Influenza: Are You Protecting Your Patients?

Abstract & Commentary

By Alison Edelman, MD, MPH, Assistant Professor, Assistant Director of the Family Planning Fellowship, Department of Obstetrics & Gynecology, Oregon Health & Science University, Portland, is Associate Editor for OB/GYN Clinical Alert.

Dr. Edelman reports no financial relationship to this field of study.

Synopsis: Influenza vaccination of pregnant women decreased the risk of influenza in both the mothers and newborns.

Source: Zaman K, et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med 2008;359:1555-1564.

Zaman et al randomized 340 pregnant Bangladeshi women to receive either inactivated influenza or pneumococcal polysaccharide (control group) vaccine.1 Women were assessed weekly until 24 weeks postpartum for any illnesses. In addition, if a febrile illness was reported in either the mother or newborn, a clinical assessment was performed and infants were also tested for influenza antigens. Influenza-vaccinated women were 36% (95% confidence interval [CI], 4%-57%) less likely to report a respiratory illness with "fever" and 43% (95% CI, -9% to 70%, not clinically significant) less likely to report a respiratory illness with a fever > 38 ° C. There were fewer cases of confirmed influenza in infants of influenza-vaccinated mothers (6 vs 16 cases), with a vaccine effectiveness of 63% (95% CI, 5%-85%).


I will admit that discussing influenza may not be in your top 10 list, but it is nonetheless a very important preventive health care issue, especially for our pregnant patients (and for you, as a health care worker). In addition, by vaccinating pregnant women we are also decreasing influenza infection in newborns. The flu vaccine is not approved for infants younger than 6 months of age even though this group has a higher rate of hospitalization if infected with influenza.2,3 Prior research has shown influenza antibody transmission from the mother to the newborn in umbilical cord blood but Zaman and colleagues have demonstrated actual disease prevention.1,4 The authors estimate that 5 pregnant women need to be vaccinated to prevent 1 respiratory illness with fever in a woman or infant.1 Whether this protection is from vertical transmission, decreased exposure risk from herd immunity, or both is unknown.

Although pregnant women are included in the recommended list of people who should receive the influenza vaccine, only a few receive it.5 This is unfortunate, as pregnant women are at higher risk for morbidity and mortality if infected with influenza.6 In addition, as shown by Zaman et al, we get a "two for one" with coverage of both mothers and newborns younger than 6 months of age.1

Myths abound regarding the flu vaccination that may prevent patients from being vaccinated. In fact, I am sure some of these sound familiar: "The vaccine gave me the flu," "I got the flu anyway even with the vaccination," "I have never had the flu so I don't need to be vaccinated," and "I am worried that the vaccine could hurt me or my baby." Here are some evidence-based responses to help combat these concerns:

  • The vaccine gave me the flu: The inactivated vaccine cannot give you the flu but you can get a post-vaccine systemic reaction that is reminiscent of a viral infection (low-grade fever and/or muscle aches).7
  • I got the flu anyway even with the vaccination: The influenza vaccine is about 70-90% effective (this depends on age and immune response) and it takes about two weeks to establish immunity.7 So it's not perfect! What in life ever is? This is because some years the strains contained in the vaccine are not well matched to the actual circulating strains. Zaman et al report an even lower vaccine effectiveness, but this may be due to the perennial vs seasonal nature of the flu in tropical regions of the world.1 Finally, many patients call any viral illness the flu and it's important to differentiate between the two. Influenza can be mild to severe but usually starts suddenly with a high fever, headache, severe muscle aches/tiredness, and a runny or stuffy nose. Gastrointestinal symptoms (nausea, vomiting, and diarrhea) can also occur but usually are seen more in children than adults.7
  • I have never had the flu so I don't need to be vaccinated: Unfortunately, influenza is not something our bodies build a lifelong immunity to because the strains change every year. So just because you haven't had it, doesn't mean you can't get it.
  • I am worried that the vaccine could hurt me or my baby: There are two types of influenza vaccinations, inactivated (killed) and live. Pregnant women should not receive the live vaccine.7 Also if you have an allergy to eggs you should not receive either type of influenza vaccine, killed or live.

There has been some unfavorable press regarding the preservative contained in the influenza vaccine, thimerosal. Thimerosal is a mercury-based preservative that prevents bacterial and fungal contamination in the vaccine.8 The level of thimerosal contained in the influenza vaccine cannot and has not caused mercury toxicity. There have been no adverse events other than local site reactions linked to thimerosal in either children or adults (pregnant or otherwise). The preservative was originally added to vaccines because of 12 deaths in the 1920s due to staphylococci contamination in a diphtheria vaccine.8 Even with the evidence supporting the safety of thimerosal, a thimerosal-free vaccine is now recommended for use in children, but the thimerosal-containing vaccine can also be used. Of note, even the thimerosal-free vaccine contains a trace amount of thimerosal.

Although nonpharmacological measures (i.e., hand washing and improved respiratory hygiene) have been shown to help decrease respiratory illnesses, they haven't been proven to specifically prevent the flu.


  1. Zaman K, et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med 2008;359:1555-1564.
  2. General Recommendations on Immunizations. Recommendations of the Advisory Committee on Immunization Practices (ACIP). Available at: Accessed Oct. 23, 2008.
  3. Neuzil K, et al. The effect of influenza on hospitalizations, outpatient visits, and courses of antibiotics in children. N Engl J Med 2000;342:225-231.
  4. Englund J, et al. Maternal immunization with influenza or tetanus toxoid vaccine for passive antibody protection in young infants. J Infect Dis 1993;168:647-656.
  5. Naleway A, et al. Delivering influenza vaccine to pregnant women. Epidemiol Rev 2006;28:47-53.
  6. Cox S, et al. Hospitalizations with respiratory illness among pregnant women during influenza season. Obstet Gynecol 2006;107:1315-1322.
  7. Centers for Disease Control and Prevention. Seasonal Flu. Available at: Accessed Oct. 23, 2008.
  8. Food and Drug Administration. Thimerosal in Vaccines. Available at: Accessed Oct. 23, 2008.