Influenza A -- Florida and Tennessee, July-August 1998, and Virologic Surveillance of Influenza, May-August 1998

During July and August 1998, the state departments of health in Florida and Tennessee each reported an outbreak of influenza. The Florida outbreak occurred in July in two residential homes for children; the Tennessee outbreak occurred in August among members of a family that vacationed together. This report summarizes the investigation of these outbreaks, which were caused by influenza type A(H3N2) viruses, and presents information on influenza isolates received by CDC during May-August 1998, 81% of which were influenza A(H3N2).

Florida

In July, an outbreak of influenza occurred in two residential homes for children with cerebral palsy; the residences are served by the same staff. On July 10, a 7-year-old resident of one home developed fever and cough. During the following 2 weeks, in both residences combined influenza-like illness (defined as fever accompanied by cough and or sore throat) developed in 20 (91%) of 22 children and 10 (18%) of 56 staff members. Eleven (55%) children were hospitalized; four had pneumonia, including one child who was in the hospital intensive-care unit for 5 days. The average length of hospitalization was 6 days (range: 2-13 days). All ill persons have recovered. Nasopharyngeal swabs were collected from three of the ill children, and all yielded influenza A virus. On analysis at CDC, the three viral isolates were similar antigenically to A/Sydney/05/97(H3N2), the strain that predominated during the 1997-98 influenza season and is included in the 1998-99 influenza vaccine.

During September and October 1997, 17 (85%) of the 20 ill children had received the 1997-98 influenza vaccine, which contained the A/Nanchang/933/94 strain as the H3N2 component (1). None of the ill staff or residents had traveled recently outside the 48 contiguous United States.

Tennessee

On August 13, a previously healthy 66-year-old man was admitted to a hospital in Tennessee after 5 days of progressive dyspnea, nonproductive cough, pharyngitis, fever, myalgias, and malaise. On admission, he was hypoxic, and a chest radiograph was suggestive of viral pneumonia. Influenza was considered among the initial diagnostic possibilities, but rapid-antigen test kits for influenza were not available in the hospital laboratory. A nasal washing was obtained on August 12, and influenza A virus was cultured from it on August 18. The isolate was characterized antigenically as A/Sydney/05/97(H3N2)-like at CDC.

On August 1, an 11-year old female relative of this patient returned to the United States after a 2-week visit to Panama. On August 3, she developed fever (104 F {40 C}), headache, myalgias, nonproductive cough, and nonexudative pharyngitis. From August 3 through August 9, she shared a beach house while on vacation with 12 family members; four of the 12, including the hospitalized man, developed similar febrile illnesses during that week. Only the man was hospitalized, and all ill persons have recovered. Two of the five ill persons had received the 1997-98 influenza vaccine.

In addition to this cluster of cases, two relatives who accompanied the 11-year-old on her return from Panama but who did not visit the beach house developed similar symptoms on August 3. None of the ill persons reported recent travel outside of the 48 contiguous United States except to Panama. Other than the two persons who accompanied the child on the plane, none of the family members in Panama visited by the 11-year-old reported any recent illness.

CDC Virologic Surveillance

During May-August 1998, CDC received 52 influenza isolates from U.S. laboratories; 44 were influenza type A viruses and eight were influenza type B viruses. Of the 42 influenza A viruses subtyped, all were influenza A(H3N2), and all were antigenically similar to A/Sydney/05/97. All eight influenza B viruses were antigenically similar to B/Beijing/184/93, which is contained in the 1998-99 influenza vaccine. Of the influenza A(H3N2) isolates, two were collected during a nursing home outbreak in Montana in May, four were collected from the outbreaks in Florida and Tennessee, and 32 were collected during an ongoing outbreak in Alaska and the Yukon Territory (2,3). Two influenza A isolates not yet subtyped also were collected during the Alaska and Yukon Territory outbreak. The eight influenza B and the four remaining influenza A(H3N2) viruses were collected from sporadic cases in seven states.

Reported by: MC Viar, Miami Children's Hospital, Miami; S Atherley, E Sfakianaki, MD, M Cruz, MPH, Miami-Dade County Health Dept, Miami; D Katz, PhD, R Hopkins, MD, State Epidemiologist, Florida Dept of Health. A Kaiser, MD, J Oates, MD, B Graham, MD, Vanderbilt Univ School of Medicine, Nashville; A Craig, MD, W Moore, MD, State Epidemiologist, Tennessee Dept of Health. Div of Applied Public Health Training, Epidemiology Program Office; Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note

Editorial Note: Although sporadic influenza infections occur in the United States throughout the year, outbreaks of influenza have been reported infrequently during the summer and early fall. However, during 1993-1997, nine summer outbreaks of influenza A have been reported to CDC (1,4-6).

