Maternal immunization remains a priority for ob.gyns. – an opportunity to provide protection against serious infectious diseases for both the mother and the baby. With influenza vaccination rates in pregnant women still hovering around 50% and the emerging public health problem of vaccine hesitancy, we must fully embrace our responsibility to recommend immunizations and to effectively communicate what is known about their efficacy and safety. Ideally, we should offer them as well.
One reason for the low rates of influenza vaccination – one of the two vaccinations routinely recommended for all pregnant women in the United States – is that pregnant women do not always know the importance of the vaccine. This is actionable: Data clearly show that the physician’s recommendation makes a difference and that a clinician’s offer to administer the vaccination has an even greater impact.
A 2017 Centers for Disease Control and Prevention analysis of data from Internet panel surveys1 shows that women who reported receiving both a clinician recommendation and offer of vaccination had higher coverage during the 2015-2016 and 2016-2017 influenza seasons (63.7% and 70.5%) than did women who reported receiving a clinician recommendation but no offer (37.5% and 43.7%) and women who reported receiving no recommendation for vaccination (12.8% and 14.8%).
The analysis suggests there are consistently missed opportunities: Fewer than 70% (67.3%) of pregnant women in the 2016-2017 flu season reported receiving a clinician recommendation for and offer of vaccination. This is similar to the prior three flu seasons, according to the CDC.
This year, with the COVID-19 pandemic ensuing, the prevention of severe influenza illness – and other vaccine-preventable illnesses – takes on even greater importance. It is not known what the impact of two potentially devastating respiratory infections could be for pregnant individuals. Therefore, maximal protection against at least influenza will be critical.
Influenza and Tdap
Poor outcomes and disproportionately high death rates for pregnant women were observed in both the influenza pandemic of 1918-1919 and the 1957 “Asian flu” pandemic. Maternal immunization for influenza has been recommended in the United States since 2004 (part of the recommendation that everyone over the age of 6 months receive an annual flu vaccine), but it was the H1N1 influenza pandemic of 2009 that reinforced its value and led our field to more fully embrace influenza vaccination as a priority for prenatal care.
Surprisingly, most of the pregnant women who became severely ill from the H1N1 virus were young and healthy and did not have a coexisting condition known to increase risk, such as asthma or diabetes. In an analysis of California epidemiologic data, 2 only one-third of 94 pregnant women who were hospitalized with 2009 H1N1 influenza had established risk factors for complications from influenza, compared with almost two-thirds of nonpregnant women of reproductive age.
Nationally, 75 deaths of pregnant women were confirmed as because of H1N1 and 34 were possibly related to H1N1, most of which (64.3%) occurred in the third trimester.3 Records of the 1957 pandemic similarly show that pregnant women in the second and third trimesters were particularly affected.
That healthy pregnant women became so ill during the H1N1 pandemic raised several flags. For one, it became clearer that pregnancy is its own significant risk factor for severe illness from the influenza virus. Physiological changes believed to make a pregnant woman more susceptible to becoming ill include decreased lung capacity, increased nasal congestion, reduced colloid oncotic pressure, and changes in the immune system. The morbidity and mortality from H1N1 influenza also increased our drive as a specialty to convince women that vaccination is an important strategy in each influenza season.
The flu vaccine can be administered at any point during pregnancy. There is no evidence that the safety profile is any different during one trimester than another.
Patients should be reassured that vaccines recommended in pregnancy have undergone rigorous testing and that the influenza vaccine has been given to millions of pregnant women over decades. They also should understand that contracting influenza has risks for the fetus; research has demonstrated that pregnant women who contract influenza are at greater risk of spontaneous abortion as well as preterm birth and low birth weight.4
In addition, the issue of flu vaccine efficacy needs to be properly teased apart. Women read every year that the vaccine is not effective, so we need to discuss with patients what efficacy means. Does the vaccine prevent illness altogether, or does it prevent severe illness? For the most part, whereas influenza vaccines often do not offer an exact match for the year’s circulating strains – and therefore may not prevent all illness – data show that the vaccine can prevent severe illness.5 That is a worthy outcome.
Also worthy is the impact of influenza vaccination on the newborn. That maternal immunization also protects the baby – it can reduce the risk for influenza in infants under 6 months of age – is underappreciated and should be part of patient counseling. There is clear evidence that maternal immunization boosts the concentration of maternal antibodies that can cross the placenta and that infants benefit from this passive antibody protection.6
The Tdap vaccine (tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis), the second vaccine routinely recommended during each pregnancy, is administered as early as possible during the third trimester precisely for this reason – to boost maternal immune response and maximize the passive transfer of antibodies to the newborn. The target is the prevention of pertussis and associated hospitalizations and death during the first 2 months of life in an era when sporadic and unpredictable outbreaks of the infection are occurring.
Data from the CDC of morbidity and mortality from pertussis in children (2001-2011) prior to routine maternal vaccination show that the highest rates of pediatric hospitalizations and deaths occurred in newborns. Research has demonstrated that the Tdap vaccine is highly effective in preventing infections and hospitalizations in newborns: Case-control and cohort studies in the United Kingdom7,8 have shown vaccine effectiveness of 91%-93%, and similar research9 done in the U.S. has demonstrated effectiveness of 78%-85%.
The Tdap vaccine is recommended for pregnant women at 27-36 weeks of gestation – in each pregnancy. The reason for revaccination with each pregnancy is that antibody levels do not remain high for too long; at 8 months post immunization, research has shown, maternal antibody levels have begun to wane.
The vaccine also is recommended for all individuals who will be in close contact with infants younger than 12 months (for example, parents, grandparents, and child-care providers) and who have not previously received it. However, “cocooning” the newborn is effective only when the mother also is immunized – a point that ob.gyns. need to better explain to their patients so that they understand the purpose of this strategy.