Influenza Summary Report, 2010–2011
Influenza and influenza-like illnesses (ILI) were last reportable by law in any county in Texas in 1993 (1). During that year, over 275,000 cases of influenza and influenza-like illness were reported to the Texas Department of State Health Services (DSHS) (legacy agency Texas Department of Health). The only influenza categories reportable by law in Texas for the 2010–11 season included influenza-associated pediatric fatalities, outbreaks associated with influenza, and novel influenza A infections in humans (reportable as an exotic disease). Because there is no current reporting requirement for the majority of influenza illnesses, it is unclear how many influenza-related illnesses, hospitalizations, and deaths occur each year in Texas residents. A small number of influenza cases are reported voluntarily through sentinel surveillance networks composed of laboratories, hospitals, physicians, nurses, schools, and universities located throughout the state. Additional resources include web-based influenza and ILI reporting systems, as well as local and regional health departments that gather data from surveillance participants in their jurisdictions. Data from all sources are reported to the DSHS Central Office in Austin, compiled, and presented weekly in the Texas Influenza Surveillance Report.
The national influenza reporting period begins in early October [Morbidity and Mortality Weekly Report (MMWR) week 40] and continues through late May (MMWR week 20). Influenza surveillance in Texas continues year-round, although in reduced capacity during the summer months. The goals of influenza surveillance are to determine when and where influenza viruses are circulating, if the circulating viruses match the vaccine strains, what changes are occurring in the viruses, what impact influenza is having on hospitalizations and deaths, and the severity of influenza activity. The three main Texas influenza surveillance components are viral, morbidity, and mortality surveillance. Viral influenza surveillance at the state level consists of influenza test results reported by Texas laboratories in the National Respiratory and Enteric Virus Surveillance System (NREVSS) and specimens sent to the DSHS Austin Laboratory and the Laboratory Response Network (LRN) laboratories for influenza surveillance testing. Morbidity surveillance consists of reports of novel influenza A virus infections in humans, reports of patients with ILI from Texas participants in the US Outpatient Influenza-like Illness Surveillance Network (ILINet) and ILI reports submitted through local and regional health departments, and reports of influenza or ILI outbreaks. Mortality surveillance consists of reports of influenza-associated deaths in children under 18 years of age and reports of influenza-associated deaths in pregnant or postpartum women. Surveillance for the latter mortality component was initiated during the 2009 influenza A (H1N1) pandemic and was discontinued as of May 21, 2011.
During the 2010-11 influenza season, Texas and US ILINet data indicated that the peak percentage of visits due to ILI and the number of weeks above baseline were lower compared to the 2009–2010 season (2-3). US data indicated that hospitalization rates in persons younger than 65 years of age were lower compared to the 2009-10 season; there were also fewer influenza-associated pediatric deaths. For both Texas and the US, influenza A (H3N2) was the predominant influenza virus detected. Resistance of 2009 influenza A (H1N1) and influenza A (H3N2) viruses to the adamantanes (i.e., amantadine and rimantadine) was widespread during the 2010–11 season.
National Respiratory and Enteric Virus Surveillance System (NREVSS)
NREVSS is an online laboratory results reporting system for several respiratory and enteric viruses; the system is maintained by the CDC. During the 2010–11 season, 24 participating laboratories in most Texas Health Service Regions (HSRs) submitted data on antigen detection, virus isolation (i.e., cultures), and polymerase chain reaction (PCR) testing for influenza. Of the 86,351 influenza tests that were reported to NREVSS from Texas laboratories, 13,006 (15.1%) were positive for influenza virus. Of the 13,006 positive tests, 9,517 (73.2%) tests were positive for influenza A and 3,489 (26.8%) were positive for influenza B. The majority (81.3%) of the positive test results for influenza A reported through NREVSS were reported as influenza A (not subtyped) because most laboratories in Texas do not perform subtyping. Of the 1,777 results for which subtyping was reported, 81.9% were identified as influenza A (H3N2) and 18.1% were identified as 2009 influenza A (H1N1). The peak of NREVSS influenza activity in Texas occurred during the week ending February 19, 2011 (MMWR week 7), when 28.0% of tests were positive for influenza virus (Figure 1).
Figure 1. Influenza types and subtypes reported by National Respiratory and Enteric Virus Surveillance System laboratories in Texas, 2010–11 season.
Texas DSHS Austin Laboratory and Laboratory Response Network (LRN) Laboratories
Influenza surveillance specimens are submitted for PCR testing to the DSHS Austin laboratory and the Texas LRN laboratories throughout the season by physicians, hospitals, clinics, and health departments across Texas. In addition to the DSHS Austin laboratory, the Texas LRN laboratories that participate in influenza surveillance are located in Corpus Christi, Dallas, El Paso, Fort Worth, Harlingen, Houston, Lubbock, San Antonio, and Tyler. The LRNs have been participating in statewide influenza surveillance since the 2008–2009 influenza season.
