A new study shows that COVID-19 restrictions significantly reduced cases of dengue fever globally in 2020.
Dengue fever is a viral infection spread by mosquitos and can be contracted in Asia, the Americas or the Caribbean. The infection is usually mild and passes after about one week without causing any lasting problems. Symptoms of dengue fever include severe headache, joint pain and a high temperature. It is one of the only infectious diseases to show a sustained increase in cases each year, and the World Health Organization (WHO) now estimates that around half the world’s population is at risk of contracting dengue.
Researchers from the London School of Hygiene & Tropical Medicine (LSHTM), Beijing Normal University and other international partners, funded by the Medical Research Council, analysed the monthly dengue fever cases from the WHO weekly reports between 2014 to 2020 from 23 countries – 16 in Latin America and seven in Southeast Asia. These are the main regions where dengue fever is endemic along with climate data on air temperature, relative humidity and precipitation.
The new study was published in Lancet Infectious Diseases.
The new study found nearly three-quarters of a million fewer global cases of dengue fever occurred in 2020, which could be linked to COVID-19 disruptions limiting human movements and contact.
The researchers found a strong association between school closures and declines in non-residential trips, such as shopping or using public transport, due to COVID-19 and reduced risk of dengue fever transmission. These findings indicate that places such as schools and commonly visited public areas could be high-risk for dengue fever transmission and play a key role in spreading the disease.
It would be beneficial for decision-makers to further research into how human movement behaviours impact the dengue fever transmission rates. Measures to mitigate dengue fever could include contact tracing, testing or quarantine.
Dr Oliver Brady, Associate Professor and MRC Fellow at LSHTM and study senior author, said: “Currently, 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.”
The fall in case numbers
The researchers found that dengue fever case numbers began to decline in April 2020 in many countries, following the introduction of public health and social measures targeting the spread of COVID-19 and the resulting change in human movement and shift to more time spent in residential places. In 2020, dengue cases decreased by 40.2% in Latin America and 58.4% in Southeast Asia, with just over two million cases recorded in the Americas and Southeast Asia in 2020.
However, in 2019, the largest global outbreak of dengue fever occurred with over 5.2 million cases recorded; therefore, this led to high levels of immunity which consequently would reduce case numbers in 2020.
Dr Brady added: “Before this study, we didn’t know whether COVID-19 disruption could increase or decrease the global burden of dengue. While we could assume a reduction in the 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.”
The research team combined all data on COVID-19 disruptions and trends and found that reduced time spent in public areas was closely linked to a reduced dengue fever risk.
Nine out of 11 countries in Central America, the Caribbean and the Philippines saw a complete suppression of their 2020 dengue season, with other countries experiencing a much-reduced season. In countries where the COVID-19 restriction measures began at the peak of dengue season, there was a sharper than expected decline in cases, despite above-average incidence being recorded earlier in the year.
However, Peru and Singapore reported worse than average dengue incidence in 2020. This could have occurred due to the unpredictable natural year-to-year variation in dengue fever incidence that occurs due to, for example, the emergence of different dengue virus variants, or could hint at the greater role being bitten by mosquitoes at home plays in spreading dengue in these countries.
As the climate recorded in 2020 was similar to the average climate of the last six years, the researchers did not find an association between climate and the reduction in dengue risk during 2020.
The researchers noted that it remains to be seen how many of the estimated 0.72 million cases were truly averted, or just delayed until later years as human movement returns to pre-COVID levels and said it is key to continue monitoring dengue trends in 2022 and beyond.
The researchers acknowledged the limitations of this study, including the lack of data on the different types of dengue, which can drive outbreaks, and the potential changes in dengue reporting resulting from COVID-19 disruptions.