Direct and total effectiveness of the intranasal, live-attenuated, trivalent cold-adapted influenza virus vaccine against the 2000-2001 influenza A(H1N1) and B epidemic in healthy children

Background: The efficacy of the intranasal, live attenuated, trivalent cold-adapted influenza virus vaccine (CAIV-T) against influenza A(H3N2) and B infections in healthy persons is established, but its effectiveness against natural influenza A(H1N1) infection is unknown. Objective: To assess the effectiveness of CAIV-T in healthy children during the 2000-2001 influenza A(H1N1) and B epidemic. Design: Community-based, nonrandomized, open label trial from August 1998 through April 2001. Setting: Intervention and comparison communities in central Texas. Participants: Healthy children, aged 1.5 to 18 years, from the intervention communities received a single dose of CAIV-T at least I time or more in 1998, 1999, and/or 2000. Main Outcome Measures: The incidence of medically attended acute respiratory illnesses during the 2000-2001 influenza epidemic was compared in 3794 health plan CAIV-T recipients with age-eligible, health plan nonrecipients in the intervention communities for direct effectiveness (n=9325), and with those in the 2 comparison communities for total effectiveness (n = 16264). Results: The 2281 CAIV-T recipients in 2000 had significant direct protection against medically attended acute respiratory illness of 18% to 20% during the biphasic influenza A(H1N1) and B epidemic, and 17% to 26% during influenza A(H1N1) predominance. The 931 recipients of CAIV-T in 1999 containing influenza A/Beijing/262/95(H1N1) and B/Beijing/184/93-like viruses had persistent heterovariant protection against the 2000-2001 influenza A/New Caledonia/20/99 (H1N1) and B/Sichuan/379/99 variants. The 616 recipients of a single CAIV-T dose in 1999 only, including those younger than 5 years with no prior natura l exposure to influenza A(HINI) viruses, showed persistent protection. Conclusion: Healthy children who received CAIV-T in 2000 or 1999 were protected against new variants of influenza A(H1N1)-and B in the 2000-2001 influenza epidemic.