Almost three-quarters of a million fewer cases of dengue were registered in 2020, which researchers suspect is linked to COVID-19restrictions on people’s movements and interactions, according to a new study. For the researchers, targeting places such as schools could greatly reduce dengue transmission hot spots and play a key role in stopping the spread of the disease.

Dengue is a big cause of acute morbidity in over 120 countries worldwide, with sustained increases year on year. Countries in Southeast Asia and the Americas regions are the worst affected, with over two million cases reported there in 2020 — but as the planet continues to heat up, more and more areas become vulnerable to dengue.

The virus isn’t transmitted human to human but by the Aedes aegypti species of mosquitoes which needs hot temperatures. Hot and humid tropical climates are ideal for transmission, and cases generally peak between June and September. Symptoms typically include a high fever, headache, vomiting, muscle and joint pains, and a characteristic skin rash. Overall, dengue infects some 400 million people a year, killing 40,000.

For dengue, the COVID-19 pandemic was a unique opportunity to better understand how different environments and human mobility contribute to transmission. That’s why an international group of researchers decided to carry out the first multi-continent study of the effects of public health and social measures on dengue incidence.

“Before this study, we didn’t know whether COVID-19 disruption could increase or decrease the global burden of dengue,” Oliver Brady, study co-author, said in a statement. “While we could assume reduction in the human movement would reduce the virus transmission, it would also disrupt the mosquito control measures already in place.”

Dengue and Covid-19

Brady and a group of researchers from Beijing Normal University and the London School of Hygiene & Tropical Medicine (LSHTM) looked at monthly dengue cases between 2014 and 2020, using data from the World Health Organization (WHO). They covered 23 countries, 16 in Latin America and seven in Southeast Asia, as well as climate data such as temperature.

The researchers then looked at two measures of Covid-19 related disruption – public health and social measures (school and public transport closure and stay-at-home requirements) and human behavior through time spent at public and residential locations. They also incorporated the strength of the restrictions in lockdowns in different countries.

By combining this data, they showed that reduced time spent outdoors was linked with reduced dengue risk. Nine out of 11 countries in the Philippines, the Caribbean, and Central America had a full suppression of their dengue season in 2020, while other countries had a much-reduced season. Countries that set their pandemic restriction measures at the peak of the dengue season had a sharper decline of dengue cases.

The decrease in cases could also be linked to a lower rate of people seeking treatment for dengue, reduced availability of laboratory testing, and a higher potential of misdiagnosis, the researchers said. However, some countries like Sri Lanka predicted this could be a problem early in the pandemic and took measures, encouraging people to get diagnosed and seek treatment. Overall, this suggests that COVID-19 lockdowns also led to drops in dengue.

“Dengue control efforts are focused on or around the households of people who get sick. We now know that, in some countries, we should also be focusing measures on the locations they recently visited to reduce dengue transmission. For all the harm it has caused, this pandemic has given us an opportunity to inform new interventions and targeting strategies to prevent dengue,” Brady said.

In the long term, more routine measurement of the prevalence for dengue as well as a better understanding of how treatment-seeking behavior changes at different phases of dengue and COVID-19 epidemics will be important, the researchers wrote. That will require continued monitoring of the dengue trends in 2021 and beyond, including the collection of human mobility data.

The study was published in the journal The Lancet.