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Update: Influenza Activity -- United States, 1997-98 Season

CDC conducts surveillance for influenza viruses and related disease activity in collaboration with the World Health Organization (WHO), its collaborating laboratories, and state and local health departments. This report summarizes influenza surveillance data in the United States from September 28, 1997, through the week ending November 8, and describes two recent cruise ship outbreaks of influenza. The findings indicate that, during this period, influenza activity in the United States was low and that influenza A predominated.

Maryland reported the first U.S. regional influenza activity * during the week ending October 25. Through the week ending November 8, two states (Maryland and New York) reported regional activity. Since September 28, the percentage of patient visits to sentinel physicians for ILI has remained under baseline levels (0-3%), and the percentage of deaths attributed to pneumonia and influenza (P&I) reported by the vital statistics offices of 122 cities has not exceeded the epidemic threshold **. During September 28-November 8, a total of 20 (0.5%) of 4477 specimens tested (by culture or direct antigen techniques) for respiratory viruses at WHO collaborating laboratories in the United States were positive for influenza virus. Of the 20 positive specimens, 19 were influenza type A, and one was type B; all influenza A isolates subtyped have been A(H3N2). Among specimens collected since September 21 and characterized by CDC (none of which came from WHO collaborating laboratories), one was an A/Nanchang/933/95-like(H3N2) virus antigenically similar to the H3N2 component in the 1997-98 influenza vaccine, and three isolates from a nursing home outbreak in Hawaii were related to A/Sydney/05/97(H3N2) (Table_1). A/Sydney/05/97-like viruses were first detected in June in Australia and New Zealand. In Australia, these viruses have accounted for 146 (29.0%) of 503 total influenza A(H3N2) isolates. A/Sydney/05/97-like viruses have been identified among isolates from Australia, New Zealand, Hong Kong, Hawaii, Puerto Rico, and a cruise ship outbreak. As of November 8, five outbreaks of influenza have been reported to CDC, including the following on two cruise ships. Cruise Ship A Outbreak

On September 10, 1997, Health Canada notified CDC that on a cruise from New York City to Montreal (cruise ship A) during August 31-September 10, a total of 39 (2.7%) of 1445 passengers and three (0.5%) of 631 crew members presented to the ship's infirmary because of acute febrile respiratory illness. All passengers disembarked in Montreal; nine (0.6%) were referred to area hospitals for respiratory complications. Influenza A was confirmed by culture.

On September 11, a new cohort of 1448 passengers boarded the same ship for the return voyage to New York City; the crew did not change. During September 11-20, a total of 19 (1.3%) passengers and 17 (2.7%) crew members presented to the infirmary because of ILI (fever greater than or equal to 100 F { greater than or equal to 38 C} and either sore throat or cough). On September 15, public health officials from Health Canada and CDC boarded the ship in Canada to investigate the outbreak and advise ship officials on control measures. On September 17, one nasopharyngeal swab was positive for influenza A by a rapid viral antigen detection test. Active surveillance for ILI was instituted among the crew; those with ILI were confined to their cabins and started on rimantadine. All non-ill crew members were started on rimantadine prophylaxis for 14 days. All 631 crew members were administered the 1997-98 influenza vaccine. On September 17, all passengers on the second cruise were notified of the outbreak, and non-ill passengers were offered rimantadine prophylaxis. Passengers presenting to the infirmary with ILI were given rimantadine for 5 days.

Based on a survey of 1284 passengers during September 17-18, a total of 994 (77.4%) were aged greater than or equal to 65 years, 336 (26.2%) had chronic health conditions associated with increased risk for severe complications of influenza, 52 (4.1%) reported an ILI, and 1020 (80.8%) of 1262 passengers reported using rimantadine prophylaxis. On September 20, two (0.1%) passengers who disembarked in New York City were referred to area hospitals for respiratory complications. Thirteen isolates received at CDC for viral culture were characterized as influenza A/Sydney/05/97-like(H3N2) (Table_1). On September 20, a new group of passengers boarded in New York City; this group was notified of the previous outbreaks. During September 21-24, no new cases of ILI were detected. Cruise Ship B Outbreak

On October 15, cruise ship B reported to CDC an outbreak of acute respiratory illness on a cruise from Tahiti to Hawaii. During October 6-18, a total of 48 (3.3%) of 1443 passengers and 16 (2.5%) of 639 crew members presented to the ship's infirmary because of acute respiratory illness. Eight (0.6%) passengers had pneumonia diagnosed; one was hospitalized. Influenza A was confirmed by rapid antigen detection and by culture.

