The Flu Shot in Pregnancy

The formulation distinction that changes the entire conversation, and the studies your provider hasn’t mentioned

Full Drug Card → Influenza Vaccine (Maternal)

Every pregnancy, every provider recommends the flu shot. What almost no provider tells you: there are two versions, and they are not the same product. One is from a multi-dose clinic vial and contains 25 micrograms of a mercury-based preservative called thimerosal. The other is a single-dose pre-filled syringe with no preservative at all. Which one you receive is rarely identified before administration — and you almost never think to ask, because no one told you there was a question to ask.

The core distinction

Two Products, Two Very Different Ingredient Lists

Feature	Multi-dose vial	Single-dose pre-filled syringe
Thimerosal (mercury preservative)	25 mcg ethylmercury per doseUsed to prevent contamination in multi-dose vials	None (preservative-free)Single-use — no preservative required
Cost to clinic	LowerWhy most high-volume settings default to this	Slightly higher
Disclosed to patient?	Almost never	Only if you ask
Thimerosal-free brands (single-dose)		Fluzone Quadrivalent, Fluarix, Flucelvax, AfluriaAll in pre-filled syringe form

Why thimerosal in pregnancy is a different conversation

Thimerosal contains ethylmercury — an organic mercury compound that is metabolized and excreted more quickly than methylmercury (the form in fish). This difference is used to reassure patients. What is rarely mentioned: a primate study by Burbacher et al. (2005, Environmental Health Perspectives) found that ethylmercury redistributed to brain tissue at approximately twice the rate of methylmercury. The shorter half-life in blood does not mean less brain exposure. The common comparison — “thimerosal is safer than fish mercury” — is not supported by this study.

At 25 mcg per dose, the thimerosal in a multi-dose flu shot is a direct parenteral (injected) mercury dose. Injected and ingested mercury are not comparable exposures — the bioavailability is fundamentally different.

What the research says

Three Studies Not Mentioned at the Appointment

Goldman & Miller (2013) — 4,000% VAERS signal

Goldman and Miller (2013, Human and Experimental Toxicology) analyzed VAERS (Vaccine Adverse Event Reporting System) reports for fetal loss following the 2009 H1N1 influenza campaign. They found a 4,000% increase in fetal-loss reports among pregnant women who received both the H1N1 vaccine and the seasonal influenza vaccine in the same season — the seasonal flu vaccine containing thimerosal.

VAERS reports are self-selected and do not establish causation. But a 4,000% increase in reported fetal losses in a specific exposure group in a specific year is a signal that belongs in the informed consent conversation. It is not disclosed when the flu shot is offered.

Donahue et al. (2017, Vaccine) — first-trimester miscarriage signal

This peer-reviewed study, published in the journal Vaccine, used data from the Vaccine Safety Datalink — a CDC-affiliated surveillance system. It found a statistically significant association between influenza vaccination in the first trimester and spontaneous abortion (miscarriage), particularly in women who had also received the influenza vaccine the prior year. The adjusted odds ratio was 7.7 (95% CI 2.2–27.3) for women vaccinated in both the current and prior seasons.

An odds ratio of 7.7 means the study found the rate of miscarriage in this specific group to be approximately 7.7 times higher than in unvaccinated women. The authors concluded the finding was unexpected and required replication. As of 2026, it has not been definitively refuted. It is not disclosed when the flu shot is offered in the first trimester.

The 28-week combined exposure

Many pregnant women are offered both the multi-dose flu shot and multi-dose RhoGAM (Rh immunoglobulin) at the same 28-week appointment. Multi-dose RhoGAM contains up to 25 mcg of ethylmercury per dose. Receiving both at the same appointment produces a combined parenteral mercury dose of up to 50 mcg. This combination has not been studied for its developmental impact. Neither provider — the OB ordering RhoGAM, the nurse administering the flu shot — typically has visibility into both exposures happening at the same visit.

What the package insert says

The Language Manufacturers Use

■The FDA prescribing information for inactivated influenza vaccines states that safety and efficacy in pregnant women were not established in randomized controlled trials conducted specifically in pregnant populations. The recommendation is based on observational data and immunogenicity studies — not RCTs in pregnancy.
■The package insert does not distinguish between multi-dose (thimerosal-containing) and single-dose (preservative-free) formulations in pregnancy counseling guidance. It is treated as one product. The clinical evidence is largely from single-dose formulations; much of the administration in high-volume settings uses multi-dose vials.
■Influenza vaccine is the single most compensated vaccine in the VICP (Vaccine Injury Compensation Program) system. The most common compensated injury is SIRVA — shoulder injury from incorrect injection angle or site — which causes prolonged or permanent shoulder pain and is frequently attributed to flu shots administered in pharmacies. Guillain-Barré syndrome (GBS) following influenza vaccination has been compensated in VICP since the 1976 swine flu campaign. VICP data: hrsa.gov/vaccine-compensation/data.

Before you decide

Questions to Bring to Your Provider

Is this vaccine from a single-dose pre-filled syringe, or a multi-dose vial? If multi-dose, does it contain thimerosal? What is the exact amount?
Can I specifically receive a thimerosal-free, single-dose formulation? Which brands are available here in single-dose form?
I received a flu shot last year as well. Has the Donahue 2017 association between consecutive-year flu vaccination and first-trimester miscarriage been ruled out?
I am also scheduled for RhoGAM at this same visit. What is the combined mercury exposure from both products if I receive the multi-dose versions of both?
What does the package insert for this specific formulation say about whether safety in pregnancy was established in randomized controlled trials in pregnant women?
If I decline this flu shot, what increased risk am I actually taking on, in my specific situation and trimester?