Pre-term delivery at 36.6 weeks of gestational age; Vaccine exposure during pregnancy; Low potassium; This observational study case was received from other health professional  on 30-Nov-2020 and concerned a 35-year-old pregnant, female subject of body weight 108 lbs, height 62 inches and body mass index (BMI) 19.8, enrolled in a prospective observational safety study on pregnancy outcomes in women immunized with Afluria QIV during pregnancy.   The subject's medical history included advanced maternal age. The subject's concomitant medications included Prenatal vitamins (unspecified vitamins and mineral) for healthy pregnancy.  The subject's obstetrical history included three previous pregnancies with one full term live birth and two elective abortions. The subject had no history of offspring with major congenital malformation (MCM). The subject had no maternal or paternal medical history with MCMs. The subject did not use tobacco, alcohol or illicit drugs during pregnancy.  The subject's last menstrual period (LMP) date was 23-Jun-2020. The subject's estimated delivery date (EDD) was reported as 30-Mar-2021 and corrected estimated date of delivery (CEDD) was not reported. The type of pregnancy was singleton.  On 02-Sep-2020, at approximately ten weeks of gestation (calculated per LMP), the subject was vaccinated with Afluria QIV (Flu Vaccine Egg Split QIV CSL) (influenza vaccine; dose, route of administration and  anatomical location: not reported) (explicitly coded as 'Vaccine exposure during pregnancy') for influenza immunisation. The batch number reported was P100239705.  On an unknown date in Sep-2020, an unknown amount of time after receiving Afluria Quadrivalent, the subject had low potassium.   On 18-Sep-2020, panorama test revealed no MCMs.    On 19-Sep-2020, first trimester screen revealed no MCMs.   On 21-Sep-2020, the subject started treatment for low potassium with potassium chlorate, at a dose of 20 mcg, three times a day, administered orally.   On 03-Nov-2020, amniocentesis revealed no MCMs.  On 06-Nov-2020, the subject finished the treatment with potassium chloride.  On 07-Mar-2021, at gestational age of 36.6 weeks (as reported), the subject delivered premature live male neonate via vaginal method of delivery. The birth weight of the neonate was 2781 g, head circumference 32 cm and length 49.5 cm. Appearance, pulse, grimace, activity and respiration (APGAR) score at one minute was 9, at five minutes was 9 and at ten minutes was not reported. No pregnancy complications were noted. No MCM was identified at birth.  The outcome of the event 'premature delivery' was not reported.  At the time of follow up reporting on 10-Dec-2020, the subject had not recovered from the event of 'potassium low'.   The reporter did not provide a causality assessment.  The event of 'premature delivery' was considered to be medically significant by a Physician within Seqirus's Pharmacovigilance and Risk Management Department.  The case is linked to the case 202101842 (corresponding baby case).  Additional information received from other health professional on 10-Dec-2020: The subject was enrolled in pregnancy registry and report type was changed from spontaneous to report from study. The subject's ID amended. LMP date and EDD were provided. The narrative was amended accordingly.  Additional information received from other health professional on 25-Feb-2021 with an additional document received on the same day (being processed together): Type of pregnancy provided and number of fetus populated in pregnancy information. Stop date for treatment medication potassium chloride provided. Narrative was amended accordingly.  Additional information received on 09-Mar-2021 from other health professional: Pregnancy outcome received. New event added, premature delivery. Stop date was added for the event of 'vaccine exposure during pregnancy' and outcome was changed from 'Not reported' to 'Resolved'. Pregnancy with advanced maternal age added in medical history. The narrative was amended accordingly.   Company comment: A 35-year-old pregnant, female subject experienced blood potassium decreased 18 days after administration of Afluria QIV and premature delivery six months after vaccination. The subject's medical history included advanced maternal age. The subject's obstetrical history included three previous pregnancies with one full term live birth and two elective abortions. Causality is assessed as unrelated due to biological implausibility and confounding factors (advanced maternal age and obstetrical history). Vaccine exposure during pregnancy is assessed as unrelated per company conventions.; Sender's Comments: A 35-year-old pregnant, female subject experienced blood potassium decreased 18 days after administration of Afluria QIV and premature delivery six months after vaccination. The subject's medical history included advanced maternal age. The subject's obstetrical history included three previous pregnancies with one full term live birth and two elective abortions. Causality is assessed as unrelated due to biological implausibility and confounding factors (advanced maternal age and obstetrical history). Vaccine exposure during pregnancy is assessed as unrelated per company conventions.