• Preterm birth after RIV4 versus IIV4 vaccination [ Time Frame: 12 months ]
  As measured by proportions of preterm birth after RIV4 versus IIV4 vaccination
• Combined fetal and neonatal death after RIV4 versus IIV4 vaccination [ Time Frame: 12 months ]
  As measured by proportions of combined fetal and neonatal death after RIV4 versus IIV4 vaccination
• Spontaneous abortion after RIV4 versus IIV4 vaccination [ Time Frame: 12 months ]
  As measured by proportions of spontaneous abortion after RIV4 versus IIV4 vaccination
• Pregnant women with moderate/severe solicited reactogenicity events (local and systemic) within 8 days after vaccination with RIV4 versus IIV4 [ Time Frame: 8 days ]
  As measured by proportions of moderate/severe solicited reactogenicity events in pregnant women vaccinated with RIV4 versus IIV4

This is a prospective, randomized clinical trial. During the study, pregnant women will be randomized (1:1) to receive RIV4 or IIV4. Vaccines will be administered by licensed providers.

Prior influenza vaccine history will be verified by medical record review when possible.

Injection-site (local) and systemic reaction data will be assessed on vaccination day and during the 8 days following vaccination using either identical web-based or paper diaries, depending on study participant preference.

Maternal serum samples will be collected for antibody titers relevant to Influenza at time points that include: prior to vaccination and ~29 days post vaccination. When feasible, maternal blood at delivery and cord blood serum will be analyzed for the same antibody titers.

Pregnant women will be followed through delivery with comprehensive obstetric and neonatal outcomes obtained from medical record review for 90 days of life.

• Safety
• Adverse Event Following Immunisation
• Birth Outcomes

• Biological: Quadrivalent Recombinant Influenza Vaccine
  The first recombinant inactivated influenza vaccine (RIV) using an insect baculovirus expression system and recombinant DNA technology
  Other Name: Flublok Quadrivalent
• Biological: Quadrivalent Inactivated Influenza Vaccine
  Standard inactivated influenza vaccine (IIV) manufactured involving the use of embryonated hen eggs.
  Other Name: Flulaval

• Experimental: RIV4
  The first recombinant inactivated influenza vaccine (RIV) using an insect baculovirus expression system and recombinant DNA technology
  Intervention: Biological: Quadrivalent Recombinant Influenza Vaccine
• Active Comparator: IIV4
  Standard inactivated influenza vaccine (IIV) manufactured involving the use of embryonated hen eggs.
  Intervention: Biological: Quadrivalent Inactivated Influenza Vaccine

Inclusion Criteria:

1. Pregnant, as determined by medical history
2. Age ≥ 18 years of age at enrollment
3. Intention of receiving influenza vaccine based on ACIP-CDC guidelines
4. Willing to provide written informed consent prior to initiation of any study procedures
5. Gestational age at vaccination ≤ 34 weeks 0 days based on reconciliation of last menstrual period and ultrasound dating. Estimated due date (EDD) and Gestational Age (GA-EDD) will be based on reconciliation of "sure" first day of the last menstrual period (LMP) and earliest dating ultrasound. If the LMP is uncertain, then the earliest dating ultrasound will be used to determine EDD and GA. If the ultrasound derived-EDD is in agreement with sure-LMP derived EDD, then the LMP-derived EDD is used to determine GA. If the ultrasound derived EDD is not in agreement with the LMP-derived EDD, the ultrasound-derived EDD is used to determine GA.
6. English or Spanish literate
7. Intention of being available for entire study period and complete all relevant study procedures, including follow-up phone calls and collection of delivery information.

Exclusion Criteria:

1. Influenza vaccine receipt during 2019-2020 or 2020-2021 influenza season prior to study enrollment.
2. Participation in this study in 2019-2020 influenza season
3. Any condition that may interfere with assessment of local injection site reactions, e.g. obscuring tattoos
4. Known or suspected immunosuppression as a result of an underlying illness or treatment
5. Use of anti-cancer chemotherapy or radiation therapy within the preceding 36 months
6. Use of oral or parenteral corticosteroids (≥ 20mg/day prednisone equivalent) or high-dose inhaled glucocorticoid for ≥ 14 consecutive days within the preceding 30 days
7. Has an active neoplastic disease (excluding non-melanoma skin cancer), a history of any hematologic malignancy, current bleeding disorder, or taking anticoagulants (a daily aspirin is acceptable)
8. Has a history of receiving immunoglobulin or other blood product (with exception of Rh immunoglobulin) within the 3 months prior to study vaccination.
9. History of febrile illness (> 100.4°F or 38°C) within the past 24 hours prior to study vaccination
10. Contraindication to IIV or RIV receipt including history of severe allergic reaction after a previous dose of any influenza vaccine; or to a vaccine component, including egg protein
11. History of Guillain-Barré syndrome within 6 weeks of a prior dose of any influenza vaccine
12. Receipt of any licensed vaccine within 7 days prior to study vaccination or intention of receiving any vaccines during 8-day post-vaccination period
13. Receipt of live vaccine during current pregnancy
14. Signs or symptoms of active preterm labor, defined as regular uterine contractions with cervical change (dilation/effacement)
15. Known multi-fetal gestation or fetal congenital anomaly, e.g. genetic abnormality or major congenital malformation based on antenatal ultrasound
16. Anyone who is already enrolled or plans to enroll in another randomized clinical trial with any drug, vaccine or medical device. Co-enrollment in observational or behavioral intervention studies are allowed at any time
17. Any condition which, in the opinion of the investigators, may pose a health risk to the participant or interfere with the evaluation of the study objectives.
18. Anyone who is a relative of any research study personnel or is an employee supervised by study staff

• Children's Hospital Medical Center, Cincinnati
• Boston Medical Center
• Centers for Disease Control and Prevention