Flu Shot Reduces Hospitalization Risk for Pregnant Women

Pregnant women vaccinated against flu had a 40% lower risk for hospitalization if they became ill with the infection compared with unvaccinated pregnant women, according to an analysis of data from six influenza seasons.

Mark G. Thompson, PhD, an epidemiologist from the Influenza Division, Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, and colleagues published their findings online October 11 in Clinical Infectious Diseases.

“Expecting mothers face a number of threats to their health and the health of their baby during pregnancy, and getting the flu is one of them,” Allison Naleway, PhD, a study coauthor from the Kaiser Permanente Center for Health Research, Portland, Oregon, explained in a news release.

"This is the first study to examine influenza vaccine effectiveness against lab-confirmed influenza-associated hospitalizations during pregnancy," Thompson and colleagues note.

"This study's findings underscore the fact that there is a simple yet impactful way to reduce the possibility of complications from flu during pregnancy: get a flu shot," Naleway adds.

The CDC, in collaboration with national and international public health and healthcare systems with integrated medical, laboratory, and vaccination records, established the Pregnancy Influenza Vaccine Effectiveness Network (PREVENT) to assess influenza vaccine effectiveness in preventing hospitalizations for laboratory-confirmed influenza during pregnancy.

The researchers examined regional public health and medical records for residents in Western Australia and the provinces of Alberta and Ontario, Canada, and looked at electronic medical records of large integrated care systems for health plan members in the United States (Kaiser Permanente in Northern California, Oregon, and Washington) and Israel (Clalit Health Services). Study sites provided data for three to six influenza seasons from 2010 through 2016 (combined total, 25 site-specific study seasons).

"Moderate" Effectiveness of Vaccine Against Hospitalization

The study included pregnant women aged 18 through 50 years whose pregnancies overlapped with local influenza seasons. The investigators used administrative data to identify hospitalizations for women with acute respiratory or febrile illness and clinician-ordered real-time reverse transcription polymerase chain reaction (rRT-PCR) testing for influenza performed within 3 days prior to admission (to capture ambulatory or emergency care testing that preceded admission) through the date of discharge. The study excluded those who were vaccinated fewer than 14 days before hospital admission and women whose influenza vaccine status was not available.

The researchers used the test-negative study design and adjusted for site, season, season timing (early, peak, late), and high-risk medical conditions.

Among 1030 hospitalizations for acute respiratory or febrile illness in which pregnant women underwent PCR-influenza testing, 58% (598) tested positive for flu virus infection, ranging from 51% to 62% across influenza seasons, and from 45% to 65% across study sites.

The number of positive influenza tests was similar during most seasons (range, n = 93-127) except for the northern hemisphere (NH) 2011-12 season, during which it was notably lower (n = 43).

Two thirds (65%) of the women were in their third trimester of pregnancy.

In total, 13% of women with rRT-PCR-confirmed influenza had been vaccinated, compared with 22% of influenza-negative women, for an unadjusted influenza vaccine effectiveness of 48%.

After adjustment, influenza vaccine effectiveness was 40% (95% CI, 12% - 59%) against hospitalization for laboratory-confirmed influenza during pregnancy.

Adjusted laboratory-confirmed influenza was lowest in Alberta (8%) and Israel (17%) and highest in Ontario (40%) and the United States. The United States was the only site for which the influenza vaccine effectiveness point estimate was significant (55%; 95% CI, 7% - 78%).

Two flu seasons were extreme outliers: southern hemisphere [SH] 2014 and NH 2014-15; when these were excluded, the combined adjusted influenza vaccine effectiveness estimate for the remaining seasons was 49%.

Vaccine effectiveness was 55% for women in their first or second trimester of pregnancy and 35% for women in their third trimester at admission.

"This moderate protection was noted during a period when A(H1N1) pdm09 viruses were a prominent strain in about half of the study seasons, A(H3N2) viruses were a prominent strain in > 70% of study seasons, and the match between vaccine strains and circulating A(H3N2) viruses varied from good to poor," the researchers explain.

Vaccine effectiveness was lower for women whose hospitalization discharge included a diagnosis of pneumonia or influenza.

Vaccine effectiveness was higher for hospitalizations in which women were admitted to an intensive care unit (ICU) and for hospitalizations where the woman's primary discharge code was acute respiratory or febrile illness.

Data Strengthen Rationale for Flu Shots in Pregnancy

The study findings are important for several reasons. Among the total cohort of more than 2 million pregnancies, 84% overlapped with an influenza season.

Although the results may be especially relevant to public health efforts in low- and middle-income countries (LMICs), which tend to prefer investing in vaccines with proven benefits against more severe outcomes, the findings may not be generalizable to LMICs.

"Access to hospital care and the severity threshold for admission likely differs for LMICs; though, it is noteworthy that influenza vaccine effectiveness unadjusted point estimates trended higher (though not significant statistically) when we limited analyses to women with ICU admissions," the researchers explain.

The results are pertinent to high-income countries as well: in the US, vaccination rates are far below national and international goals.

"In addition to the ample data on the safety" of inactivated flu vaccination during pregnancy, given the "mounting evidence" that immunization against flu reduces the risk of mild to moderately severe lab-confirmed influenza during pregnancy and offers secondary protection to infants during the first months of life, "our finding of 40% influenza vaccine effectiveness" in preventing lab-confirmed flu hospitalization during pregnancy "further strengthens the rationale for influenza vaccination programs for pregnant women," the researchers conclude.

One author reports serving as a content advisor on respiratory virus testing to Johnson & Johnson (Janssen Pharmaceuticals). Several authors report grants from companies including Pfizer, MedImmune/AstraZeneca, Merck, GlaxoSmithKline, Sanofi Pasteur, Protein Science, Merck, Novartis (now GlaxoSmithKline), and Dynavax. A complete list of disclosures is available on the journal website.

Clin Inf Dis. Published online October 11, 2018. Full text

For more news, join us on Twitter and Facebook