Should a pregnant patient get the flu vaccine with thimerosal (preservative)?

Should a Pregnant Patient Receive Flu Vaccine with Thimerosal (Preservative)?

Yes, pregnant women should receive influenza vaccine regardless of thimerosal content—the benefits of preventing influenza-related complications and death far outweigh any theoretical risk from thimerosal exposure. 1

Evidence-Based Rationale

Maternal Risk Without Vaccination

Pregnant women face substantially elevated risks from influenza infection:

• Hospitalization risk increases dramatically by trimester: relative risk rises from 1.4 at weeks 14-20 of gestation to 4.7 at weeks 37-42 compared to postpartum women 1
• Third-trimester hospitalization rates (250/100,000 pregnant women) match those of non-pregnant women with high-risk medical conditions 1
• Historical pandemic data documented excess deaths among pregnant women during the 1918-1919 and 1957-1958 pandemics 1
• Physiologic vulnerability stems from increased heart rate, stroke volume, oxygen consumption, decreased lung capacity, and immunologic changes 1

Thimerosal Safety Profile

The evidence strongly supports safety of thimerosal-containing vaccines in pregnancy:

• No evidence of harm exists from the level of mercury exposure through influenza vaccination—thimerosal has been used in U.S. vaccines since the 1930s without documented adverse effects 1
• Substantial safety margin has been incorporated into health guidance values for organic mercury exposure 1
• No scientific evidence demonstrates that thimerosal-containing vaccines cause adverse events among children born to vaccinated mothers 2
• The benefit of influenza vaccine with reduced or standard thimerosal content outweighs the potential risk, if any, for thimerosal 1

Vaccine Safety in Pregnancy

Multiple studies confirm influenza vaccine safety:

• Study of >2,000 pregnant women demonstrated no adverse fetal effects associated with influenza vaccine 1
• Matched case-control study of 252 pregnant women found no adverse events after vaccination and no difference in pregnancy outcomes compared with 826 unvaccinated pregnant women 1
• 2000-2003 surveillance of an estimated 2 million vaccinated pregnant women reported only 20 adverse events to VAERS, including nine injection-site reactions and eight systemic reactions; three miscarriages were reported but not causally related to vaccination 1
• No increased risk of spontaneous abortions, preterm birth, low birth weight, congenital malformations, or cesarean section has been demonstrated 3, 2, 4

Clinical Algorithm for Vaccination

Timing Recommendations

• All pregnant women who will be beyond the first trimester (>14 weeks gestation) during influenza season should be vaccinated 1
• Some providers prefer second trimester administration to avoid coincidental association with spontaneous abortion (common in first trimester) and because vaccine exposures have traditionally been avoided in the first trimester 1
• High-risk pregnant women (those with medical conditions increasing complication risk) should be vaccinated before influenza season regardless of pregnancy stage 1
• Influenza vaccine can be safely administered during any trimester of pregnancy 1, 5, 3, 2

Vaccine Selection

• No preference exists for thimerosal-free versus thimerosal-containing vaccine for pregnant women 1
• Either preparation may be used depending on availability 1
• Inactivated influenza vaccine (TIV) only—live attenuated influenza vaccine (LAIV) is not licensed for use in pregnant women 1, 3

Expected Benefits

Vaccination prevents significant morbidity:

• An average of 1-2 hospitalizations can be prevented for every 1,000 pregnant women vaccinated 1
• Maternal protection reduces rates and severity of influenza-like illness 4
• Infant protection occurs through transplacental antibody transfer and breast milk, protecting infants for several months after birth who cannot be vaccinated before 6 months of age 3, 6, 7, 4

Common Pitfalls to Avoid

• Do not delay vaccination waiting for thimerosal-free formulations—the risks of influenza infection far exceed any theoretical thimerosal risk 1
• Do not avoid first-trimester vaccination in high-risk women—the benefits outweigh concerns about coincidental association with spontaneous abortion 1
• Do not use LAIV in pregnant women—only inactivated vaccine is appropriate 1, 3
• Do not miss the opportunity to vaccinate during prenatal visits—provider recommendation is the most important factor in vaccine acceptance 4