Pediatric Vaccine Efficacy Does Not Vary in Flu Subtype

Chandresh Ladva, PhD

An early assessment into the effect of the A(H3N2) influenza (flu) subtype found limited variation in vaccine effectiveness (VE) in potentially exposed children.

A new study presented at IDWeek 2019 in Washington, DC, last week found the uncommon, burdensome influenza A strain—which was associated with greater hospitalization rates in recent years—did not respond differently to vaccine among children aged 6 months to 17 years old in the 2016-17 and 2017-18 flu seasons.

The findings—presented by author Chandresh Ladva, PhD, Epidemic Intelligence Service Officer for the US Centers for Disease Control and Prevention (CDC)— warrant greater, and longer, assessments of how A(H3N2) affects vaccinated flu patients different.

In the 2017-18 flu season, VE against A(H3N2) illness was observed to be highest among children younger than 5 years, compared to all other pediatric ages. As investigators noted, a child’s first infection with the flu can eventually shape their later immune responses.

“The emergence of antigenically distinct influenza A(H3N2) viruses in 2014—15 provided an opportunity to explore potential effects of first virus infection on vaccine effects,” Ladva and colleagues wrote. “We compared VE against influenza A(H3N2) during 2016–17 and 2017–18 among children born after and before 2014.”

The team enrolled outpatient children aged 6 months to 17 years old, with acute respiratory illness with cough, to the US Influenza Vaccine Effectiveness Network. Flu infection was tested for via reverse-transcriptase PCR (RT-PCR), while vaccination status was derived through patient medical records and immunization registries.

Investigators excluded children with partial or unknown vaccination status. A test-negative design provided Ladva and colleagues with VE estimates and logistic-regression, adjusted 95% confidence intervals (CIs).

The cohorts were defined by birth before or after June 2014, with clinical assumption being made to A(H3N2) effect among children born after that date.

Among 2545 children observed during 2016-17, 445 (18%) tested positive for A(H3N2). Investigators did not observe any VE differentiation among children born after June 2014 (49%; 95% CI, -12 to 77) versus those born before June 2015 (43%; 95% CI, 27-55; P <.75).

Among 2936 patients observed during 2017-18, 631 (22%) tested positive for A(H3N2). VE against the subtype was 59% (95% CI, 36 — 74) among children born after June 2014, and 20% (95% CI, -1 to 37) among those born before that date (P <.01).

Investigators’ findings were not a consistent conclusion that VE against A(H3N2) was positively beneficial for either observed age group of pediatric patients. That said, they noted limitations including few seasons of observed A(H3N2) effect, as well as potential error in exposure assignment to the virus.

“Future study will include additional A(H3N2) seasons as initial exposures to current circulating viruses increase among young children,” they wrote. “Alternative explanations for age-related differences will also be explored, such as prior seasonal vaccination.”

The study, "Lack of influence of early exposure to influenza A(H3N2) viruses on vaccine effectiveness against A(H3N2)-associated illness in U.S. children <18 years, 2016—18," was presented at IDWeek 2019.