Using Sentinel Surveillance System to Monitor Seasonal and Novel H1N1 Influenza Infection in Houston, Texas: Outcome Analysis of 2008–2009 Flu Season

The advent of the novel H1N1 virus prompted the Houston Department of Health and Human services (HDHHS) to use the existing
sentinel surveillance system to effectively monitor the situation of novel H1N1 virus in the Houston metropolitan area. The
objective of this study was to evaluate the demographic characteristics and common symptoms associated with confirmed cases
of seasonal influenza and Novel H1N1 virus reported to HDHHS between October 2008 and October 2009. A total of 30 providers
were randomly selected using the probability proportional to size (PPS) sampling technique to participate in a sentinel surveillance
system. The system was used to effectively monitor both seasonal and novel H1N1 virus in the Houston metropolitan area. These
providers collected and submitted specimens for testing at HDHHS laboratory from patients with influenza-like illness (ILI)
symptoms who visited their clinics during the period, October 2008 and October 2009. These data formed the basis of the current
study. Data obtained were subjected to both descriptive and inferential statistical analyses using SAS software version 9.1.3.
Overall a total of 1,122 ILI cases were reported to HDHHS by sentinel providers and tested by HDHHS laboratory. Of this number
296 (67.5%) specimens tested positive for influenza A; 140 (32.0%) for influenza B, and 2 (0.46%) for influenza A/B. Two hundred
and fifty-nine (59%) were confirmed cases of seasonal influenza and 179 (41%) were novel H1N1 subtype, respectively. The median
ages for seasonal influenza and novel H1N1 virus were 7 and 8 years, with majority of the cases reported among children of
age 5–9 years. Fever was the most common symptom reported among patients with seasonal flu and novel H1N1 virus, followed
by cough. Twenty-three percent (23%) of patients who were vaccinated against seasonal flu prior to the epidemic were infected
with seasonal flu virus. The sentinel surveillance system provided timely data on the circulating ILI that assisted in making
decisions regarding response activities for both seasonal and novel H1N1 influenza.