Influenza vaccination during pregnancy protected infants against the illness for a limited duration of 8 weeks, a recent study reported.
“The incidence of influenza illness among infants is high and is associated with a substantial increase in outpatient visits and hospitalizations during the influenza season, especially among infants younger than 6 months of age,” Marta C. Nunes, PhD, of the respiratory and meningeal pathogens research unit at Chris Hani Baragwanath Hospital in Johannesburg, South Africa, and colleagues wrote. “While active annual influenza vaccination is the most efficient mode for the prevention of influenza, current vaccines are poorly immunogenic and not licensed for use for infants younger than 6 months of age.”
To determine the influence of maternal influenza immunization on infants, the researchers enrolled two cohorts of pregnant women in a randomized, double blind clinical trial during 2011 and 2012.
The women were divided into two groups to evaluate the safety, immunogenicity and efficacy of trivalent inactivated influenza vaccine (IIV) during pregnancy. One group was vaccinated with IIV; the other was assigned a placebo. For the first 6 months after birth, the infants were followed and assessed for PCR-confirmed influenza. In October 2014, researchers measured the infants’ hemagglutination inhibition (HAI) antibodies.
There were 1,026 infants (47.2% females) born to mothers in the IIV group and 1,023 infants (47.3% females) born to those assigned placebo. Infants aged 8 weeks or younger were most strongly protected against PCR-confirmed influenza illness (85.6%; 95% CI, 38.3-98.4). Among infants aged 8 to 16 weeks (vaccine efficacy, 25.5%) and those aged 16 to 24 weeks (efficacy, 30.3%), Nunes and colleagues said there was no protective effect.
As age increased, the vaccine’s efficacy waned, decreasing to 53.9% for infants aged 16 weeks or younger (95% CI, 10.4-77.4) and to 49.5% for infants aged 24 weeks or younger (95% CI, 9.9-72.6). In addition, the percentage of infants in the IIV group with HAI titers of at least 1:40 to influenza vaccine strains decreased with age (first week of life: 56%; 16 weeks: less than 40%; 24 weeks: less than 10%).
“To enhance the concentration of the antibodies transferred transplacentally and decrease the period of vulnerability to disease in young infants, there is the need to find more immunogenic vaccines for pregnant women,” Nunes and colleagues concluded. “Alternatively, more immunogenic vaccines that can generate a protective immune response in infants, beginning at 8 weeks of age, need to be identified.” – by Alaina Tedesco
Disclosure: Nunes reports no relevant financial disclosures. Please see the full study for a list of all other authors’ relevant financial disclosures.