BC) and considering the low platelet count and temporal relationship to vaccination (BC Criteria level 1), the s events are assessed to have a plausible relationship with vaccination. Reporter contact information was not provided which precludes meaningful additional information in this version.