irregular periods; painful periods; prolonged heavy periods/ longer and heavier periods; this is a spontaneous report received from a contactable reporter (consumer) from the regulatory authority. other case identifier(s): gb-mhra-vac-202202261833416530-g7mih, gb-mhra-adr 26655622. a 23 year-old female patient (not pregnant) received bnt162b2 (comirnaty) (batch/lot number: unknown) as dose number unknown, single for covid-19 immunisation. the patient's relevant medical history and concomitant medications were not reported. patient last menstrual period date was 09jan2022. patient was not currently breastfeeding. the following information was reported: heavy menstrual bleeding (medically significant) with onset 24aug2021, outcome "not recovered", described as "prolonged heavy periods/ longer and heavier periods"; menstruation irregular (medically significant), outcome "unknown", described as "irregular periods"; dysmenorrhoea (medically significant), outcome "unknown", described as "painful periods". patient experienced longer and irregular periods: heavier and painful. the reporter did not think this reaction occurred as a result of a mistake made in the administration of the vaccine. no follow-up attempts are possible; information about lot/batch number cannot be obtained. no further information is expected