Maternal influenza vaccination reduces ILI risk in infants

Administration of maternal influenza vaccine during the first- or second-trimester of pregnancy reduces the risk of influenza-like illness (ILI) in the offspring, according to a study presented at ESPID 2023.

“Infants are at high risk of influenza and its complications because of their underdeveloped immune system and their lack of prior exposure,” said lead author Dr Anne-Marie Rick from UPMC Children’s Hospital of Pittsburgh, Pittsburgh, Pennsylvania, US.

“Maternal influenza vaccine can protect infants from influenza. However, it is unknown whether the timing of the vaccine during pregnancy impacts the degree and duration of protection for the infant,” she noted.

Hence, Rick and her team conducted a retrospective study involving 44,132 mother-infant pairs, with infants born between 2012 and 2019 and had longitudinal care in a single healthcare system. Mothers were categorized according to the timing of their influenza vaccine exposure during pregnancy, which was classified as during the first (16.5 percent), second (17.1 percent), or third (15.1 percent) trimesters. Approximately 51.3 percent of the mothers did not receive maternal vaccination. [ESPID 2023, abstract O0002]

Overall, 2,163 infants with ILI episodes were identified during the first year of life.

Younger infants (aged 0 to <6 months) born to mothers who were vaccinated during the first or second trimesters had a significantly decreased risk of ILI by 39–79 percent, while there was no effect observed in those born to mothers who were vaccinated during the third trimester.

“We know that just in terms of how antibody transfer works, it really starts to ramp up in the second trimester, and while it continues into the third trimester, there may not be enough time, if you have been vaccinated in the third trimester, to get sufficient antibody for protection,” said Rick.

Among older infants (aged ≥6 to 12 months), the third-trimester vaccine was associated with a 63 percent reduction in the risk of ILI, whereas the first-trimester vaccine had no impact.

“However, the effect seen with trimester vaccines seems to decrease at 4–8 months, which we think makes sense as those maternal antibodies start to decay and we are seeing that fairly consistently with the other studies that were presented today,” Rick noted.

Of note, these effects were age-dependent, as first and second-trimester vaccines significantly reduced ILI risk among infants aged 0 to <6 months (adjusted hazard ratios, 0.61 and 0.21, respectively), but not among those aged ≥6 to 12 months.

Overall, infants exposed during the first- or second-trimester vaccines had an early benefit in terms of protection during their first 6 months of life, Rick said.

“Moreover, infants exposed in the second trimester had a slightly longer duration of protection, and this evidence highlights that vaccination during pregnancy may impact protection duration,” she added.