Correspondence

Effectiveness of Maternal Influenza Immunization

N Engl J Med 2009; 360:537-538January 29, 2009

Article

To the Editor:

In their article on maternal influenza vaccination, Zaman et al. (Oct. 9 issue)1 address a very important preventive health intervention that had substantial benefits to both mothers and infants. However, the potential effect of breast-feeding on the outcome remains unclear. Was there an association between breast-feeding and apparent immunity to respiratory illness in the two study groups? Though the authors mention that the mean numbers of weeks of exclusive breast-feeding were similar in the two groups, the range varied from 1 to 25 weeks. Was any subgroup analysis done to investigate for an association between the duration of breast-feeding and the development of infection? This is an important factor to consider, since previous studies have shown that breast-feeding may decrease susceptibility to influenza as a result of passive antibody transfer.2 It would be interesting to note whether breast-feeding provided an additional protective factor to the infants in this study, as has been observed in epidemiologic data previously.3

Saira S. Tandon, M.D.
Lutheran Medical Center, Brooklyn, NY 11220
dr_saira_ahmad@hotmail.com

3 References

1. 1

   Zaman K, Roy E, Arifeen SE, et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med 2008;359:1555-1564
   Full Text | Web of Science | Medline

2. 2

   Ali HM, Scott R, Toms GL. The effect of foster feeding and bottle feeding expressed breast-milk on the susceptibility of guinea-pig infants to influenza virus. Br J Exp Pathol 1989;70:183-191
   Web of Science | Medline

3. 3

   Chantry CJ, Howard CR, Auinger P. Full breastfeeding duration and associated decrease in respiratory tract infection in US children. Pediatrics 2006;117:425-432
   CrossRef | Web of Science | Medline

Author/Editor Response

Although breast-feeding is known to have an effect on the infant immune response,1 its effect should be equal in the two study groups in a randomized trial. In Table 1 of our article, we report similar mean numbers of weeks of exclusive breast-feeding in the two study groups (P=0.18).

We have now explored in detail the distribution of the duration of exclusive breast-feeding in each study group (Figure 1Figure 1Distribution of the Mean Number of Weeks of Exclusive Breast-feeding.). Both groups showed a left-skewed distribution, with a peak around the 20th week. However, the distribution in the influenza-vaccine group appears bimodal, with an additional peak around the fifth week. The comparison of means and distribution does not suggest that breast-feeding is a significant confounder. If anything, the slightly shorter duration of breast-feeding in the influenza-vaccine group would tend to reduce the observed effect of influenza immunization.

Effect modification by breast-feeding cannot be ruled out. We plan to conduct a detailed secondary analysis to explore differential vaccine effectiveness according to breast-feeding status in a variety of infant age groups and to assess the concentration of IgA antibodies in breast milk.

Saad B. Omer, M.B., B.S., Ph.D.
Emory University, Atlanta, GA 30322

K. Zaman, M.B., B.S., Ph.D.
International Centre for Diarrheal Disease Research, Dhaka 1212, Bangladesh

Mark C. Steinhoff, M.D.
Cincinnati Children's Hospital, Cincinnati, OH 45229
m.steinhoff@gmail.com

1 References

1. 1

   Katona P, Katona-Apte J. The interaction between nutrition and infection. Clin Infect Dis 2008;46:1582-1588
   CrossRef | Web of Science | Medline