brain swelling; she developed a difuse erythematous rash; cerebellar ataxia/she wasn't walking or crawling and her legs were uncoordinated and shaking/felt like she was more wobbly in terms of her gait/the patient preferred to crawl and only take 2-3 steps at a time; high grade fever; allergic reaction to VARIVAX; This spontaneous report was received from a registered nurse (the patient's mother), the physician and healthcare professionals (HCPs) via company representative, and refers to a currently 2-year-old, female patient. The patient's birth weight was 3500 g (reported as 3.5 kg). She had no siblings. No pre-existing diagnosed allergies, birth defects, or medical conditions. There were no concomitant medications or vaccines. Laboratory tests were unknown.  On 19-MAY-2020, at the age of 12 month-old, the patient was vaccinated at private doctor's office/hospital with the first dose of varicella virus vaccine live (oka/merck) (VARIVAX) lot # S025913, expiration date was reported and also validated as 20-AUG-2021, subcutaneously (provided as subq), single use (dose was unknown, strength and anatomical location were not provided) for immunization. The vaccine was purchased with private funds. It was only vaccine given at the time of the visit as the patient's mother preferred to split vaccines. No illness at time of vaccination.  On the same day at the evening (also provided as one or two hours after the administration), the patient developed a fever (fever) of 100.7 Fahrenheit degree (F). On that date, the temperature measured as well 103-104 F. She continued with fever for about 4 days with the temperature of 103 F on day 2 of illness (considered as 21-MAY-2020). The treatment for high grade fever included paracetamol (TYLENOL) and ibuprofen (MOTRIN). On 21-MAY-2020 (day 2 after administration), the patient's mother noticed, that the patient was not walking or crawling and her legs were uncoordinated and shaking. She was more wobbly in terms of her gait. On 23-MAY-2020, provided as on day 4 of illness, she recovered from the fever. Also on that day, the patient developed a diffuse erythematous rash (rash erythematous) (discrepancy, onset was also provided as 19-MAY-2020). On approximately 24-MAY-2020 (by day 5) of the illness, the patient preferred to crawl and only take 2-3 steps at a time. Prior to the vaccine, the patient was walking regularly. It took approximately one week for her to stand on her own. It took approximately three weeks for her to get back to full walking capacity. The event was diagnosed as cerebellar ataxia (cerebellar ataxia). Prior to this diagnosis, the daughter had seen three different doctors to rule out other diagnosis, such as urinary tract infection (UTI), pertaining to the symptoms the patient was experiencing. The neurologist was trying to rule out if the patient experienced an allergic reaction to varicella virus vaccine live (oka/merck) (VARIVAX) (allergy to vaccine) and referred her to an allergist regarding an allergen test. On an unspecified date, the patient developed also brain swelling (brain swelling).  It was stated, that no temperature excursion occurred as a reading from a digital data logger. The patient's mother sought medical attention. On an approximately 28-MAY-2020, 9 days after vaccination, the patient was seen virtually by neurologist. She was observed to have a normal gait for her age. No hospitalization was needed due to the events. It was also reported, that the events were not connected with any cancer or overdose. The cerebellar ataxia recovered on approximately 11-JUN-2020 (also reported as middle of June 2020, after three weeks) (conflicting information, it was also reported, that the events of cerebellar ataxia, fever and erythematous rash started on 19-MAY-2020 and the stop date was on 28-MAY-2020).   No treatment was given for the events except for the fever. The physician did not report these events previously. Regulatory Authority information was sent to the physician and it was received by the clinical staff member. It was not aware, if the physician would like to join this program and send the sample. Additional information has been requested.  The outcome of brain oedema, rash erythematous, pyrexia, allergy to vaccine and cerebellar ataxia was reported as recovered/resolved.    The causality between the suspect vaccination and the events of cerebellar ataxia, rash erythematous, fever and allergy to vaccine were considered by the reporter as possibly related. The causality between the vaccine and the event of brain swelling was not provided.   Upon internal review, the event brain swelling was determined to be medically significant.