Her menstrual period stopped at day 4 / usually had a 7-8 days cycle for her entire life; This is a spontaneous report from a contactable consumer (patient). A 37-year-old female patient received second dose of bnt162b2 (Batch/Lot number was not reported), via an unspecified route of administration, administered in left arm on 08Aug2021 at 15:15 at age of 37-year-old as single dose for covid-19 immunisation. The patient previously received first dose of bnt162b2 (Batch/Lot number was not reported), via an unspecified route of administration, administered in left arm on 19Jul2021 at age of 37-year-old as single dose for covid-19 immunisation. Medical history was reported as none. There were no concomitant medications.  No history of all previous immunization with the Pfizer vaccine considered as suspect. No additional vaccines administered on same date of the Pfizer suspect. No vaccine administered at military facility. No prior vaccinations within 4 weeks. The patient received second dose of vaccine on 08Aug2021. She was on her 4th day of her period. One hour later her menstrual period stopped at day 4 and that never happens. The patient usually had a 7-8 days cycle for her entire life. It had never been 4 days her entire life. This was the first time in her entire life that this happened. She would like to know what information was available about menstrual changes after receiving the vaccine. She was trying to find out if it was a side effect, or if something was happening. The reporter didn't have a bad reaction to the COVID vaccine. Her second vaccine was at a different location. The event didn't require a visit to emergency room/ physician office. The outcome of event was unknown.  Response: The authorized EUA Prescribing Information for the Pfizer-BioNTech COVID-19 Vaccine does not list menstrual changes as adverse reaction from clinical studies and post authorization experience. The authorized Product Monograph for the Pfizer BioNTech COVID-19 Vaccine does not list menstrual changes as adverse reaction from clinical studies and post-market experience. Pfizer has received reports of menstrual cycle (period) changes since the Pfizer-BioNTech COVID-19 Vaccine was authorized for use. These cases have not been proven to be a result of vaccination of the Pfizer-BioNTech COVID-19 Vaccine. an organization that provides leadership in education, research and gynecologic care to improve the reproductive health of young people. states that: [1] here have been anecdotal and media reports of irregular menstrual cycles, including skipped periods, prolonged periods, spotting after COVID-19 vaccines in adults and similar changes in menstrual cycles have been reported after COVID-19 infection. This is not surprising because infections, immune reactions, and fevers can cause short-term, self-limited changes in cycles. However, further studies are needed to verify these changes in people who have received COVID-19 vaccines. In teens, it is also important to note the natural variability in cycles due to the normal maturation process and the large range of normal menstrual cycles in the first few years after the onset of menses. Stress and weight changes, which have also occurred in many adolescents during the pandemic, can themselves cause these changes. COVID-19 vaccines can be used during puberty and that irregular periods are a common part of adolescence and usually have a wide normal range. Your healthcare provider knows your medical history, can discuss the risks and benefits of the vaccine, and can provide recommendations to you. Pfizer is independent of these recommendations.  The lot number for the vaccine BNT162B2, was not provided and will be requested during follow up.