Update: Influenza Activity -- United States, 1992-93 Season

Influenza activity in the United States increased from December 1992 through mid-February 1993; during this period, influenza type B viruses circulated at high levels nationwide. However, since late February, high levels of influenza type A have been reported. This report updates surveillance for influenza during the 1992-93 season.

Of the 4252 influenza viruses reported from September 27, 1992, through May 15, 1993 (CDC surveillance week 19), 3086 (73%) were type B and 1166 (27%), type A. Of the 640 influenza type A viruses that were subtyped, 71 (11%) were A(H1N1) and 569 (89%), A(H3N2).

The total number of influenza isolates reported per week peaked at 443 during the week ending February 13 (week 6) then decreased steadily. Influenza type A virus circulation increased substantially after January 1993, and type A was the predominant isolate reported after March 20 (week 11) (Figure 1).

Throughout the season, virtually all influenza type B viruses isolated in the United States and characterized at CDC have been antigenically similar to the B/Panama/45/90-like virus included in the 1992-93 influenza vaccine. All characterized influenza A(H1N1) viruses have been related to the A/Texas/36/91-like virus included in the 1992-93 vaccine or the related A/Taiwan/1/86 strain (1). Of the 103 influenza A(H3N2) viruses isolated and characterized this season, 13 (13%) have been antigenically similar to A/Beijing/353/89, the strain included in the 1992-93 influenza vaccine, and 90 (87%) have been similar to the more recently detected antigenic variant A/Beijing/32/92 (1). Laboratory studies suggest that this variant is sufficiently different from the vaccine component to result in diminished effectiveness of the 1992-93 vaccine against infection with the A/Beijing/32/92 subtype virus (1).

Since February, outbreaks of influenza A(H3N2) have been reported in nursing homes and other institutions, particularly in areas where surveillance indicated the highest levels of influenza A(H3N2) activity (the New England, Mountain, Middle Atlantic, and South Atlantic regions). Subsequent increased influenza A(H3N2) activity was concurrent with an increase in the proportion of total deaths associated with pneumonia and influenza (P&I) reported through CDC's 121-city mortality reporting system. This proportion exceeded the upper threshold * of expected levels for this time of year for 10 consecutive weeks beginning the week ending March 13 (week 10) (Figure 2).

The number of states and territories reporting outbreaks of widespread ** or regional influenza-like illness (ILI) peaked at 26 during each of the 2 weeks ending February 13 and February 20 (weeks 6 and 7). By the weeks ending May 8 and May 15 (weeks 18 and 19) no widespread activity was reported, but regional activity was reported each week in one state. Nationwide, the average percentage of patients examined for ILI by sentinel physicians peaked at 5.8% during the week ending February 6 (week 5) and declined to levels of less than 2% by the week ending April 24 (week 16).

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. WHO collaborating laboratories. Sentinel Physicians Influenza Surveillance System of the American Academy of Family Physicians. WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Influenza Br, and Epidemiology Activity, Office of the Director, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The excess P&I mortality reported late in the 1992- 93 influenza season reflects mortality among the elderly that has historically been attributable to circulation of influenza A(H3N2) viruses. This indicator of excess influenza-associated mortality tends to lag behind surveillance indicators of influenza-associated morbidity or strain circulation by several weeks.

Although influenza A(H3N2) viruses similar to the 1992-93 vaccine component A/Beijing/353/89 continue to be isolated, antigenic analysis indicates that the majority of recent isolates are similar to the antigenic variant A/Beijing/32/92 -- a component of the 1993-94 influenza vaccine. Although the relative predominance of influenza virus subtypes during future influenza epidemics cannot be reliably predicted, this late-season increase in isolation of a previously nondominant subtype suggests that influenza A(H3N2) viruses could predominate next season.

Reference

1. CDC. Update: influenza activity -- United States and worldwide, and composition of the 1993-94 influenza vaccine. MMWR 1993;42:177-

2. 
   

• The epidemic threshold is 1.645 standard deviations above the seasonal baseline calculated using a periodic regression model applied to observed percentages since 1983. This baseline was calculated using a robust regression procedure. ** Levels of activity are 1) sporadic -- sporadically occurring ILI or culture-confirmed influenza, with no outbreaks detected; 2) regional -- outbreaks of ILI or culture-confirmed influenza in counties having a combined population of less than 50% of the state's total population; and 3) widespread -- outbreaks of ILI or culture-confirmed influenza in counties having a combined population of 50% or more of the state's total population.

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