One of the leading causes of morbidity and mortality in children worldwide is lower respiratory tract infection. Influenza is responsible for many hospitalizations in infants, and preschool and school-age children have the highest attack rates for influenza. During the 2000–2001 influenza epidemic, school-age children missed 63 days of school per 100 children, and, because of secondary infections, parents missed 20 days of work per 100 children. A live-attenuated, cold-adapted, trivalent influenza virus vaccine delivered by the intranasal route was approved recently for use in healthy children and adults five through 49 years of age. Studies have shown this vaccine to be effective in preventing a significant number of infections from certain influenza A and B viruses. However, during a recent influenza epidemic, a resurgence of influenza A (H1N1) occurred. This strain had not been prevalent since the 1995–1996 influenza season, and the intranasal vaccine had not been tested against this strain. Gaglani and associates evaluated the efficacy of the intranasal influenza vaccine against the new variants of influenza A and B in healthy children.

The community-based, nonrandomized, open-label study lasted from August 1998 through April 2001. Investigators assessed the intranasal influenza vaccine against certain influenza strains in healthy children 1.5 to 18 years of age. The study population included children from several communities who received the vaccine and children from several other communities who served as the control group.

Exclusion criteria for the study were underlying medical illnesses that would require administration of the standard influenza vaccine and asthma that required the use of routine medications. If children had been febrile within the previous 48 hours or had significant nasal discharge, administration of the intranasal vaccine was delayed. Included in the data analysis were children who received at least one dose of intranasal influenza vaccine during the study period. The main outcome measure was the incidence of medically attended acute upper respiratory illnesses during the 2000–2001 influenza epidemic. The intervention and control groups were compared.

During the three-year study period, 9,765 children received at least one dose of intranasal influenza vaccine. The intervention group was similar to the control group, except that blacks were underrepresented in the study population compared with the general population in these communities. During the 2000 influenza epidemic, those who received the intranasal influenza vaccine had significant direct protection against influenza A and B viruses and against the influenza A (H1N1) virus during that period compared with the control group. Children who received the intranasal influenza vaccine in 1999 had protection against the different strains that were present during the 2000 epidemic period. There were 616 children who received only one dose of the intranasal vaccine in 1999 who still had persistent protection against the influenza A (H1N1) strain during the 2000–2001 influenza season.

The authors conclude that healthy children who received the intranasal influenza vaccine were protected from the new variants of influenza A and B during the 2000–2001 influenza epidemic. They add that this protection was present even in children who received only one vaccine and then went through two different influenza seasons.