heavy menstrual bleeding; off label use/maternal exposure during breast feeding; drug use in unapproved population/maternal exposure during breast feeding; irregular periods; this is a spontaneous report received from a contactable reporter(s) (consumer or other non hcp) from the regulatory authority. the reporter is the patient. regulatory number: gb-ra-vac-202202111924006420-bzy3n (ra). other case identifier(s): gb-mhra-adr 26595195 (ra). a 37 year-old female patient received bnt162b2 (comirnaty) (batch/lot number: unknown) as dose 3 (booster), single for covid-19 immunisation. relevant medical history included: "breast feeding" (ongoing). there were no concomitant medications. vaccination history included: covid-19 vaccine (dose 1, manufacturer unknown), for covid-19 immunization; covid-19 vaccine (dose 2, manufacturer unknown), for covid-19 immunization. the following information was reported: menstruation irregular (medically significant) with onset 06dec2021, outcome "recovered", described as "irregular periods"; heavy menstrual bleeding (medically significant), outcome "recovered", described as "heavy menstrual bleeding"; off label use (non-serious), outcome "unknown", described as "off label use/maternal exposure during breast feeding"; product use issue (non-serious), outcome "unknown", described as "drug use in unapproved population/maternal exposure during breast feeding". the patient had heavier periods since the booster. one was a week after her last period about 5 days after the booster. it was bright red blood and very heavy.the patient did not think this reaction occurred as a result of a mistake made in the administration of the vaccine. it was also reported that the has been no more bleeding at the time of the report. no follow-up attempts are possible; information about lot/batch number cannot be obtained. no further information is expected. sender's comments: linked report(s) : gb-pfizer inc-202200323081 baby case