HYPOXEMIA; UNABLE TO FOCUS; COLD CHILLS; WEAK; TIRED; FEVER; PAIN; VERY FORGETFUL; HARD TO TALK; LOSS OF CONSCIOUSNESS; FOGGY THINKING; SHORTNESS OF BREATH; SHORT-TERM MEMORY LOSS; MUSCLE CRAMPS IN FEET, TOES AND ARMS; FALLING OVER; CONFUSED; DIZZY; This spontaneous report received from a patient concerned a 43 year old female. The patient's weight was 245 pounds, and height was 70 inches. The patient's past medical history included stroke, and concurrent conditions included chronic bronchitis, anxiety, emphysema, dizziness, unspecified lung disorder and handicapped (uses a walker) due to a stroke, the patient had history of abandonment disorder. Patient was on oxygen (since last year) due to lung problems.  The patient received covid-19 vaccine ad26.cov2.s (suspension for injection, route of admin not reported, batch number: 041A21A, and expiry: Unknown) dose was not reported, 1 total administered on 19-MAY-2021 14:00 for prophylactic vaccination on right arm. No concomitant medications were reported.  On 20-MAY-2021 at 1:30, patient had very intense cold chills from hell and unable to focus. The next day the patient's sister had to help patient to stay upright in bed because patient kept falling over in bed and could not stay upright. The patient's sister was concerned and insisted that the patient go to the hospital to seek medical care but the patient refused. Patient then experienced periods of loss of consciousness and patient's sister was having a hard time waking patient up and then checked oxygen saturation and noted that it was low at 72%. The patient's sister subsequently managed to wake patient up and turned up patient's oxygen tank from a previous setting of 2 to a higher setting of 3. At this point the patient's sister managed to help patient sit up. The patient has been confused and dizzy and oxygen kept dropping and it will go back up then go back down. The patient then stated it was not as bad as it was, more like it was a side effect. The patient called doctor on 20-MAY-2021 at 19:30 and told what had happened and what was going on. The patient's doctor told patient she had hypoxemia and gave instructions to keep oxygen on and to see if patient was feeling better. Patient felt very confused and found hard to talk, very tired and not focused at that moment. Patient could not remember what was told 15 seconds ago. Patient had oxygen currently in the low 80s to 96 depending on the lungs. Patient started to fall and lean over. Patient's sister had a hard time waking up patient. After the episode when patient's sister could not wake patient up, patient had developed a fever of 101.9. Patient alternated Ibuprofen and Tylenol as treatment for fever. Patient was short of breath and it was twice as worse as it was. Patient also mentioned that eyelid on left eye went down. Patient could not seem to return to normal and had written notes so that could keep on track and that patient had become very forgetful and that patient's sister had moved in to help after patient received the vaccine since patient was weak. Patient stated that had severe muscle cramps in feet, toes, arms and that hands clench up like a fist for a minute due to the pain. The clenching relieved the pain but as patient unclenches the pain started again. Patient stated that was in pain and hurting, had focus fog. The HCP had told patient of side effects and that symptoms would last for about 24 hours but patient had been experiencing them for a few days. Patient was worried and fever kept coming and going. Patient took meclizine for dizziness, as since patient received the vaccine, the dizziness was much worse.  The action taken with covid-19 vaccine ad26.cov2.s was not applicable. The patient had not recovered from hypoxemia, and the outcome of muscle cramps in feet, toes and arms, cold chills, falling over, loss of consciousness, confused, dizzy, unable to focus, foggy thinking, fever, shortness of breath, pain, short-term memory loss, tired, weak, very forgetful and hard to talk was not reported. This report was serious (Other Medically Important Condition).; Sender's Comments: V0 - 20210544835 - Covid-19 vaccine ad26.cov2.s - HYPOXEMIA , LOSS OF CONSCIOUSNESS and FOGGY THINKING . This event(s) is considered unassessable. The event(s) has a compatible/suggestive temporal relationship, is unlabeled, and has unknown scientific plausibility. There is no information on any other factors potentially associated with the event(s).