Public health measures to prevent the spread of Covid-19 had an unforeseen consequence in Latin America and Southeast Asia in 2020: dengue virus infections were averted in hundreds of thousands of people, according to a study published by the journal scientific The Lancet this month.
The research offers clues to new strategies to combat this dangerous tropical disease that infected more people each year.
The study found a sharp drop in infections from April 2020 in many regions where dengue is transmitted by mosquitoes; he estimates there were 720,000 fewer dengue cases worldwide in the first year of the pandemic, due to restrictions on movement.
“We’ve found really unexpected benefits from Covid restrictions that will help us better fight dengue in the future,” said Dr Oliver Brady, an epidemiologist at the London School of Hygiene and Tropical Medicine and lead author of the study.
More than 5 million people were infected with dengue — also known as “breakbone fever” for the severe joint and muscle pain it causes — in 2019.
Brady said that at the start of the pandemic, he and other infectious disease researchers feared disaster as resources were diverted to Covid-19, and other disease control measures, such as mosquito spraying, were halted.
The huge decline in dengue cases came as a happy surprise and left them eager to find out what could have caused it. They eliminated other potential factors, including environmental changes and the decrease in dengue reporting by public health agencies. That left only the severe disruption to the displacement of people as a plausible explanation, he said.
School closures, in particular, appear to have played a key role in reducing dengue cases. The main vector of dengue, the Aedes aegypti mosquitoes, feed during the day. Most dengue control programs focus on homes — spraying to kill mosquitoes and monitoring standing water where they can breed — assuming that’s where transmission takes place.
“But if the house was really the place of risk and the mosquitoes were just biting in the house, you would expect stay-at-home orders to increase the risk, but we just haven’t seen that in many countries,” he said.
The researchers are not suggesting that stay-at-home orders should continue, but the extraordinary circumstance allowed for an unexpected result. Brady said his findings suggest that bites happen at school or workplaces, meaning mosquito control should focus in public places.
Dengue may also have declined during stay-at-home orders because when people became infected, they didn’t go to places where new mosquitoes could bite them and then pass the virus on to other people.
The dengue findings may be relevant to other related mosquito-borne viruses, including Zika and chikungunya, the study suggests. But Brady warned that dengue data for 2021, due soon, and for a post-pandemic period, could bring bad news: infection rates could return to pre-Covid levels or worse if control programs vectors are stopped, while immunity levels may have dropped because fewer people were exposed, he said.
Translated by Luiz Roberto M. Gonçalves​
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