Update: Influenza Activity --- United States, 2001--02 Season

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This report summarizes influenza activity in the United States during the weeks ending October 5--November 23, 2002*. Influenza activity was low during this period but is expected to increase during the coming weeks. Vaccine supplies are plentiful this year, and vaccination should continue throughout December and the remainder of the influenza season, particularly among persons at high risk for complications from influenza.

As of November 23, the World Health Organization (WHO) and the National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories in the United States tested 10,036 respiratory specimens for influenza viruses, of which 47 (0.5%) were positive. For the weeks ending October 5--November 23, the weekly percentages of respiratory specimens testing positive for influenza viruses ranged from 0 to 2.3%. During the 1999--2000, 2000--01, and 2001--02 influenza seasons, respectively, the peak percentages of specimens testing positive for influenza ranged from 23% to 31% (2; CDC, unpublished data, 2002).

Of the 47 positive isolates, 20 were influenza A and 27 were influenza B viruses. Six of the 20 influenza A isolates were subtyped as influenza A(H1) viruses. The A(H1) isolates were reported from North Carolina and Florida. One influenza A(H3N2) virus was reported from Hawaii. Influenza A isolates not yet subtyped were reported from Louisiana, Nebraska, New York, South Carolina, and Virginia. Influenza B isolates were identified in Louisiana, New York, South Carolina, and Texas. In addition to the isolates reported through WHO and NREVSS laboratories, California reported an influenza B isolate.

CDC has characterized antigenically seven influenza viruses submitted by U.S. laboratories since September 29: five influenza B, one influenza A(H3N2), and one influenza A(H1N2). The influenza B viruses, the A(H3N2) virus, and the hemagglutinin protein of the A(H1N2) virus were similar antigenically to the corresponding vaccine strains B/Hong Kong/330/01, A/Panama/2007/99 (H3N2), and A/New Caledonia/20/99 (H1N1), respectively.

Since 1977, two influenza A virus subtypes, A(H1N1) and A(H3N2), have co-circulated in humans. During the 2001--02 season, influenza A viruses with the H1 hemagglutinin and the N2 neuraminidase protein were identified. These A(H1N2) viruses appear to have resulted from the reassortment of genes between the currently circulating A(H1N1) and A(H3N2) viruses (3). Influenza A(H1N2) viruses have been identified in several countries, including the United States. Four of the six H1 viruses reported in the United States since September 29 have been subtyped; one virus from North Carolina was an A(H1N2) virus, and three from Florida were A(H1N1) viruses. The 2002--03 vaccine contains A(H1N1) and A(H3N2) viruses and is expected to provide protection against influenza A(H1N2) viruses (4).

During the weeks ending October 5--November 23, the weekly percentages of patient visits for influenza-like illness (ILI)^§ to approximately 750 sentinel providers ranged from 1.0% to 1.3%. For the week ending November 23, the percentage of patient visits for ILI was 1.3%, which is less than the national baseline of 1.9%^¶. During the 1999--2000, 2000--01, and 2001--02 seasons, the peak percentages of patient visits for ILI ranged from 3.2% to 5.6% (2; CDC, unpublished data, 2002).

During the week ending November 23, influenza activity** was reported by state and territorial epidemiologists as regional in Louisiana and Texas and as sporadic in 18 states (Colorado, Florida, Georgia, Hawaii, Indiana, Kansas, Kentucky, Maine, Michigan, Missouri, Nebraska, Nevada, New Mexico, Pennsylvania, Tennessee, Utah, West Virginia, and Wyoming) and the District of Columbia; 28 states and New York City reported no influenza activity. Two states did not report. Since the week ending October 5, Florida, Louisiana, and Texas have reported regional activity for at least 1 week.

During October, Louisiana reported an influenza outbreak in schools in Lafourche Parish. The outbreak peaked during the week ending October 26 and affected primarily persons aged 5--24 years. Since October 5, a total of 102 cases of influenza B have been diagnosed by rapid test or viral isolation in Lafourche Parish.

During October 1--17, a total of 30 patients with ILI were admitted to the infirmary of a north Florida prison, which employs 300 staff and houses 1,000 prisoners. Influenza A(H1) was confirmed by culture in three cases. The estimated number of patients in the outbreak was 100--150, including two staff.

For the week ending November 16, Texas reported an outbreak of respiratory illness affecting mostly elementary school children. On November 19, a total of 12 specimens collected from children aged 6--11 years with ILI attending an east Texas school in Smith County were submitted to the Texas Department of Health Virology Laboratory for culture confirmation. Ten of these cultures were positive for influenza B, and two are pending. During the outbreak, absentee levels for local schools in Smith County approached 25%.