On October 18, a new cohort of 1477 passengers, most unvaccinated, boarded the ship in Honolulu and were informed of the outbreak. Active surveillance was initiated among the crew, and influenza vaccine was administered to 631 (97.8%) of 645 crew members. Rimantadine was administered to all non-ill crew members. During October 18-27, a total of 29 (2.0%) passengers and 39 (6.0%) crew members reported acute respiratory illness. Ill crew members were confined to their cabins and administered rimantadine for 5 days. Ill passengers were offered rimantadine treatment. There were no reported severe complications. One isolate received at CDC was characterized as influenza A/Nanchang/933/95-like(H3N2), antigenically similar to the H3N2 component in the 1997-98 influenza vaccine.

Reported by: T Tam, MD, J Hockin, MD, Field Epidemiology Training Program; D Kertesz, MD, Div of Respiratory Diseases, Bur of Infectious Diseases; R Nowak, MD, T Nguyen, MHA, Quarantine Health Svcs; R St John, MD, Office of Special Health Initiatives; L Ouellette, MEd, G Lynch, MD, Occupational and Environmental Health Svcs, Health Canada, Ottawa; M Libman, MD, Montreal General Hospital, Montreal; P Rene, MD, Royal Victoria Hospital, Montreal; M Miller, MD, Jewish General Hospital, Montreal; J MacDonald, MD, Montreal Children's Hospital, Montreal; J Carsley, MD, Disease Control, L Mathieu, Occupational and Environmental Health Svcs, Quebec District, F Saintonge, MD, L Valiquette, MD, P Leguerriur, MD, Infectious Disease Unit, Montreal Regional Public Health Dept, Canada. S Kuhr, Mayor's Office of Emergency Management, New York; JR Miller, MD, Bur of Communicable Diseases, F Winters, New York City Dept of Health. KF Gensheimer, MD, State Epidemiologist, Bur of Health, Maine Dept of Human Svcs. MA Barry, MD, Communicable Disease Control, Boston Public Health Commission, Boston Massachusetts. U Bandy, MD, Rhode Island Dept of Health. R Ueki, G Kunimoto, Virology Section, State Laboratories Div; C Wakida, M Ching-Lee, MPH, PV Effler, MD, State Epidemiologist, Hawaii State Dept of Health. Participating state and territorial epidemiologists and state public health laboratory directors. World Health Organization Collaborating Center for Reference and Research on Influenza, Parkville, Australia. World Health Organization collaborating laboratories. Div of Quarantine and Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The overall level of influenza activity in the United States as described in this report is typical for fall months. During September 28-November 8, all but one of the 20 influenza viruses reported by U.S. WHO collaborating laboratories were influenza A, and all subtyped isolates were A(H3N2). Influenza A(H3N2) virus infections have been associated with increased morbidity and mortality among the elderly (1-3); the increased risk underscores the need for all elderly and other persons at high risk for complications of influenza to receive influenza vaccination.

The primary characteristics of the two cruise ship outbreaks described in this report were 1) most passengers on both ships were aged greater than or equal to 65 years and were at risk for severe influenza-related complications; 2) most crew members and passengers on both ships were unvaccinated; and 3) control measures on both ships included the combination of active surveillance, cohorting of ill crew members, vaccination of crew members, and use of antiviral therapies to treat cases and to prevent disease in non-ill persons; these measures were successful in controlling the outbreak (4).

