Research Article: Incidence, Seasonality and Mortality Associated with Influenza Pneumonia in Thailand: 2005–2008

Date Published: November 11, 2009

Publisher: Public Library of Science

Author(s): James Mark Simmerman, Malinee Chittaganpitch, Jens Levy, Somrak Chantra, Susan Maloney, Timothy Uyeki, Peera Areerat, Somsak Thamthitiwat, Sonja J. Olsen, Alicia Fry, Kumnuan Ungchusak, Henry C. Baggett, Supamit Chunsuttiwat, Landon Myer.

Abstract: Data on the incidence, seasonality and mortality associated with influenza in subtropical low and middle income countries are limited. Prospective data from multiple years are needed to develop vaccine policy and treatment guidelines, and improve pandemic preparedness.

Partial Text: Influenza is a common vaccine preventable viral infection that can cause severe or fatal disease in the elderly, the very young and those with underlying illness [1], [2], [3], [4], [5]. In temperate climates of Europe and North America, wintertime seasonal influenza epidemics often result in dramatic increases in hospitalization, death and significant economic losses due to workplace absenteeism [4], [6], [7]. Much less is known about the burden of influenza morbidity and mortality in tropical and subtropical countries [8], [9]. Data from Hong Kong and Singapore suggest that influenza is an important cause of illness with rates of hospitalization and mortality comparable to that of the United States [10], [11], [12], [13]. The contribution of influenza virus infection to pneumonia, the leading cause of pediatric mortality, has also not been determined. This question is particularly important for developing countries in tropical and subtropical climates where most childhood pneumonia occurs and where influenza has historically been perceived as a mild or uncommon disease. Prospective, multi-year, population-based data on laboratory-confirmed influenza infection are needed to describe the disease burden and inform prevention and control strategies during both interpandemic and pandemic periods.

We prospectively identified all hospitalized pneumonia patients during January 2005 through December 2008 using an active, population-based surveillance system carried out through collaboration between the Thailand MOPH and the U.S. Centers for Disease Control and Prevention (CDC) in Sa Kaeo province in eastern Thailand and Nakhon Phanom province in northeast Thailand (Figure 1)[21], [22] . Sa Kaeo (pop 526,432) and Nakhon Phanom (pop 734,000) are rural provinces with age distributions that are similar to the national population [23]. The 2007 population densities of Sa Kaeo and Nakhon Phanom were 75 and 126 persons per km2 respectively, compared to 111 per km2 for all of Thailand outside of Bangkok. The same year among 76 Thai provinces, Nakhon Phanom ranked 74th with a per capita GDP of $891 while Sa Kaeo ranked 50th with a per capita GDP of $1,632 (national range $858–$30,457 USD) [24]. During 2008 the mean daily temperature in Sa Kaeo province was 26.8 degrees Celsius with 1602 mm of total rainfall. The daily mean temperature in Nakhon Phanom was 25.4 degrees Celsius and 2786 mm of rainfall was recorded. For comparison, Thailand’s mean daily temperature was 26.9 degrees and an average of 914.7 mm of rain fell during 2008 [25].

From January 2005 through December 2007, we enrolled 10,075 (50.1%) of 20,100 patients. Specifically, we enrolled 3,292 patients in 2005, 3,633 patients in 2006 and 3,150 patients in 2007. In 2008 when chest radiographs were no longer required for eligibility, 13,425 patients were eligible and 50% were invited to participate. Among 7,205 patients with a chest radiograph, 1,796 (24.9%) were enrolled and among 6,220 without a chest radiograph, 1,248 (20.1%) were enrolled (Figure 2). Overall, 1,346 (10.3%) of enrolled clinical pneumonia patients tested positive for influenza by RT-PCR or cell culture. Among the 1,346 pneumonia patients with influenza, 707 (52.5%) had results evaluated by the radiologist panel. Four hundred and eight of 670 (60.9%; 37 missing) had evidence of pneumonia, including 40 of 387(5.9%; 21 missing) with consolidation and 327 of 393 (83.2%; −15 missing) with interstitial infiltrates.

During 2005–2008, across all age groups influenza virus infection was associated with 10.4% of pneumonia cases identified in our study population. These findings extend our 2004 report in which 80 (11%) of 761 pneumonia inpatients had laboratory-confirmed influenza infection [15]. The annual adjusted incidence of hospitalized influenza pneumonia across all age groups was 83, 22, 42 and 66 per 100,000 persons in 2005, 2006, 2007 and 2008, respectively. For comparison, in 2004 we estimated an adjusted incidence of hospitalized influenza pneumonia of 111 per 100,000 across all age groups in Sa Kaeo province [15]. The Thailand MOPH passive surveillance system reported 4.8 cases of clinically diagnosed influenza in hospitalized patients per 100,000 persons in 2005, 4.7 cases per 100,000 in 2006, and 5.7 cases per 100,000 in 2007 [30]. Using U.S. National Hospital Discharge Survey and WHO laboratory data during 1979–2001, Thompson and colleagues estimated that influenza was associated with 8.6% of all primary pneumonia and influenza hospitalizations and the average annual incidence across all age groups was 36.8 per 100,000 (range; 7.8–69.5)[31].



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