Influenza Activity -- United States and Europe, 1993-94 Season

In collaboration with the World Health Organization (WHO) international collaborating laboratories and with state and local health departments in the United States, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. Laboratory surveillance indicates the predominance of influenza type A so far this season. This report summarizes influenza activity in the United States and Europe from mid-September through mid-November 1993. United States

From September 19 through November 6, nearly all state and territorial epidemiologists reported either sporadic * levels of influenza-like illness (ILI) or no activity. The first reports of regional activity associated with laboratory-confirmed outbreaks of influenza were from Wyoming and Montana for the week ending November 13 and from Idaho for the week ending November 20. From October 3 through November 13, an average of less than 3% of all patient visits to family practitioners participating in the CDC sentinel physician surveillance system was for ILI.

From September 23 through November 23, 14 states (Alaska, California, Colorado, Connecticut, Hawaii, Louisiana, New Mexico, New York, North Carolina, Ohio, Oregon, South Dakota, Texas, and West Virginia) reported sporadic isolates of influenza type A. The outbreaks of culture-confirmed influenza A in Montana and Wyoming were the first since August-September, when three outbreaks of influenza A(H3N2) were reported in Louisiana (1). The outbreaks in Montana and Wyoming were associated with high absentee rates in two neighboring schools in those states. The first outbreak was recognized on November 4, when 18 (45%) of 40 students in an elementary school in Wyoming were absent. During November 4-11, the neighboring school in Montana (302 students in grades kindergarten through 12) reported daily absentee rates of 8%-14%. Seven of 10 nasopharyngeal specimens collected from students, household contacts, and others living in the community were positive for influenza type A by antigen detection as of November 30; two of these were confirmed by viral culture. In mid-November, Idaho reported outbreaks of ILI in schools in two southern counties; daily absentee rates were high (10%-44%), and influenza type A viruses were isolated from four students aged 9-15 years.

Of the influenza A viruses reported since late September, eight were subtyped; seven were identified as type A(H3N2) and one as type A(H1N1). Four of these viruses were further characterized at CDC and are antigenically related to the A/Beijing/ 32/92(H3N2) strain, the type A(H3N2) strain included in the 1993-94 influenza vaccine.

As of November 19, WHO collaborating laboratories in the United States have not reported influenza type B viruses. Europe

In all European countries except the United Kingdom, influenza activity occurred at low levels from October 1 through mid-November. Influenza type A(H3N2) has been the predominant virus isolated. In addition, sporadic cases of influenza type B have been diagnosed by antigen detection. One isolate of influenza type A(H1N1) was reported from France.

During the week ending November 13, influenza activity in the United Kingdom became widespread. Influenza activity began early in October with outbreaks of ILI in Scotland and England. In Scotland, an outbreak that began among university students and staff extended into the surrounding community, and an outbreak in a residential home for the elderly affected both residents and staff. In England, outbreaks occurred in a residential home and in a boarding school. Influenza type A(H3N2) was isolated from ill persons in all four of these outbreaks. Outbreaks of ILI were reported in additional institutions, and general practitioners have reported increased levels of ILI in communities. The Central Public Health Laboratory in London analyzed 136 influenza isolates from sporadic cases and from outbreak-related cases; all were antigenically related to the A/Beijing/32/92(H3N2) virus.

In Finland, beginning the week ending October 30, outbreaks of ILI among children were associated with absentee rates of 50% in some elementary schools; influenza type A was diagnosed in two patients by antigen detection. In Sweden, the incidence of ILI was increasing among all age groups by the end of October; influenza type A(H3N2) was isolated from two patients, and influenza A was diagnosed by antigen detection in three. France, Czechoslovakia, and the Netherlands have reported either antigen detection of influenza type A or isolation of influenza type A(H3N2) from sporadic cases. Reported by: P Farmer, MD, P Coleman, Mammoth Clinic, Mammoth; J Alston, Yellowstone National Park; S Zanto, TA Damrow, PhD, State Epidemiologist, Montana State Dept of Health and Environmental Svcs. S Music, MD, State Epidemiologist, Wyoming Dept of Health. R Medlin, C Sandaval, Idaho Dept of Health and Welfare. WHO Influenza Centers. Communicable Diseases Div, World Health Organization, Geneva. Central Public Health Laboratory, Public Health Laboratory Svc, Communicable Disease Surveillance Center and Virus Reference Div, London. Dept of Virology, National Public Health Institutes, Helsinki. Swedish Institute for Infectious Disease Control, Stockholm. 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 findings in this report indicate that levels of influenza activity during November were higher than usually seen during this time of year. Reports of confirmed influenza outbreaks in early November illustrate the importance of prompt vaccination of unvaccinated high-risk persons before widespread activity occurs. Because protective levels of antibody develop within 2 weeks after vaccination, vaccine ideally should be administered at least 2 weeks before influenza outbreaks are expected. However, influenza vaccine should continue to be offered to high-risk persons after influenza activity is documented in a community. Because early viral surveillance has indicated the predominance of influenza type A, the antiviral drugs amantadine and rimantadine, which are effective against influenza type A viruses, can be used for prevention and treatment. When vaccine is administered after influenza type A has begun to circulate in a community, amantadine or rimantadine can be administered for 2 weeks after vaccination to provide protection until vaccine-induced antibody has developed (2,3).

The increased circulation of influenza type A(H3N2) virus may increase the risk for outbreaks in nursing homes and facilities housing elderly persons; such outbreaks were reported during the latter half of the 1992-93 season and during August-September in Louisiana (1). Therefore, such facilities should now ensure that their residents have received influenza vaccine and also should develop contingency plans for rapid administration of amantadine or rimantadine in the event of suspected or confirmed influenza type A outbreaks.

Influenza surveillance findings are updated at least every other week throughout the influenza season, and summaries are available by computer to subscribers of the Public Health Network and to health-care providers and the public through the CDC Voice Information System, telephone (404) 332-4555.

References

1. CDC. Influenza A outbreaks -- Louisiana, August 1993. MMWR 1993;42:689-92.

2. ACIP. Prevention and control of influenza: part I, vaccines -- recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1993;42(no. RR-6).

3. ACIP. Prevention and control of influenza: recommendations of the Immunization Practices Advisory Committee (ACIP). MMWR 1992;41(no. RR-9).

• Levels of activity are: 1) sporadic -- sporadically occurring influenza-like illness (ILI) or culture-confirmed influenza, with no outbreaks detected; 2) regional -- outbreaks of ILI or culture-confirmed influenza in counties with 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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