In late October 1987, influenza A(H3N2) was isolated from a throat culture specimen collected from an 83-year-old woman in Los Angeles County, California. The patient had been hospitalized with pneumonia and congestive heart failure 1 day after returning from a 19-day tour and cruise in Asia. When interviewed in November, the woman said many tourists had been ill with an acute respiratory illness (ARI) while aboard the cruise ship. The patient's physician became ill with an ARI 36 hours after examining the woman. Influenza A(H3N2) virus was also isolated from his throat culture.

The tour group convened in Hong Kong on October 7 and toured the city for 2 days before boarding a cruise ship on October 9. The ship departed the following day and visited Shanghai, People's Republic of China (PRC), on October 13-14; Pusan, Korea, on October 17; and Yantai and Qinhuandao, PRC, on October 19 and 20, respectively. The tour group disembarked at Qinhuandao for an overland journey and a 3-day visit in Beijing before returning to the United States by air.

A telephone survey was conducted to determine the impact of ARI among the cruise-ship passengers. There were 427 passengers (median age, 66 years): 222 were residents of California, 201 were residents of 31 other states and the District of Columbia, three were from foreign countries, and the residence of one was not identified. Between November 21 and December 15, attempts were made to contact each of the 423 U.S. residents by telephone. A standard questionnaire was administered to the 277 (65%) persons who were interviewed; 19 persons (5%) refused to be interviewed; and 127 (30%) could not be reached.

One hundred four (38%) of the persons interviewed reported an ARI during the period October 1-30. Symptoms for 94 of these persons included fever or feverishness with either cough or sore throat; 10 persons reported cough, sore throat, and myalgias. The peak of the outbreak occurred on October 20 (Figure 1), 6 days after the ship's visit to Shanghai and 3 days after the visit to Pusan, Korea. Seven passengers (2.5%) reported that they were diagnosed by a physician as during the period October 1-30. Symptoms for 94 of these persons included fever or feverishness with either cough or sore throat; 10 persons reported cough, sore throat, and myalgias. The peak of the outbreak occurred on October 20 (Figure 1), 6 days after the ship's visit to Shanghai and 3 days after the visit to Pusan, Korea. Seven passengers (2.5%) reported that they were diagnosed by a physician as having pneumonia after their return home; five (71%) of the seven were hospitalized. No deaths were reported.

To determine the potential for secondary spread in the community, passengers were asked to identify nontourist contacts who had onset of an ARI within 3 days of the tourist's return home. Ten (36%) of the 28 households with a tourist who reported late onset of symptoms* reported that at least one nontourist contact had become ill. Two (5%) of the 42 households without a patient with late onset of symptoms reported contacts who had become ill (relative risk = 7.5, 95% confidence interval = 2.3 to 24.5).

Data from persons over 64 years of age were analyzed separately because this age group is considered to be at increased risk for complications following influenza infection (1). To decrease this risk, the Immunization Practices Advisory Committee (ACIP) recommends that all persons over 64 receive influenza vaccine annually (1). In calculating vaccine efficacy, investigators assumed that 1) all cases of ARI were due to influenza and 2) risk factors for influenza-related complications were similar for the vaccinated and unvaccinated groups. The attack rate among the 36 tourists who were over 64 years of age and had received the 1987-88 influenza vaccine in August or September 1987 was 36%; the rate was 37% among the 127 unvaccinated travelers in this age group. Duration of illness, defined as the number of days before patients felt that they had returned to their normal level of health, did not differ significantly between the two groups. The mean duration was 19.3 days for the vaccinated group and 21.9 days for the unvaccinated group (t test = 0.62, pless than 0.40). The hospitalization rate was the same (3%) among the vaccinated and unvaccinated passengers in this age-group.

