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Letter Modeling the Impact of Pandemic Influenza on Pacific IslandsNick Wilson,* AppendixAdditional MethodsThe output of the FluAid model is the number of additional deaths, hospitalizations, and illness attributable to pandemic influenza that will require medical consultations. The model assumes no effective public health interventions to control disease spread (such as quarantine, access to appropriate vaccine, or widespread use of antiviral drugs). Specific details about the FluAid software and the various assumptions in the model are detailed on the Centers for Disease Control and Prevention (CDC) Web site (1) and in other documents (2,3). Given the lack of accessible data for specific Pacific Island countries and areas, the default values used in FluAid were used to determine the proportion of the population in the high-risk category for each age group, when calculating the death rates, the hospitalization rates, and the illness rates. The model's parameters were based on the available data (mostly from North America and some from Europe) from the 1957 pandemic and subsequent nonpandemic data (2). Persons categorized as high-risk have a preexisting medical condition (e.g., diabetes) that makes them more susceptible to developing medical complications due to influenza. The proportions in this high-risk category used in the model were 6.4% for persons birth–18 years of age, 14.4% for those 19–64 years of age, and 40% for those ≥65 years of age. The output values from the model were for most likely, minimum, and maximum values (each for death, hospitalization, and illness requiring medical consultations). The FluAid model provides the total disease impact but does not specify its time distribution. The length of influenza epidemics is highly variable (4,5), but for this analysis, the first pandemic wave was assumed to span 8 weeks and have the same distribution over time as a model of a stochastically simulated influenza epidemic (6) i.e., 32.3% of all cases in week 4, the peak week. The countries and areas included in this analysis were the 20 tropical Pacific Island countries and areas that are members of the Secretariat of the Pacific Community (SPC) (excluding Papua New Guinea, and Pitcairn Island). Mid-2004 total population estimates from SPC (7) were used and adjusted according to World Health Organization (WHO) data on population distribution (8) to fit the age categories required by the model. The total number of hospital beds in each Pacific Island country and area was based on WHO data (9). However, the data for Tonga were updated from more recent information (S. Kupu, pers. comm.) and updated similarly for Niue as well (10). When countries' data were missing, the information was found through additional Internet searches (i.e., for Nauru and Tokelau). The number of doctors per capita was also obtained from WHO data (9). Limitations with the ModelingThe uncertainties associated with pandemic influenza mean that any modeling of its future impact is relatively crude. The FluAid model also has a number of specific limitations, which may lend an underestimation of the next pandemic's impact. First, the new strain may be particularly infectious, virulent, or both. The model's upper incidence rate for clinical illness was 35%, when higher rates (e.g., 50%) are plausible. Second, the proportions of the population in various high-risk groups in Pacific Island countries and areas may be larger than used in the model. This is plausible given the relatively high prevalence of chronic conditions such as diabetes in some Pacific Island countries and areas (e.g., a 23% prevalence rate for diabetes amongst adults aged 25–64 years in Samoa [11]). Finally, the level of antimicrobial drug resistance (e.g., Streptococcus pneumoniae) may continue to increase globally, thus the actual death rate may be higher than used in this model (i.e., if alternative treatments for the secondary microbial infections after influenza infection are not readily available). In contrast, the results could also be overestimated for the following reasons: First, the use of international level public health interventions as recommended by WHO (12) may prevent or delay pandemic influenza reaching some Pacific Island countries and areas or particularly remote island groups (e.g., the provision of health alert notices to incoming travelers and entry screening). Improvements in surveillance systems (with access to rapid detection kits) over time may increase the chances of control measures being successful or subsequent pandemic waves being delayed. Second, some Pacific Island countries and areas could possibly avoid the first pandemic wave and might have access to a vaccine for protection from subsequent pandemic waves (though this may take 6–9 months from the time that a new virus variant is first identified (13). In addition, the use of antiviral agents (14) could prevent infection and reduce illness among key personnel and also those with high-risk conditions, but only if supplies are adequate. A recent study suggests that pandemic influenza could be contained with "the use of antiviral prophylaxis, if 80% of the exposed persons maintained prophylaxis for up to 8 weeks" (6). Also, improved treatment in the community and hospital settings could lower hospitalization and death rates (relative to those used in this model). Finally, the geographic dispersal of some Pacific Island countries and areas may mean that spread within the country is much slower than the 8 weeks used in this model. This would reduce the peak demand on health services. Possible Roles of WHO, SPC, and Donor Nations and AgenciesAgencies such as WHO and SPC play valuable roles in improving influenza surveillance, which may facilitate the control of pandemic influenza. Donor nations and agencies can potentially contribute to enhancing regional surveillance efforts by sharing their experience with developing national influenza pandemic plans and by conducting pandemic planning exercises (New Zealand has experience with both of these [15,16]). Donor support for increasing the size of the health workforce could assist not only at a time of a pandemic but also when dealing with the current threats to health. Such support could include additional funds for health workforce training and placing and retaining staff in areas of greatest need (e.g., remote locations). Reducing the burden of chronic disease (e.g., donor support for enhanced tobacco control) may also reduce the proportion of the population at increased risk of adverse sequelae from infection with influenza. Investment by donors in expanding current hospital bed capacity in Pacific Island countries and areas may be of value. However, this is likely a lower priority than strengthening primary healthcare services in Pacific Island countries and areas and making plans to mobilize effective community care. Funding for antiviral drugs and influenza vaccine (when available) to healthcare workers and high-risk groups could also be considered. Appendix References
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This page posted January
21, 2005 |
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Emerging
Infectious Diseases Journal |
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