Chlamydia; Vaccine exposure during pregnancy; This is an observational study case, initially received from other health professional on 17-Jan-2020, concerning a 27-year-old, female pregnant subject of body weight: 131 lbs, height: 63.6 inches and body mass index (BMI): 22.8, enrolled in a prospective observational safety study.  The subject's relevant medical history included post-traumatic stress disorder (PTSD), severe depression, urinary tract infection (UTI), Herpes simplex 2, morning sickness, bacterial vaginosis and candidiasis. The subject was group B streptococcal bacteriuria carrier (as reported).  The subject's past drugs included cyclobenzaprine for an unknown indication, valacyclovir for Herpes simplex 2 and terconazole for candidiasis. The concomitant medications of the subject included Nitrofurantoin monohydrate/ Macrocrystals (nitrofurantoin monohydrate) for UTI, Diclegis (doxylamine succinate, pyridoxine hydrochloride) for morning sickness, lidocaine for Herpes simplex 2, metronidazole for bacterial vaginosis, Phenergan (promethazine) for nausea and vomiting, unspecified vitamins as obstetric (OB) supplement and sertraline for severe depression.  The subject's obstetrical history included two previous full term live births. The subject had no history of offspring with major congenital malformation (MCM) and no relevant maternal and paternal medical history. The subject did not use alcohol or illicit drugs during pregnancy, but she used cigarettes (one pack per week).    The subject's last menstrual period (LMP) date was 18-Apr-2019. The type of pregnancy was singleton. The subject's estimated date of delivery (EDD) and corrected EDD (CEDD) were reported as 23-Jan-2020.   On 12-Jul-2019, the subject underwent first trimester screen and no MCM was noted.   On 12-Aug-2019, the subject underwent second trimester screen and no MCM was noted.   On 03-Sep-2019, the subject underwent anatomy ultrasound and no MCM was noted.  On 12-Dec-2019, approximately at 34 weeks of gestation, the subject was administered Flucelvax QIV (Flu Vaccine Cell Subunit QIV NVD) [influenza vaccine, subunit influenza virus vaccine polyvalent, batch number: 261223, dose: 0.5 ml, route of administration, anatomical location and expiration date: not reported] for an influenza immunization (explicitly coded as ''Vaccine exposure during pregnancy'').  On 02-Jan-2020, 21 days after vaccination, the subject experienced chlamydia and she was treated with azithromycin at dose of two tablets, every day, orally form 02-Jan-2020 to 03-Jan-2020.   At the time of this report, the outcome of chlamydia was not reported.   Per company, the event of chlamydia was considered as serious due to criterion of medical significance.   The reporter did not provide causality assessment to Flucelvax QIV.  Company Comment: The subject experienced chlamydial infection 21 days after vaccination with the suspect product, Flucelvax QIV. Reportedly, vaccination occurred during pregnancy at gestational age of approximately 34 weeks. Chronology is plausible. Medical history of urinary tract infection, bacterial vaginosis and candidiasis may have contributed to development of the event. Causality is also confounded by therapy with multiple concomitant medications. Vaccine exposure during pregnancy is assessed as not related per company's conventions. Causality between suspect product and the event chlamydial infection is assessed as not related due to biological implausibility.; Sender's Comments: The subject experienced chlamydial infection 21 days after vaccination with the suspect product, Flucelvax QIV. Reportedly, vaccination occurred during pregnancy at gestational age of approximately 34 weeks. Chronology is plausible. Medical history of urinary tract infection, bacterial vaginosis and candidiasis may have contributed to development of the event. Causality is also confounded by therapy with multiple concomitant medications. Vaccine exposure during pregnancy is assessed as not related per company's conventions. Causality between suspect product and the event chlamydial infection is assessed as not related due to biological implausibility.