In this study the investigators propose a randomized controlled trial to evaluate the effect of influenza vaccination on clinical pregnancy rates among women undergoing in vitro fertilization.

Since 2004 flu vaccination has been recommended by American College of Obstetricians and Gynecologists (ACOG) Advisory Committee on Immunization Practices (ACIP) for all pregnant women during flu season, regardless of the trimester of pregnancy. Many studies have examined the safety of influenza vaccination and found benefits beyond the simple prevention of maternal infection including the reduction of still birth. Recently evidence has emerged that women who have received a flu vaccination were less likely to experience premature labor. The investigators hypothesize that some of the maternal benefits of influenza vaccination might be based on other than prevention of the flu, possibly secondary to the induction of immune system pathways that favor immunological tolerance. Induction of immune system pathways that favor immunological tolerance has long been considered a possible path to improved embryo implantation and reduce miscarriages, though the topic is still somewhat controversial.

Patients undergoing IVF will be randomized in two strata:

1. History of recurrent miscarriage or implantation failure
2. No history of recurrent miscarriage of implantation failure

Consenting patients will be randomized to receive either influenza vaccine or a saline placebo. Vaccination will occur 10 days before initiating anticipated menses or fertility treatment cycle start.

All patients will be receiving an offered a second injection at the time of negative pregnancy test or at the time of sign-out with a clinical pregnancy. The second injection will either be placebo or influenza vaccination depending on the original randomization. Patients and clinical staff will remain blinded to the original treatment assignment, except for the study coordinator and the single staff member assigned to administer the injection.

Inclusion Criteria:

1. All women preparing to undergo an IVF cycle
2. Women will only be allowed to participate in one treatment cycle
3. Willingness to have an influenza vaccination
4. Signed informed consent

Exclusion Criteria:

1. Any contraindication for flu vaccination
2. Unwillingness to have a flu vaccination
3. Unwillingness to sign informed consent
4. Previous diagnosis of unfavorable endometrial development
5. Unresolved uterine condition that significantly compromises the endometrial cavity

The investigators do not plan to share individual participant data (IPD)