Influenza vaccination reduced the rate of influenza illness by nearly two thirds in infants up to 6 months of age and averted roughly one third of all respiratory illnesses with fever in mothers and young infants, report Dr. Mark C. Steinhoff from Cincinnati Children's Hospital, Ohio, who conducted the study along with colleagues in Dhaka and Baltimore.
The study results were released online, in advance of print in the October 9th issue of The New England Journal of Medicine, to coincide with presentation of the results before the National Vaccine Advisory Committee at the US Department of Health and Human Services.
In the study, 340 women in the third trimester of pregnancy were randomly assigned receive either the inactivated influenza vaccine or a 23-valent pneumococcal polysaccharide vaccine (the control group).
Steinhoff's team observed fewer cases of laboratory-confirmed influenza before 24 weeks of age among the infants of influenza-vaccinated mothers than among infants in the control group (six vs. 16), for a vaccine effectiveness of 63 percent.
There were also fewer cases of respiratory illness with fever in infants in the influenza-vaccinated group than among infants in the control group (110 vs. 153), for a vaccine effectiveness of 29 percent.
Among the mothers, there was also a reduction in the rate of respiratory illness with fever of 36 percent.
"In a tropical setting of perennial transmission of influenza virus," write Steinhoff and colleagues, "maternal influenza immunisation for much of the year had a substantial protective effect in both mothers and their young infants."
The researchers point out that in regions with limited financial resources, the strategy of immunising pregnant women is widely used for tetanus prevention and prenatal immunisation programs are in place.
Based on the study findings, they suggest that the strategy of maternal immunisation "should be evaluated further for the prevention of influenza."
The investigators also remind healthcare providers that infection with influenza virus is associated with serious illness and hospital visits among pregnant women and young infants.
The inactivated trivalent influenza vaccine has been recommended for pregnant women in the US for more than a decade, yet "few mothers" receive it. The vaccine is also recommended for infants between six months and 23 months of age.