Influenza must be considered as a potential cause of any outbreak of febrile respiratory illness, even during summer months. Tests for the rapid diagnosis of influenza A and influenza B infections aid in early detection and enable initiation of appropriate control measures and treatment. Although influenza vaccines usually are not available during the summer months, the antiviral medications rimantadine and amantadine are available. These drugs are effective for prophylaxis and for treatment of influenza type A when administered within 48 hours of illness; neither drug is effective against influenza type B viruses (7). Use of these drugs and implementation of other outbreak-control measures, such as cohorting of ill persons, should effectively decrease the morbidity associated with influenza outbreaks.

Influenza virus strains associated with summer outbreaks are important indicators of the strains likely to predominate during the fall and winter months. In 1997, a summer outbreak of influenza aboard a cruise ship traveling between New York and Montreal yielded early North American isolates that were A/Sydney/05/97(H3N2)-like. This strain became the predominant circulating influenza virus in the United States during the 1997-98 season, but was not well matched antigenically with the H3N2 component in that year's vaccine (1,8).

Summer outbreaks of influenza may become more common with increases in international travel. A 1997 outbreak aboard a cruise ship was associated with viruses from the southern hemisphere, where it was winter and influenza activity was elevated for that hemisphere. In addition, influenza can circulate year-round in the tropics. The investigation of the outbreak in Tennessee suggested that the three family members who traveled from Panama may have had a common exposure in Panama or during the return trip.

The Florida outbreak underscores that particular groups of persons aged less than 65 years are at high risk for severe complications of influenza, and annual influenza vaccination is recommended (7). Although 85% of the children in the Florida outbreak had been vaccinated during the previous year, vaccine coverage among high-risk groups aged less than 65 years typically is much lower (9). Persons aged less than 65 years and at increased risk for influenza-related complications include those who reside in nursing homes or chronic-care facilities; persons with chronic cardiovascular or pulmonary disorders (including asthma); persons who required medical follow-up or hospitalization during the previous year because of diabetes or other chronic metabolic diseases, renal dysfunction, hemoglobinopathies, or immunosuppression; children and teenagers (aged 6 months-18 years) who are receiving long-term aspirin therapy (and who therefore may be at risk for developing Reye syndrome after influenza); and women who will be in the second or third trimester of pregnancy during the influenza season. Because persons who are clinically or subclinically infected can transmit influenza virus to high-risk persons, vaccination also is recommended for health-care workers and other persons, including household members in frequent contact with persons at high risk for influenza-related complications (7). Influenza vaccine is recommended annually because the protective antibody levels provided by vaccine wane during the year. In addition, continual antigenic drift among influenza viruses frequently results in the circulation of new strains that may not be adequately covered by older vaccine. Both of these factors probably contributed to the high attack rate in the Florida outbreak.

The optimal time for organized influenza vaccination campaigns is October through mid-November; however, beginning in September, health-care providers should offer influenza vaccine to persons at high risk who are seen for routine care or as a result of hospitalization. Because influenza viruses can circulate in the spring, health-care providers should continue to offer influenza vaccine to unvaccinated high-risk persons after influenza activity has been documented in the community (7).

Information about influenza surveillance is available through the CDC Voice Information System (recorded message), telephone (888) 232-3228 ({888} CDC-FACT), fax (888) 232-3299 ({888} CDC-FAXX) (document no. 361100), or the Internet at http://www.cdc.gov/ncidod/diseases/flu/weekly.htm. From October through May, the information is updated weekly.

References

1. CDC. Update: influenza activity -- United States, 1997-98 season. MMWR 1997;46:1094-8.

2. CDC. Update: outbreak of influenza A infection -- Alaska and the Yukon Territory, July-August 1998. MMWR 1998;47:685-8.

3. CDC. Outbreak of influenza A infection -- Alaska and the Yukon Territory, June-July 1998. MMWR 1998;47:638.

4. CDC. Update: influenza activity -- worldwide, March-August 1997. MMWR 1997;46:815-8.

5. CDC. Update: influenza activity -- worldwide, 1996. MMWR 1996;45:816-9.

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

7. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1998;47(no. RR-6).

8. CDC. Update: influenza activity -- United States and worldwide, 1997-98 season, and composition of the 1998-99 influenza vaccine. MMWR 1998;47: 280-4.

9. Greby SM, Singleton JA, Strikas RA, Williams WW. Influenza and pneumococcal vaccination -- progress toward Healthy People 2000 goals {Abstract}. In: Abstracts from the 32nd National Immunization Conference. Atlanta, Georgia: 32nd National Immunization Conference, 1998.

   
   

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