The first PCR positive influenza specimen of the season was collected on October 4, 2010 (MMWR week 40) from Cameron County and identified as influenza A (not subtyped) by the South Texas Laboratory in Harlingen (Figure 2). The first specimens positive for influenza A (H3N2) and influenza B were collected during week 41 from Bell and Starr counties, respectively. The first specimen that was PCR positive for 2009 influenza A (H1N1) was collected during the week ending December 4, 2010 (week 48) from a resident of Travis County; however, this subtype was detected only sporadically until the week ending January 8, 2011 (week 1). All three virus types and subtypes co-circulated throughout the remainder of the season; however, influenza A (H3N2) was the predominant subtype of influenza A that was detected during the 2010–11 season in Texas.
Figure 2. Influenza types and subtypes identified by the DSHS Austin Laboratory and the Texas Laboratory Response Network, 2010–11 season
Submission of specimens for influenza surveillance began to increase during the week ending December 11, 2010 (week 49). The largest number of specimens collected for influenza surveillance, 346, occurred during the week ending January 29, 2011 (week 4); the peak percentage of specimens positive for influenza, 87%, also occurred during this week. The proportion of specimens positive for influenza virus in the 2010–11 season exceeded 10% for 25 weeks. Specimen submission began to decline sharply beginning in the week ending March 12, 2011 (week 10).
Over the course of the 2010–11 influenza season, the DSHS Austin laboratory and the LRN laboratories received 3,495 specimens for influenza surveillance that met specimen testing and handling requirements; of those, 2,039 (58.3%) were positive for influenza virus. Of those that were positive for influenza virus, 1,480 (72.6%) were identified as influenza A viruses and 559 (27.4%) were identified as influenza B viruses. Sixty percent of influenza A isolates were subtyped: 329 (36.9%) were identified as 2009 influenza A (H1N1) and 562 (63.1%) were identified as influenza A (H3N2).
Antigenic Characterization of Influenza Isolates Received by the DSHS Austin Laboratory
Like other state virology laboratories in the country, DSHS submits early, mid, and late-season as well as unusual isolates to the CDC for strain characterization. Sixty-seven influenza viruses from Texas were antigenically characterized during the 2010–11 season: 15 influenza A (H1N1) viruses, 34 influenza A (H3N2) viruses, and 18 influenza B viruses. Fourteen of the influenza A (H1N1) viruses were characterized as A/California/07/2009 (H1N1)-like, the 2010–11 Northern Hemisphere vaccine A (H1N1) component; the remaining influenza A (H1N1) virus could not be grown in culture but was identified as 2009 influenza A (H1N1) by PCR testing. Of the 34 influenza A (H3N2) viruses, 32 were characterized as A/Perth/16/2009-like (H3N2), the 2010–11 Northern Hemisphere influenza A (H3N2) vaccine component; the remaining two influenza A (H3N2) viruses could not be grown in culture but were identified as influenza A (H3N2) by PCR testing. Of the 18 influenza B viruses characterized, 17 (94.4%) were characterized as B/Brisbane/60/2008-like (part of the B/Victoria lineage), the 2010–11 Northern Hemisphere vaccine B component. One (5.6%) virus was characterized as similar to B/Wisconsin/01/2010 (a B/Yamagata lineage virus). These results were similar to those found in the summary of all influenza isolates characterized during the 2010–11 season in the United States (3).
US Outpatient Influenza-like Illness Surveillance Network (ILINet)
Texas participants in ILINet report weekly on the number of patient visits for ILI by age group and the total number of patients seen for any reason. For ILINet reporting, ILI is defined as “fever (≥100°F [37.8°C], oral or equivalent) and cough and/or sore throat in the absence of a known cause other than influenza (4).” These data are used to calculate a weekly percentage of visits due to ILI.
One hundred and thirty-two providers in Texas submitted data to ILINet for at least one week during the 2010–11 season (i.e., 2010 MMWR week 40 to 2011 week 39) (Figure 3). During the official influenza reporting season (i.e., 2010 week 40 to 2011 week 20), an average of 85 providers submitted data on an average of 34,438 patient visits each week.
Figure 3. Number of active Texas participants per county in the US Outpatient Influenza-like Illness Surveillance Network, 2010–11 influenza season*
*Participants did not necessarily report every week of the influenza reporting season.
The HHS Region 6 (Arkansas, Louisiana, New Mexico, Oklahoma, Texas) ILI baseline calculated by CDC was 4.9% for the 2010–11 influenza season. The Texas baseline for the 2010–11 season was 6.09%. The baseline is the mean percentage of patient visits for ILI during non-influenza weeks for the previous three seasons plus two standard deviations.