During the week ending November 23, a total of 7.1% of the deaths in the 122 Cities Mortality Reporting System were attributed to pneumonia and influenza (P&I). This percentage was below the epidemic threshold of 7.4% for that week. The percentage of P&I deaths has been below the epidemic threshold each week since the week ending October 5. The percentage of P&I deaths exceeded the epidemic threshold for 5 weeks during the 2001--02 season, for no weeks during the 2000--01 season, and for 22 weeks during the 1999--2000 season (2,5,6).

Reported by: J Wright, DVM, A Postema, MPH, L Brammer, MPH, S Harper, MD, T Uyeki, MD, E Murray, MSPH, C Bridges, MD, K Fukuda, MD, N Cox, PhD, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

All four national influenza surveillance system components indicated that levels of influenza activity were low in the United States during the weeks ending October 5--November 23. Both influenza A and B viruses have been identified this season, but it is too early to determine which strain(s) will predominate. Influenza activity is expected to increase during the coming weeks and months.

The best prevention against influenza is annual vaccination. Although the optimal time to receive influenza vaccination is during October--November, CDC encourages continuing vaccination efforts in December and throughout the influenza season. The American Medical Association, with support from CDC, has declared December 2--13 as "National Influenza Vaccination Catch-up Fortnight" because millions of persons at high risk for complications from influenza remain unvaccinated each year. It is particularly important to vaccinate those at high risk for serious complications from influenza (including persons aged >65 years and those with certain underlying medical conditions), and health-care providers (7). Such persons might benefit from vaccination even after influenza has been detected in their community (8). Vaccine supplies continue to be plentiful. The manufacturers estimate that approximately 93 million doses of influenza vaccine were produced this year, compared with approximately 87 million doses produced during the 2001--02 season (7,9).

On November 19, Wyeth Vaccines announced that it will no longer produce inactivated influenza vaccine after the 2002--03 influenza season. This development will not jeopardize this year's vaccination programs or vaccine availability. The two remaining influenza vaccine manufacturers in the U.S. market, Aventis Pasteur, Inc. and Evans Vaccines, Ltd., have indicated that they plan to increase influenza vaccine production for the 2003--04 season to make up for the loss of the Wyeth Vaccines product.

Influenza surveillance reports for the United States are published weekly during October--May and are available at http://www.cdc.gov/ncidod/diseases/flu/weekly.htm or through CDC's voice (telephone, 888-232-3228) and fax (telephone, 888-232-3299, document number 361100) information systems.

This report is based on data contributed by C Scott, MPH, R Ratard, MD, R Tapia, MPH, Louisiana Dept of Health; H Melancon, C Petre, MD, Lafourche Parish. C Blackmore, DVM, P Colarusso, MSH, Florida Dept of Health; S Straub, Florida Dept of Corrections. N Pascoe, P McGaha, DO, Texas Dept of Health. J Engel, MD, North Carolina Dept of Health. Participating state and territorial epidemiologists and state public health laboratory directors. WHO collaborating laboratories. National Respiratory and Enteric Virus Surveillance System collaborating laboratories, U.S. Influenza Sentinel Provider Surveillance System. Div of Public Health Surveillance and Informatics, Epidemiology Program Office, DJ O'Mara, Immunization Svcs Div, National Immunization Program, CDC.

References

* Data are reported as of December 1, 2002. The four components of the influenza surveillance system have been described previously (8).
Includes both the A(H1N1) and A(H1N2) influenza virus subtypes.
^§ Temperature of >100.0^o F (>37.8^o C) and either cough or sore throat in the absence of a known cause other than influenza.
^¶ The national baseline was calculated as the mean percentage of visits for ILI during noninfluenza weeks, plus two standard deviations. Wide variability in regional data precludes calculating region-specific baselines and makes it inappropriate to apply the national baseline to regional data.
** Levels of activity are 1) no activity, 2) sporadic---sporadically occurring ILI or laboratory-confirmed influenza with no outbreaks detected, 3) regional---outbreaks of ILI or laboratory-confirmed influenza in counties with a combined population of <50% of the state's population, and 4) widespread---outbreaks of ILI or laboratory-confirmed influenza in counties with a combined population of >50% of the state's population. The expected baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I during the previous 5 years. The epidemic threshold is 1.654 standard deviations above the seasonal baseline (1).

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