Fort Sam Houston, TX—The U.S. military’s response to the Ebola crisis in Liberia, beginning in 2014 to 2015, was the first Department of Defense operation to support a disease-driven foreign humanitarian assistance mission.

One requirement for personnel who assisted was mass yellow fever vaccination (YF17D). A recent study describes how that often occurred fewer than 4 weeks after the routine mass live-attenuated influenza vaccination (LAIV) for military personnel.

The report in the journal Vaccine says that situation prompted concerns for immune interference. Researchers from Brooke Army Medical Center in Texas and Walter Reed National Military Medical Center and the Uniformed Services University of the Health Sciences, both in Bethesda, Maryland, worked with colleagues to compare YF17D seroconversion rates in personnel who received YF17D as recommended—vaccinated by guidelines (VBG)—to those who received the vaccine outside the recommended timing following LAIV—not vaccinated by guidelines (NVBG).

The study included Operation United Assistance deploying personnel who received LAIV simultaneously or before YF17D and had pre- and postvaccination archived serum.

For purposes of the study, VBG was defined as YF17D given concurrently or 30 days or more post-LAIV; and NVBG was defined as YF17D given 1 to 29 days post-LAIV. Seroresponse to YF17D was determined by enzyme-linked immunosorbent assay (ELISA) and confirmed with plaque reduction neutralization testing (PRNT) on positive ELISA samples. Exclusion criteria were prior YF17D and prevaccination YF17D positive PRNT.

Of the 660 personnel included, 507 were VBG and 153 were NVBG. Median age was 25 years for both groups. Men made up 84% of those VBG, and 79% of those NVBG (P = .194).

The results indicated that seroconversion rates were 97.8% for VBG and 95.4% for NVBG (P = .15). Multivariate logistic regression reveals that YF17D on Days 7 through 21 post-LAIV (adjusted odds ratio [aOR] 0.304, P = .017; CI 0.114-0.810) and female sex (aOR 0.330, P = .026; CI 0.124-0.879) were associated with decreased seroresponse, the researchers advised.

“In this healthy, young adult military population, there was high seroconversion following YF17D when administered simultaneously and at various time points after LAIV,” the authors explained. “Slight decreases in seroresponse were seen in women and those receiving YF17D 7-21 days following LAIV.”

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