According to data from Texas ILINet participants, the percentage of visits due to ILI first exceeded the HHS Region 6 baseline during the week ending December 25, 2010 (MMWR week 51), with 5.25% of visits due to ILI (Figure 4). Influenza-like illness peaked during the week ending February 19, 2011 (week 7). During that week, ILINet providers reported that influenza-like illness accounted for 10.02% of all patient visits. The percentage of visits due to ILI fell below the regional baseline in the week ending March 12, 2011 (week 10); the percentage of visits due to ILI continued to decrease and remained below the regional baseline for the remainder of the season. Overall, ILI activity in Texas exceeded the HHS Region 6 baseline for nine weeks (eight of those were consecutive). The percentage of visits due to ILI exceeded the Texas baseline for six consecutive weeks beginning with the week ending January 22, 2011 (week 3).
Figure 4. Percentage of visits for influenza-like illness reported by the US Outpatient Influenza-like Illness Surveillance Network in Texas, 2010–11 season
*Baselines are calculated for official influenza reporting season only (i.e., MMWR weeks 40 through 20)
School Closures and Institutional Outbreaks
A total of five ILI or influenza outbreaks in a school setting were reported during the 2010–11 season, and all were reported by HSR 4/5N. Two school closures due to influenza B were reported early in the season, during the week ending October 23, 2010 (MMWR week 42) and during the week ending December 12, 2010 (week 49). High absenteeism and rapid influenza tests positive for influenza A and B were reported during the week ending February 5, 2010 in two schools, one of which closed. An outbreak of ILI was reported in a school during the week ending May 7, 2011 (week 18).
Eleven institutional outbreaks were reported in long-term care facilities, a nursing home, and a hospital unit in HSRs 2/3, 7, 8, and 9/10. Reports of institutional outbreaks began in the week ending January 8, 2011 (week 1) and continued through the week ending February 19, 2011 (week 7). Two additional outbreaks were reported during the week ending March 5, 2011 (week 9) and the week ending March 12, 2011 (week 10). The etiologic agents responsible for these outbreaks included influenza (no typing or subtyping performed) for three outbreaks, influenza A (no subtyping performed) for six outbreaks, influenza A (H3N2) for one outbreak, and 2009 influenza A (H1N1) for one outbreak.
Influenza-Associated Pediatric Mortality
Influenza-associated pediatric mortality is defined as a death resulting from an illness that is laboratory confirmed as influenza in a person under the age of 18 years (5). Twelve influenza-associated pediatric fatalities were reported to DSHS during the 2010–11 influenza season. The reported deaths occurred during the week ending October 30, 2010 (MMWR week 43) through the week ending March 12, 2010 (week 10). These twelve total deaths were reported in residents of HSRs 2/3, 4/5N, 6/5S, and 8. Ten patients had confirmed influenza A infections and two patients had influenza B infections. Subtyping was performed for five of the ten influenza A results—two were influenza A (H3N2) and three were 2009 influenza A (H1N1).
The median age at death was 1 year, with patients ranging in age from 5 months to 17 years. Of the 10 reported cases, one patient was less than 6 months of age, seven patients were 7 months to 2 years of age, and four patients were 5–17 years of age. The influenza vaccination status for the 2010–11 season was known for eight of the twelve cases; of those eight, one was too young to be vaccinated for influenza (i.e., under 6 months of age). Of the remaining seven who were old enough to be vaccinated and for whom vaccination status was known, four (57%) were not vaccinated for influenza. Nine (75%) of the twelve patients had significant underlying medical conditions and/or a bacterial co-infection. Two of the twelve patients had a methicillin-resistant Staphylococcus aureus (MRSA) co-infection.
Influenza-Associated Deaths in Pregnant and Postpartum Women
No influenza-associated deaths were reported in pregnant or postpartum women during the 2010–11 influenza season.
- Texas Department of Health. Epidemiology in Texas 1993 Annual Report. Available at: http://www.dshs.state.tx.us/idcu/data/documents/1993%20Texas%20Annual%20Report.pdf.
- Centers for Disease Control and Prevention. 2009–2010 Influenza Season Summary. Available at http://www.cdc.gov/flu/weekly/weeklyarchives2009-2010/09-10summary.htm. Accessed on March 19, 2012.
- Centers for Disease Control and Prevention. 2010–11 Influenza Season Summary. Available at http://www.cdc.gov/flu/weekly/weeklyarchives2010–11/10-11summary.htm. Accessed on November 17, 2011.
- Centers for Disease Control and Prevention. U.S. Outpatient Influenza-like Illness Surveillance Network: 2010–11 Workfolder. 55.20E, Rev. 06/2011.
- Centers for Disease Control and Prevention. Influenza-Associated Pediatric Mortality, 2004 case definition. Available at: http://www.cdc.gov/osels/ph_surveillance/nndss/casedef/Influenza-Associated_current.htm. Accessed on November 17, 2011.