The influenza strain identified from cruise ship A (A/Sydney/05/97-like {H3N2}) is related but antigenically distinguishable from A/Nanchang/933/95, which is the A(H3N2) component included in the 1997-98 influenza vaccine. This antigenic variant has not yet been detected in Africa, Europe, South America, or in the continental United States, and the extent to which this variant will circulate during the 1997-98 season cannot be predicted. In addition, the effect of this virus' circulation on vaccine effectiveness is also unknown. However, because vaccine effectiveness is dependent, in part, on the match between the vaccine and circulating strains, protection could be less than optimal if this variant circulates widely (5-7). Even when vaccine and epidemic strains match closely, outbreaks can occur among vaccinated groups. When feasible, measures should be taken to reduce contact between symptomatic and asymptomatic persons during outbreaks. In addition, chemoprophylaxis of all non-ill persons with antiviral drugs rimantadine or amantadine should be considered during influenza A outbreaks in closed or semi-closed settings where persons at risk for influenza-related complications may be in close proximity (e.g., nursing homes and cruise ships); contingency planning is needed to ensure rapid administration of rimantadine or amantadine. These drugs also can be used to reduce the severity and shorten the duration of influenza A illness when treatment is initiated within 48 hours of illness onset (4).

Throughout the season, influenza surveillance data are updated weekly and are available through CDC's fax information system, telephone (888) 232-3299, by requesting document number 361100, or through CDC's National Center for Infectious Diseases, Division of Viral and Rickettsial Diseases, Influenza Branch World-Wide Web site Information about local influenza activity is available from some county and state health departments.


  1. Lui KJ, Kendal AP. Impact of influenza epidemics on mortality in the United States from October 1972 to May 1985. Am J Public Health 1987;77:712-6.

  2. Noble GR. Epidemiological and clinical aspects of influenza. In: Beare AS, ed. Basic and applied research. Boca Raton, Florida: CRC Press, 1982:11-50.

  3. Simonsen L, Clarke MA, Williamson GD, Stroup DF, Arden NH, Schonberger LB. The impact of influenza epidemics on mortality: introducing a severity index. Am J Public Health (in press).

  4. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1997;46(no. RR-9).

  5. Stiver HG, Graves P, Eickhoff TC, Meiklejohn G. Efficacy of "Hong Kong" vaccine in preventing "England" variant influenza in 1972. N Engl J Med 1973;289:1267-71.

  6. Couch RB, Keitel WA, Cate TR, Quales JA, Taber LA, Glezen WP. Prevention of influenza virus infections by current inactivated influenza A virus vaccines. In: Brown LE, Hampson AW, Webster RG, eds. Options for the control of influenza III. Amsterdam, Netherlands: Excerpta Medica/Elsevier Science Publishers BV, 1996:97-106.

  7. Sugaya N, Nerome K, Masatoshi I, Matsumoto M, Mitamura K, Nirasawa M. Efficacy of inactivated vaccine in preventing antigenically drifted influenza type A and well-matched type B. JAMA 1994;272:1122-6.

* Levels of activity are 1) no activity; 2) sporadic -- sporadically occurring influenza-like illness (ILI) or culture-confirmed influenza with no outbreaks detected; 3) 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 4) widespread -- outbreaks of ILI or culture-confirmed influenza in counties with a combined population of greater than or equal to 50% of the state's total population. 

** The epidemic threshold is 1.645 standard deviations above the seasonal baseline. The expected seasonal baseline is projected using a robust regression procedure in which a periodic regression model is applied to observed percentages of deaths from P&I since 1983.

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Hemagglutination-inhibition titers of influenza type A(H3N2) viruses
 with serum specimens from infected ferrets *
                                        Ferret antiserum
Viral antigen        A/Wuhan/359/95    A/Nanchang/933/95   A/Sydney/05/97
Reference antigens
A/Wuhan/359/95            1280                1280               160
A/Nanchang/933/95         1280                2560               160
A/Sydney/05/97             160                 160              1280
Recent isolates
A/Canada/11/97+             80                  80              1280
A/Hawaii/05/97&             80                  80              2560
A/Guangzhou/133/97        1280                2560               320
A/Chile/3252/97            640                1280               320
A/Hawaii/06/97@            640                1280               320
* A fourfold or greater difference in hemagglutination-inhibition titers
  between two viruses is indicative of antigenic variation between viruses.
+ Virus identified in cruise ship A outbreak.
& Virus identified in a nursing home outbreak in Hawaii.
@ Virus identified in cruise ship B outbreak.

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