Modeling the impact of pandemic influenza on pacific islands.

To the Editor: Many Pacific Island countries and areas have been severely impacted in influenza pandemics. The 1918 pandemic killed substantial proportions of the total population: Fiji [approximately equal to] 5.2%, Tonga [approximately equal to] 4.2% to 8.4%, Guam [approximately equal to] 4.5%, Tahiti [approximately equal to] 10%, and Western Samoa [approximately equal to] 19% to 22% (1,2). Thirty-one influenza pandemics have occurred since the first pandemic in 1580 (3); another one is likely, if not inevitable (4). The potential use of influenza as a bioweapon is an additional concern (5).

The scale of an influenza pandemic may be projected on the basis of the available historical data that have been built into a computer model, e.g., FluAid (6). FluAid uses a deterministic model to estimate the impact range of an influenza pandemic in its first wave. Given the lack of accessible data for specific Pacific Island countries and areas, the default values used in FluAid were used for the proportion of the population in the high-risk category for each age group, for the death rates, hospitalizations, and illness requiring medical consultations. Country-specific population data were obtained from the Secretariat of the Pacific Community, and hospital bed data were obtained from the World Health Organization (WHO) (7,8). The FluAid model was supplemented by a model of an 8-week pandemic wave and modeling of hospital bed capacity. Further methodologic details are provided in the online Appendix (available from htttp:// www.cdc.gov/ncidod/EID/vol11no02 /04-0951_app.htm).

The results indicate that at incidence rates of 15% and 35%, pandemic influenza would cause 650 and 1,530 deaths, respectively, giving crude death rates of 22 to 52 per 100,000 (see the Table in the online Appendix). Most deaths (83%) would occur in the high-risk group, 60% of whom would be 19 64 years of age, and 22% would be [greater than or equal to] 65 years of age. Additionally, 3,540 to 8,250 persons would be hospitalized, most of whom (78%) would not have high-risk conditions. Also, 241,000 to 563,000 medical consultations would occur. Most (87%) consultations would be for patients without high-risk conditions (50% birth-18 years of age and 46% 19-64 years of age).

In the peak week of the pandemic (week 4), from 15% to 34% of all hospital beds would be required for patients with influenza (Table). The upper end of impact on hospital beds at >40% would occur for Guam, Kiribati, Marshall Islands, Northern Mariana Islands, and Tonga. Assuming all consultations required doctors, 42 to 99 influenza consultations per doctor would be required during the peak week (Table). The upper end of impact on consultations for individual Pacific island countries and areas would vary from 31 (New Caledonia) to 524 (Vanuatu); Fiji, Kiribati, Samoa, Solomon Islands, Tonga, and Vanuatu would have rates >150 consultations per week.

The uncertainties associated with pandemic influenza mean that any modeling of its future impact is relatively crude. For example, the new strain may be particularly infectious, virulent, or both. In contrast, the use of international-level public health interventions as recommended by WHO (9) may prevent pandemic influenza from reaching some Pacific Island countries and areas or particularly remote island groups. These issues and other limitations with the model are detailed in the online Appendix.

Nevertheless, if the death rate is in the range suggested by the model, this outcome would make it the worst internal demographic event since the 1918 influenza pandemic for many Pacific Island countries and areas. The lower death rate (albeit for a single wave) is similar to the U.S. rates for the 1957 influenza pandemic (22 per 100,000) and the 1968 influenza pandemic (14 per 100,000) (10). The upper end is considerably lower than for the 1918 pandemic, which suggests that the range indicated is reasonably plausible. Although relatively high, the death toll from pandemic influenza would still be less than the typical annual impact for some Pacific Island countries and areas from other infectious diseases (including malaria and diarrheal diseases) and from such fundamental determinants of health status such as poor sanitation, poor diet, and tobacco use.

The predicted range of hospitalizations attributable to pandemic influenza would likely overwhelm hospital capacity in many of the Pacific Island countries and areas. Rapid response at the onset of the pandemic could ensure efficacious use of hospital beds and resources, e.g., cancel elective procedures and early discharge to community care. Other contingency plans by hospitals could facilitate lower hospital admission rates (e.g., strengthening the primary care response).

Planning and capacity building could be provided by WHO, the Secretariat of the Pacific Community, and donor nations and agencies with support for improving surveillance and other preventive measures for disease control (see the online Appendix for details). A combination of national capacity building with international support will maximize the capacity to respond to the next influenza pandemic as well as other potential communicable disease threats.

Acknowledgments

Helpful comments were provided by Debbie Ryan, George Thomson, and Seini Kupu.

This work was funded in part by the New Zealand Ministry of Health. The views expressed are those of the authors and do not necessarily represent those of the Ministry of Health or the Secretariat of the Pacific Community.

References

(1.) Herda PS. The 1918 influenza pandemic in Fiji, Tonga, and the Samoas. In: Bryder L, Dow DA, editors. New countries and old medicine: proceedings of an international conference on the history of medicine and health. Auckland (NZ): Pyramid Press; 1995. p. 46-53.

(2.) Crosby AW. America's forgotten pandemic: the influenza of 1918. Cambridge, UK: Cambridge University Press; 2003. p. 233-6.

(3.) Lazzari S, Stohr K. Avian influenza and influenza pandemics. Bull WHO. 2004;82:242.

(4.) Webster RG. Predictions for future human influenza pandemics. J Infect Dis. 1997;176:S14-9.

(5.) Madjid M, Lillibridge S, Mirhaji P, Casscells W. Influenza as a bioweapon. J R Soc Med. 2003;96:345-6.

(6.) Meltzer MI, Shoemake HA, Kownaski M, Crosby R. FluAid 2.0: A manual to aid state and local-level public health officials plan, prepare, and practice for the next influenza pandemic (Beta test version). Atlanta: Centers for Disease Control and Prevention; 2000.

(7.) Secretariat of the Pacific Community Demography/Population Programme. Pacific Island populations 2004, prt 1. Available from http://www.spc.org.nc/ demog/English 01-02/Recent Stats/ 2004/Pacific%201sland%20Populations%2 02004.xls

(8.) World Health Organization. The Work of WHO in the Western Pacific Region. Report of the regional director--1 July 2002-30 June 2003. Manila: The Organization; 2003:p. 217. [cited 2004 Jul 20]. Available from http://www.wpro. who.int/pdf/rcm54/en/rdr/19_stat_annex.pdf

(9.) World Health Organization. WHO consultation on priority public health interventions before and during an influenza pandemic. Geneva: The Organization; 2004. [cited 2004 Jul 20]. Available from http://www.who.int/csr/disease/avian_influ enza/en/final.pdf

(10.) Glezen WP. Emerging infections: pandemic influenza. Epidemiol Rev. 1996;18:64-76.

Address for correspondence: Nick Wilson, Department of Public Health, Wellington School of Medicine and Health Sciences, Otago University, PO Box 7343, Wellington South, New Zealand; fax: [phone]; email: [e-mail]

Nick Wilson, * Osman Mansoor, ([dagger]) Douglas Lush, ([double dagger]) and Tom Kiedrzynski ([section])

* Otago University, Wellington, New Zealand; [dagger] Public Health Consulting Ltd, Wellington, New Zealand; [double dagger] New Zealand Ministry of Health, Wellington, New Zealand; and [section] Secretariat of the Pacific Community, Noumea, New Caledonia