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.
Disclaimer
All MMWR HTML documents published before January 1993 are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.
**Questions or messages regarding errors in formatting should be addressed to mmwrq@cdc.gov.