Antigenic analysis of the isolates from the two culture-confirmed index cases was performed by reciprocal hemagglutination-inhibition tests with sera taken from infected ferrets during the convalescent stage of illness. The results indicate that the isolates are type A(H3N2) variants similar to influenza viruses isolated in Asian and Pacific countries since April 1987 (2). Reported by: CD Berlingberg, MD, FH Kahn, MD, Los Angeles; LY Chun, MPH, JM Cruz, G Gellert, MD, MP Giles, MS, L Mascola, MD, FJ Sorvillo, MPH, M Tormey, MPH, SH Waterman, MD, Los Angeles County Dept of Health Svcs; RA Murray, DrPH, KH Acree, MDCM, MPH, JD, Acting State Epidemiologist, California Dept of Health Svcs. Office of the Director, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: This is the second outbreak of ARI among U.S. tourists who have traveled aboard cruise ships in the Pacific Basin during the last 6 months. The previous outbreak was also identified following reports of a traveler who was acutely ill upon returning from a trip and for whom influenza A(H3N2) infection was confirmed (3). The extent of illness among nontourist household contacts in the present outbreak suggests that an infectious agent that is transmitted from person to person was responsible. The following evidence suggests that influenza virus was involved: 1) influenza A(H3N2) was isolated from a tourist and her physician, 2) viruses from these index cases were antigenically similar to strains present in Hong Kong (4) and Shanghai during September 1987 (Health and Antiepidemic Station, Shanghai, PRC, unpublished data) and 3) the sharp peak in the epidemic curve and high attack rates are typically seen in influenza outbreaks in partially closed settings, including military vessels (5,6), aircraft (7,8), and institutions (9).

Most tourists with ARI had onset of illness in late October and were ill either during the flight home to the United States or within 3 days of returning home. Although the duration of illness following influenza infection for persons over 64 years of age may be longer than the 5-7 days usually experienced by younger adults (10), the longer durations reported in this outbreak may be partially due to travel-related factors such as jet lag. Nevertheless, these data suggest that influenza vaccine did not attenuate illness duration in this group.

There are at least three possible reasons for the lack of vaccine efficacy demonstrated: 1) repeated exposures to infectious persons or different dynamics of transmission (11,12) occurring in a population in a partially closed setting may overcome levels of immunity that might be protective in other settings, 2) some of the illnesses may have been caused by other respiratory pathogens circulating at the same time (13), or 3) an influenza virus representing a clinically significant antigenic drift from the vaccine strain caused the outbreak.

The observations in this investigation support the results of laboratory studies (2) that suggest that the A/Leningrad/86(H3N2) component of the vaccine may not provide optimal protection against the strains of virus recently identified in the Pacific Basin and now present in the United States. The need for long-term care facilities housing high-risk patients to develop contingency plans for rapidly initiating amantadine prophylaxis in the event of influenza A outbreaks should be reemphasized (2), particularly in light of continuing reports of influenza A(H3N2) outbreaks in such institutions this winter.

References

1. Immunization Practices Advisory Committee. Prevention and control of influenza. MMWR 1987;36:373-80,385-7.

2. Centers for Disease Control. Antigenic variation of recent influenza A(H3N2) viruses. MMWR 1988;37:38-40,46-7.

3. Centers for Disease Control. Outbreak of influenza-like illness in a tour group--Alaska. MMWR 1987;36:697-8,704.

4. World Health Organization. Influenza. Wkly Epidem Rec 1987;47:359.

5. Olson JG, Ksiazek TG, Irving GS, Rendin RW. An explosive outbreak of influenza caused by A/USSR/77-like virus on a United States naval ship. Milit Med 1979;144:743-5.

6. Ksiazek TG, Olsen JG, Irving GS, Settle CS, White R, Petrusso R. An influenza outbreak due to A/USSR/77-like (H1N1) virus aboard a US Navy ship. Am J Epidemiol 1980;112:487-94.

7. Moser MR, Bender TR, Margolis HS, Noble GR, Kendal AP, Ritter DG. An outbreak of influenza aboard a commercial airliner. Am J Epidemiol 1979;110:1-6.

8. Centers for Disease Control. Influenza activity in civilian and military populations and key points for use of influenza vaccines. MMWR 1986;35:729-31.

9. Patriarca PA, Weber JA, Parker RA, et al. Efficacy of influenza vaccine in nursing homes: reduction in illness and complications during an influenza A(H3N2) epidemic. JAMA 1985;253:1136-9.

10. Barker WH, Mullooly JP. Impact of epidemic type A influenza in a defined adult population. Am J Epidemiol 1980;112:798-813.

11. Alford RH, Kasel JA, Gerone PJ, Knight V. Human influenza resulting from aerosol inhalation. Proc Soc Exp Biol Med 1966;122:800-4.

12. Douglas RG Jr. Influenza in man. In: Kilbourne ED, ed. The influenza viruses and influenza. New York: Academic Press, 1975:395-447.

13. Monto AS, Cavallaro JJ. The Tecumseh study of respiratory illness. II. Patterns of occurrence of infection with respiratory pathogens, 1965-69. Am J Epidemiol 1971;94:280-9. *Less than 4 days before the end of the tour or within 3 days of returning home.

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