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Update: Influenza Activity -- United States -- and Influenza Type B Virus Drift

Influenza Activity in the United States. For the week ending February 7, 1986, 21 states and the District of Columbia reported widespread outbreaks of influenza-like illness, and 14 states reported regional outbreaks. Tallies of patients with influenza-like illnesses seen by the network of family physicians* nationwide continued to increase from an average of 9.1 for the reporting week ending January 22, to an average of 10.5 for the week ending January 29 (Figure 1). During the influenza epidemics of the past two winters, this surveillance system indicated maximum values of between 11 and 12 cases per week.

Type B influenza viruses represent more than 75% of isolates reported from collaborating diagnostic laboratories where outbreaks are occurring; the remaining isolates are type A(H3N2). Three states reported their first isolates for the season: Indiana and Maryland--type B; and Massachusetts--types B and A(H3N2). Twenty states have now reported both types B and A(H3N2) viruses. The percentage of pneumonia and influenza deaths reported from the 121 U.S. cities for the week ending February 7 was 5.8%, the same percentage reported for the preceding week (Figure 1).

Antigenic Drift in Circulating Type B Influenza Strains. Preliminary characterization of type B viruses isolated in the United States this season indicates that B/Georgia/1/86 and B/Ann Arbor/1/86 exhibit some antigenic drift from B/USSR/100/83, the type B virus component of the currently available influenza vaccine. Among 25 persons (12 adults and 13 high-risk children) known to respond to one dose of 1985-1986 vaccine, the serum antibody titer to type B/USSR/100/83 influenza virus increased to a value of 1:40 or greater for 92% of the group (Table 3). In contrast, 28% of those responding to the vaccine reached the 1:40 or greater antibody titer to the B/Georgia/1/86 or B/Ann Arbor/1/86 virus strains. Similar patterns were seen at the other titer levels. Reported by State and Territorial Epidemiologists; State Laboratory Directors; Statistical Svcs Br, Div of Surveillance and Epidemiologic Studies, Div of Field Svcs, Epidemiology Program Office, WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: No absolute level of serum antibody from influenza vaccination has been identified that can guarantee complete protection against influenza infection, although titers higher than 1:20 have been generally associated with significantly (up to about 70%) decreased rates of infection or illness. The above analysis demonstrates the existence of antigenic drift, using sera from a selected subgroup of vaccinees known to respond to the influenza B vaccine component. These data cannot be accurately extrapolated to predict vaccine efficacy rates, although protection afforded by the 1985-1986 vaccine used in programs during late 1985 will probably be lower than if the type B component of the vaccine more closely represented current isolates. Recent laboratory data indicate a close antigenic similarity between the type A(H3N2) virus strains now circulating and the A/Philippines/2/82 component of the 1985-1986 vaccine.

If each season's vaccine is to be available for use by late summer/early fall, production (and, therefore, strain selection) must begin by February. In past epidemics, for example, influenza B outbreaks often occurred toward the end of the influenza season (March/April), rather than during the January/February peak months; thus, it has not been possible to examine influenza isolates from these late outbreaks in time for vaccine formulation decisions. *Cases reported by those members of the American Academy of Family Physicians Research Panel who serve as sentinel physicians for influenza.

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