Disruptions related to the COVID-19 pandemic led to steep declines in cases of dengue in endemic regions, with more than 723,000 cases averted in 2020, researchers reported in Lancet Infectious Diseases.

Oliver J. Brady, DPhil, associate professor and fellow at the London School of Hygiene & Tropical Medicine, and colleagues pooled data on monthly dengue incidence from WHO weekly reports, climatic data and population variables for 23 countries — 16 in Latin America and seven in Southeast Asia — from 2014 to 2019. The 23 countries reported at least 2,000 cases of dengue per year.

The researchers compared model predictions with reported 2020 dengue data and assessed whether there were differences in projected incidence from March 2020 through the end of the year. They observed differences following pandemic measures, such as lockdowns and social distancing.

Nineteen of the 23 countries reported a lower dengue incidence in 2020 compared with the average incidence rates from 2014 to 2019. Compared with 2019, there was a 44.1% decrease in incidence across all areas of the study — 2.28 million cases in 2020 compared with 4.08 million cases in 2019.

Specifically, in Latin America, there was a 40.2% decrease in cases — 569.26 per 100,000 cases in 2019 to 340.33 per 100,000 in 2020. Likewise, in Southeast Asia, there was a 58.4% decrease — 297.31 per 100,000 cases in 2019 to 123.58 per 100,000 in 2020.

“Before this study, we didn’t know whether COVID-19 disruption could increase or decrease the global burden of dengue,” Brady said in a statement. “While we could assume reduction in human movement would reduce the virus transmission, it would also disrupt the mosquito control measures already in place. This disruption may result in long-term impacts on dengue cases, which might not be evident until the next epidemic.”

According to the authors, the decrease in cases was even more pronounced among the 19 countries beginning in April 2020, following the implementation of COVID-19 mitigation strategies. Nine countries experienced complete suppression of the 2020 dengue season, the authors wrote, whereas most other countries experienced highly suppressed seasons.

“These abnormal declines coincide with the introduction of public health and social measures and the subsequent shift of human movement behaviors toward time spent in residential premises in late March to April,” the authors wrote.

Assessing other potential variables, the researchers reported that the climate in 2020 was similar to the average climate in the previous 6 years. There was also no evidence that the decline in cases was due to underreporting.

The authors compared observed and predicted dengue cases between April and December 2020 and estimated that there were 0.72 million (95% CI, 0.12-1.47) fewer dengue cases— a 35% (95% CI, 9-56) decrease potentially attributed to pandemic-related disruptions.

School closures represented 70.95% (95% CI, 55.55-80.48) of the reduction in cases, and reductions in movement in nonresidential locations accounted for 30.95% (95% CI, 15.57-43.65), the researchers said.

“Currently dengue control efforts are focused on or around the households of people who get sick,” Brady said. “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.”