platelets completely crashed; arm got a little sore; felt a little lousy and had very mild flu-like symptoms; felt a little lousy and had very mild flu-like symptoms; This is a spontaneous report received from a contactable consumer (reporting on himself). A 24-years-old male patient received first dose of BNT162B2 (PFIZER-BIONTECH COVID-19 VACCINE, Formulation: solution for injection, lot number: SH1555218 and expiration date was not reported) via an unspecified route of administration in Arm Left on 21Mar2021 at single dose for COVID-19 immunization. Medical history included ongoing 22Q (diagnosed with this condition when he was about 4 years old). Immunodeficiency from an unknown date and unknown if ongoing. Concomitant medication(s) included rituximab (RITUXAN) taken for immunodeficiency, start and stop date were not reported. Patient states he received the 1st dose on 21Mar2021 about 9:00am or a little before that time and had a very bad side effect and was in the hospital for a couple of days. His weight was 162 or 163 pounds. Patient was immunocompromised and has issues regarding that lately and received Rituximab which is like a chemo; states after he had the Pfizer Vaccine on 21Mar2021, his arm that the vaccine was administered into got a little sore relatively soon after the shot and the main thing he was reporting that his platelets completely crashed which was the reason for the hospitalization (from 24Mar2021 to 26Mar2021). Patient states for the Rituximab he gets due to having some issues being immunocompromised, it was his 4th treatment with the Rituximab and states he has no lot numbers, dosages, expiry dates or NDC numbers to provide and states he was on the Rituximab for his immunocompromised status. His arm soreness was in the same arm that he had the Pfizer vaccine administered into and would say the following day after getting the Pfizer vaccine, he felt a little lousy and had very mild flu-like symptoms that resolved completely like 2 days later and these symptoms went away. Patient was immunocompromised and has fairly serious health issues since he was age 4-5 and was able to get the vaccine with his HCP. Rituxan was the only medication he can think of to include in this report. Patient did not require a visit to the emergency room or the HCP office but was actually just getting the last dose of his treatment with Rituxan and then they drew his labs and the labs stated his platelet count had completely crashed the last time they checked his lab work. His platelet count was 3000 on 26Mar2021 and the platelets went up relatively quickly after that. Patient did not have treatment other than just the steroid of the very high dose Dexamethasone. Patient would like Pfizer's opinion and was not entirely sure he can get the 2nd dose of the Pfizer Vaccine and was asking what was recommended; patient aware that Pfizer was unable to offer medical advice or treatment recommendations and was referred to his HCP. Patient still on steroids very high dose Dexamethasone and got IVIG in the hospital; states he has no lot numbers, expiry dates, dosages or NDC numbers for the Dexamethasone or IVIG to provide. He has his patient card for the Pfizer Vaccine and states for the lot number, he thinks it was SH1555218 but does not locate other information including other numbers that could be the lot or the expiry date or the NDC number; states the patient card information was not handwritten and he can't tell what the lot number was and states this information was embedded like a bar code on his patient card. The outcome for the events platelets completely crashed, arm got a little sore was unknown and for events felt a little lousy and had very mild flu-like symptoms was recovered on 24Mar2021.