Vaccination with LAIV and TIV. Vaccine administration began on 26 October and continued until 19 December 2007. Scott and White clinical research teams supported by Bell County Health Department nurses and student nurses from Mary Hardin Baylor University enrolled students at 25 public elementary schools and 3 parochial schools. A total of 4951 (47.5%) of 10,418 students were vaccinated in the public schools. Catch-up visits to schools, community outreach clinics, and weekend clinics at Scott and White increased the total number to 6191, of whom 5247 (84.8%) received LAIV and 944 (15.2%) received TIV. Vaccine coverage for all Scott and White patients at both the intervention and comparison sites was estimated from the Scott and White immunization registry for children and from administrative records. The proportion of Scott and White patients vaccinated in each age group for the intervention and comparison sites is illustrated in Figure 1. The influenza immunization rates were comparable for all age groups except the 5–11-year-old students (75.8% of Scott and White patients at the intervention site and 24.5% at the comparison sites), who included most of the elementary-school children.
Influenza virus surveillance. Epidemic activity began during week 52 of 2007 at the intervention site and during week 1 of 2008 at the comparison sites and ended on week 11 of 2008 (). During that time, 95.5%, 97.4%, and 98.5% of all influenza viruses were detected by surveillance at the Temple-Belton intervention site and at the Bryan-College Station and Waco comparison sites, respectively.
Figure 2. Biweekly rates of medically attended acute respiratory illness for members of the Scott and White Health Plan (top) and number of Scott and White patients with positive culture results for influenza viruses at intervention and comparison sites (bottom (more ...)
Virus characterization. A sample of 50 influenza viruses recovered throughout the epidemic was characterized at the CDC. The viruses included 22 influenza A(H1N1), 12 influenza A(H3N2), and 16 influenza B viruses. All 50 were classified as new antigenic variants—A/Brisbane/59/2007(H1N1), A/Brisbane/10/2007(H3N2), and B/Florida/04/2005—not included in the 2007–2008 vaccine. For the United States, the CDC reported the new variants for only 24%, 69%, and 95%, respectively, of all viruses characterized. The influenza B/Florida virus was from the Yamagata lineage, whereas the influenza B component of the vaccine was from the distinctly different Victoria lineage. Viruses antigenically similar to the 3 new variants prevalent in Central Texas in 2007–2008 were chosen for the 2008–2009 vaccine.
Indirect protection. Biweekly MAARI rates for SWHP members presenting to intervention and comparison site clinics for the 2007–2008 epidemic year are illustrated in Figure 2. Early during the epidemic, influenza A(H1N1) contributed to influenza A activity, as shown in the bar graph, but peak activity consisted mainly of influenza A(H3N2), followed by influenza B activity. The biweekly MAARI rates for intervention and comparison groups were almost identical for the first 15 weeks of the year, as documented for the prevaccine period in Table 1. MAARI rates were lower at the intervention site from week 43 through week 51 when 5247 doses of LAIV were administered mainly to the elementary-school children. The RR was 0.89 (95% CI, 0.86–0.91) for the period during vaccine administration. Indirect protection was evident during the epidemic period and persisted during the early weeks of the postepidemic period; the rates for intervention and comparison sites were comparable for the last 12 weeks of the year (). The age-specific MAARI rates for SWHP patients from intervention and comparison sites during the epidemic period are shown in Table 2. Significant indirect protection was detected for all age groups except the 12- 17-year-olds, who were not offered free vaccine in the schools; these students have multiple opportunities for exposure to infectious contacts at school and during other activities.
The expected number of illnesses for the intervention site (12,464) can be estimated by applying the MAARI rate for the comparison sites (20.5 episodes per 1000 person-weeks) to the 607,980 person-weeks of observation of the intervention communities (). Subtracting the observed number of MAARIs (11,152) from this number suggests that 1312 SWHP medical encounters were prevented by influenza immunization of the elementary-school children. Given that SWHP provides health care for approximately one-half of the people in the intervention area, an estimated 2500 medical encounters were avoided in the Temple-Belton area.
Risk Ratios (RRs) for Age-Specific Rates of Medically Attended Acute Respiratory Illness (MAARI) for Intervention and Comparison Sites during the Influenza Epidemic, 23 December 2007 to 15 March 2008, Central Texas Field Trial
Direct effectiveness. At the intervention site, direct effectiveness was influenced by the indirect benefit of vaccination of a large proportion (47.5%) of elementary-school children, who typically have the highest influenza virus infection rate. Extensive vaccination coverage reduces exposure of unvaccinated persons to infection, resulting in indirect protection (in addition to direct protection for the vaccinated). This was evident from the age distribution and frequency that cultures were obtained for surveillance at the intervention and comparison sites. Both the number of patients of all ages for whom cultures were performed and the proportion positive tended to be lower at the intervention site. Of 482 persons cultured at intervention clinics, 236 (48.5%) yielded an influenza virus; 524 were cultured at the comparison clinics, and 288 (55.0%) were positive for an influenza virus (P = .066). Of patients presenting to clinics with no history of current influenza vaccination, 203 (51.1%) of 397 were culture positive at the intervention clinics, compared with 250 (55.7%) of 449 cultured at the comparison clinics (difference not significant). For recipients of both vaccines combined, the proportion with positive culture results— 31 (37.3%) positive of 83 cultured—was significantly lower, compared with those obtained from unvaccinated patients cultured at intervention clinics (P = .052) but not at comparison sites, where 38 (50.7%) were positive of 75 cultured (P = .370). Influenza B accounted for only 84 (18.5%) of isolates from unvaccinated subjects at all sites and was recovered from 39 (28.1%) of persons who received TIV. Influenza B virus was not isolated from LAIV recipients.
LAIV protection is more evident when the data for the target population (5–11 years old) are examined (). The culture rate for LAIV recipients was only 2.5 per 1000 persons, significantly lower than for unvaccinated children at the intervention site and children who received TIV or no vaccine at comparison sites (P< .01). At the intervention site, only 1 positive culture result each was found for 11 LAIV and 6 TIV recipients; the numbers were too small to be compared separately. When combined, the proportion positive (11.8%) for vaccinated children was significantly lower than that (46.5%) for unvaccinated children at the intervention site (P = .023). In contrast, at the comparison sites the proportion of positive culture results for vaccinated children was not different than that for unvaccinated children (P = .323). For the comparison sites, the proportion of Scott and White children given LAIV was only 15.2%, compared with 73.8% for those at the intervention site.
Surveillance of Influenza Virus Infections in a Sample of Age-Eligible Persons, 5–11 Years Old, from Intervention and Comparison Clinics of Scott and White, 2007–2008
The 3 groups defined by vaccine status and at least 1 MAARI were compared using Poisson regression for the number of visits per week during the epidemic period (). The subgroup of LAIV recipients had a significantly lower weekly visit rate than the subgroups with no vaccine or TIV recipients. The combined surveillance and MAARI visit data suggest that good protection is provided by a single dose of LAIV and that marginal protection is provided by TIV.
Table 4. Number of Medically Attended Acute Respiratory Illness (MAARI) Events for Schoolchildren Aged 5–11 Years by Vaccine Status Who Presented for Care at the Intervention Site during the Influenza Epidemic, 23 December 2007 to 15 March 2008, Central (more ...)