ARACHNOID GRANULATION; This spontaneous report was originally received from the patient's husband and concerned a 45-year-old female patient. Initial information was received and processed with additional information received in discussion with treating physician on 15-APR-2021.  The patient's weight 69 kg and height 178 cm. Patient's medical history included smoking for 10 years and quit 10 years ago. Subject's concurrent conditions included insomnia. The patient was reported to be in good health, Body Mass Index (BMI) reported as 21.7. The patient had no family history of blood clots.  On 08-APR-2021, the patient received vaccination with Janssen COVID-19 vaccine Ad26.CoV2.S (suspension for injection, route of administration not reported; batch number not reported and requested), dose unspecified, for COVID-19 vaccination. Batch number was not reported, will be requested. Non-company suspect drug included unspecified contraceptive pill (unknown, oral) for unspecified indication. Concomitant medications included cannabis sativa for insomnia.  On an unspecified date 'just following' vaccination, the patient had influenza-like symptoms . On 10-APR-2021, patient started experiencing headaches.  On 14-APR-2021 (6th day post-vaccination), the patient presented to Emergency Room (ER) with four days of worsening headache and diagnosed with suspected right transverse sinus thrombosis (later ruled out.) The patient was transferred to a second hospital's intensive care unit (ICU) on an unspecified date. On 15-APR-2021 (7th day post-vaccination), a magnetic resonance imaging (MRI) scan of the brain showed suspected cerebral venous sinus thrombosis (CVST). No other symptoms or neurologic deficits reported, and her platelet count was reported as normal at 268,000 on admission (units and normal range not reported). The admitting physician initiated argatroban whilst closely following the patient's platelet count. Testing for a panel for heparin-induced thrombocytopenia including anti-platelet factor 4 (anti-PF4) antibodies and other unspecified workup were also planned. On an unspecified date, MRV of brain showed an absence of flow in right transverse sinus highly suggestive of thrombosis, sluggish flow in sigmoid sinus and superior IJ.  MRA showed non-visualized of A1 segment of right ACA, more distal segment supplied by Left ACA, this is a normal anatomic variant. Also non visualization of R vertebral artery which is felt to be a normal anatomic variant as well. This read was reviewed on 15-APR with neuro-interventionalist physician and 3 radiologist who were all in agreement that this was instead a normal variant (arachnoid granulation.) Report was addended to clarify that there was never any thrombosis. Patient's blood pressure was 120/86 and heart rate 88. Patient was fully anticoagulated with argatroban. No heparin was received. Heparin induced thrombocytopenia (HIT) panel was sent and lab was requested to urgently process. Platelet count remained normal from 268 to 227 on 15-APR-2021.   On 15-APR-2021, a CT venogram was performed which again confirmed no thrombosis; rather there was arachnoid granulation (normal variant). All imaging was confirmed with neuro-interventionalist physician and 3 radiologist who were all in agreement.  Treatment with agratroban was discontinued on 15-APR-2021. As of 15-APR-2021, patient was clinically improving. Headache was resolved and neurologically intact. Patient was eating and was planned to discharge home later in the day.  The action taken with COVID-19 vaccine Ad26.CoV2.S was not applicable. Action taken with contraceptive pill was unknown.  The event arachnoid granulation was recovering This report was serious (hospitalization/prolonged hospitalization).; Sender's Comments: V0: A 45-year-old female experienced headache 2 days after vaccine and was hospitalized for suspected transverse venous sinus thrombosis (which was ruled out) 6 days after vaccine. There was no secondary hemorrhage, and after the imaging was reviewed with a neuroradiologist, it was determined that there was no thrombosis at all, but rather an arachnoid granulation (normal anatomic variant.)  This was confirmed on follow up CT venogram the following day. Relevant concomitant medications included oral contraceptives. Platelet count was normal (268,000) on admission. No heparin was received, but a panel for heparin-induced thrombocytopenia, including ELISA for anti-PF4 antibodies is pending. The patient was initially anticoagulated on argatroban and symptoms improved one day after admission; the headache is resolved, she is neurologically in tact, and discharge is planned to home. Although the use of oral contraceptives is a risk factor for thrombosis, there was no thrombosis in this case.