In a study published in The Lancet, researchers found that the number of cases of dengue fever went from 4.08 million in 2019 to 2.28 million in 2020 in these two regions, mostly owing to restrictive COVID-19 measures that closed schools and cut crowds in nonresidential areas.

Insights from the study may provide some guidance on how to combat dengue fever, which has been infecting an increasing number of people each year. The number of dengue cases reported to the World Health Organization increased more than eightfold over the past two decades, affecting mostly younger age groups.

“We found really unexpected net benefits from COVID restrictions that are going to help us better fight dengue in the future,” Oliver Brady, an epidemiologist at the London School of Hygiene and Tropical Medicine who was senior author of the study, told the New York Times.

Dengue fever, which is transmitted by the Aedes aegypti mosquito, causes high fever, muscle and joint pain, and a characteristic skin rash. In 2019, more than 5 million people were infected with the disease across the world.

Brady noted that early in the pandemic, he and other infectious disease researchers worried that resources for fighting dengue fever, like spray for mosquitoes, would be diverted to treating COVID-19. However, the severe disruption in the movement of people has resulted in “historically low” dengue incidence, the Lancet study says, with 720,000 fewer confirmed dengue fever cases during the first year of the pandemic directly attributable to COVID-related restrictions.

Moving forward

The study findings have important implications for the fight against dengue fever, which globally claims around 4,000 lives each year, according to the World Health Organization.

School closures in particular appear to have a significant impact on curbing cases of dengue. The study notes that while before COVID-19, mosquito control was mainly targeted within households, with people spraying to kill mosquitoes around homes and looking to remove standing water that they breed in, the new findings suggest transmission mainly occurs in shared spaces.

Brady said because mosquitoes appear to be transmitting the virus to humans in public places like schools or workplaces, mosquito control should be concentrated in those areas.

Cases of dengue fever may have also declined during COVID-19 restrictions because infected people were not going out and passing the virus to others.

The findings of the Lancet study may prove relevant to other mosquito-borne viruses, including Zika and chikungunya, the study suggests. But it is unclear whether the decline in cases is “truly averted or just delayed,” the study notes, and Brady says cases may rise to even higher levels once public spaces are filled and immunity levels have fallen.