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COVID-19's impact on dengue transmission

A recent study has demonstrated that physical distancing measures resulted in a significant increase in reported cases of dengue in Thailand but unchanged rates in Malaysia and Singapore.

Dengue is transmitted to humans through the bite of certain species of Aedes mosquitos that carry one of the virus’s four serotypes: dengue viruses 1–4. These mosquitos mostly inhabit subtropical and tropical places worldwide, including some hot, humid parts of the United States.

Globally, dengue is commonplace in more than 100 countries, with 40% of the world’s population living in at-risk areas. Outbreaks of the disease are a significant health problem in the Caribbean, Africa, the Pacific Islands, Southeast Asia, the Americas, and the Middle East.

Approximately 400 million individuals worldwide contract dengue each year. About 1 in 4 people develop a resulting illness, with 22,000 individuals dying from severe disease anually.

Southeast Asia and the Western Pacific region are disproportionately burdened by dengue, with 75% of the cases arising in these countries.

In Southeast Asia, all four serotypes of the dengue virus are in active circulation, causing multiple outbreaks, which can be persistent, each year. In Malaysia, Thailand, and Singapore, the activity of all four serotypes results in substantial increases in dengue cases.

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Dengue symptoms

With an initial infection, a person usually either has no symptoms or mild ones. Subsequent infections with a different serotype increase the risk of severe disease.

Severe dengue is a medical emergency and can cause internal bleeding, shock, and sometimes death.

Mild cases of dengue cause a fever accompanied by a rash, nausea, and sometimes vomiting. Often, there is pain in the muscles, joints, bones, and behind the eyes.

Warning signs of severe dengue usually begin 24–48 hours after the fever resolves. People with any of the following symptoms require immediate medical attention:

• abdominal tenderness and pain
• bleeding from the gums or nose
• vomiting three or more times in 24 hours
• agitation, irritability, or fatigue
• blood in vomit or stool

There is no cure for dengue, and treatment only addresses the symptoms. In most individuals, dengue symptoms last 2–7 days.

While there is a vaccine for dengue, the varying rates of antibodies among different populations pose a challenge to vaccination efforts.

Dengue is primarily controlled through mosquito management. This involves tracking mosquito populations, removing areas of standing water where they lay eggs, using pesticides to kill larvae and adults, and introducing mosquito predators.

Another option involves introducing mosquitoes that carry Wolbachia, a type of bacteria that blocks the insects’ ability to carry harmful viruses such as dengue.

Singapore, Malaysia, and Thailand focus on mosquito control in primarily residential areas, where the risk of exposure is highest. These countries also target high-density urban areas with the greatest incidences of dengue cases.

COVID-19 measures

During the ongoing COVID-19 pandemic, countries worldwide have implemented physical distancing measures to reduce the spread of SARS-CoV-2, the virus responsible for the disease.

Government agencies strictly enforced near-complete lockdowns in Singapore, Malaysia, and Thailand for 2–3 months before gradually loosening physical distancing measures.

The effects of reduced mobility and having fewer people in workplaces on dengue transmission are unknown. For this reason, lead study author Jue Tao Lim, Ph.D., and colleagues at the National University of Singapore conducted a study to measure the impact on dengue virus case numbers in Thailand, Malaysia, and Singapore.

The study, which appears in PLOS Neglected Tropical Diseases, examined dengue case counts from national surveillance data and data from the World Health Organization’s Institutional Repository for Information Sharing. The data run from 2010 for Thailand and Malaysia and from 2012 for Singapore through to mid-2020.

The team also factored in the start and end dates of physical distancing measures and their strictness, using information from local and national government news websites.

In addition, the study incorporated population census data reported from 2010 to 2020 for Thailand and from 2010 to 2019 for Singapore and Malaysia using national data sources.

The researchers also collected climate data, such as total precipitation, humidity, and dewpoint temperatures, from the European Centre for Medium-Range Weather Forecasts’ ERA5 data set. They also took into account respective monsoon and wet and dry seasons.

The scientists adjusted for the effects of climate and seasonality on the mosquito populations, dengue transmission trends, and country-specific geographic patterns that may have confounded the results to ensure a robust identification of the impact of physical distancing.

What was the effect?

In 2019, the average number of reported dengue cases per 10,000 individuals was 1.15, 2.15, and 0.31 in Thailand, Singapore, and Malaysia, respectively.

After controlling for time trend and climate seasonality, the number of dengue cases increased significantly — by 0.431 cases per 10,000 individuals per province in Thailand after the introduction of physical distancing.

The expected absolute increase in the average number of dengue cases each month in each province of Thailand was 26.1, with the highest average number of additional cases, in Bangkok, being 171 and the lowest, in Samut Sakhon, a city in the Bangkok Metropolitan Region, being 5.8. Unlike in Thailand, physical distancing measures did not significantly affect the number of dengue cases in Singapore or Malaysia.

Although transmission in workplaces is possible, dengue predominantly transmits through exposure in the home. There is a greater risk of contracting dengue in urban and rural areas with low-rise residential buildings and more drainage networks.

The researchers explained, “Across country disparities in social distancing policy, effects on reported dengue cases are reasoned to be driven by differences in workplace-residence structure, with an increase in transmission risk of arboviruses from social distancing primarily through heightened exposure to vectors in elevated time spent at residences.”

Limitations of the study include the possibility of underreporting cases due to fear of leaving one’s home to seek medical treatment and the burden of the COVID-19 pandemic on healthcare systems.

In countries with a high risk of dengue, it is a leading cause of disease and can have devastating consequences. As the team observed:

“[This demonstrates] the need to understand the effects of location on dengue transmission risk under novel population mixing conditions, such as those under social distancing policies.”

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