Climatic-driven seasonality of emerging dengue fever in Hanoi, Vietnam
Thi Thanh Toan Do
0
Pim Martens
Ngoc Hoat Luu
0
Pamela Wright
Marc Choisy
0
Biostatistics and Medical Informatics Department, Institute of Training for Preventive Medicine and Public Health, Hanoi Medical University
,
Hanoi
,
Vietnam
Background: Dengue fever (DF) has been emerging in Hanoi over the last decade. Both DF epidemiology and climate in Hanoi are strongly seasonal. This study aims at characterizing the seasonality of DF in Hanoi and its links to climatic variables as DF incidence increases from year to year. Methods: Clinical suspected cases of DF from the 14 central districts of Hanoi were obtained from the Ministry of Health over a 8-year period (2002-2009). Wavelet decompositions were used to characterize the main periodic cycles of DF and climatic variables as well as the mean phase angles of these cycles. Cross-wavelet spectra between DF and each climatic variables were also computed. DF reproductive ratio was calculated from Soper's formula and smoothed to highlight both its long-term trend and seasonality. Results: Temperature, rainfall, and vapor pressure show strong seasonality. DF and relative humidity show both strong seasonality and a sub-annual periodicity. DF reproductive ratio is increasing through time and displays two clear peaks per year, reflecting the sub-annual periodicity of DF incidence. Temperature, rainfall and vapor pressure lead DF incidence by a lag of 8-10 weeks, constant through time. Relative humidity leads DF by a constant lag of 18 weeks for the annual cycle and a lag decreasing from 14 to 5 weeks for the sub-annual cycle. Conclusion: Results are interpreted in terms of mosquito population dynamics and immunological interactions between the different dengue serotypes in the human compartment. Given its important population size, its strong seasonality and its dengue emergence, Hanoi offers an ideal natural experiment to test hypotheses on dengue serotypes interactions, knowledge of prime importance for vaccine development.
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Background
Dengue Fever (DF) has recently been recognized by WHO
as the fastest spreading tropical disease across all
continents. Bhatt et al. [1] estimated that the global number of
new infections per year (390 millions, 95% confidence
interval: 284528) is largely underestimated: only 96 millions
(95% confidence interval: 67136) being declared yearly.
DF is one of the many symptoms (ranging from mild fever
to hemorrhagic fever and shock syndrome) caused by one
of the four serotypes of dengue virus (Flaviridae family).
Even though recovery from dengue confers life-long
immunity against the infecting serotype, immunological
interactions between the different serotypes are not fully
understood. The virus is transmitted by bites of female
Aedes aegypti or albopictus mosquitoes in the inter-tropical
regions of the world. In absence of vaccine (under
development), mosquito control is the only available method of
prophylaxy.
In Vietnam, dengue is recognized as a major cause of
mortality and morbidity and ranks amongst the top ten
communicable diseases in terms of overall health burden
[2]. All four dengue virus serotypes have been found
circulating in Vietnam with the dominant one varying
over time. Reports from the National Institute of
Hygiene and Epidemiology show that DENV-1 and
DENV-2 have been the predominant circulating viruses
almost every year. DENV-3 emerged in the late 1990s
and was responsible for the large outbreak of 1998,
whereas DENV-4 was also detected between 1999 and
2003 [3]. Dengue transmission occurs throughout the
year in Vietnam, with peaks in the numbers of cases (72%
of total cases) reported between June and November [4].
There are regional variations in the seasonality of dengue
epidemiology in Vietnam. In the Northern and Central
Highland regions, dengue notifications are low during the
winter time from December to March, while in southern
regions, dengue transmission occurs throughout the year,
even if it sharply increases during the rainy season from
July to September. Given that dengue is a vector-born
disease and that vector population dynamics are strongly
dependent on climatic factors, the diversity of climates in
Vietnam may explain the observed diversity of dengue
epidemiological dynamics.
Hanoi, the capital of Vietnam, is located in the North
of the country and known as a low transmission setting
of DF [5]. Hanoi experiences annual seasonal dengue
outbreaks with the pinnacle of epidemics usually falling in
September/October and ending in November/December.
Over the last decade, the number of DF cases has been
increasing from year to year, reaching a peak in 2009.
According to the Ministry of Healths statistics, the
outbreak in Hanoi in 2009 is the most important outbreak
of the last decade, with 384 notified cases per 100,000
individuals. Interestingly, 2009 was also the year El Nio
increased actively [6,7]. There are only few studies
published on dengue epidemiology in Hanoi that are
based on the public hea (...truncated)