Nearly three-quarters of a million fewer global cases of dengue occurred in 2020, which could be linked to COVID-19 disorders that limit human mobility and contact, according to a new study published in the Lancet Infectious Diseases.
Researchers from the London School of Hygiene & Tropical Medicine (LSHTM), Beijing Normal University and other international partners, funded by the Medical Research Council, analyzed the monthly dengue cases from the World Health Organization (WHO) weekly reports between 2014 and 2020 from 23 countries – 16 in Latin America and seven in Southeast Asia, the main regions where dengue is endemic, as well as climate data on air temperature, relative humidity and precipitation.
They found a strong association between school closures and declines in out-of-home trips, such as shopping or using public transportation, due to COVID-19 and reduced risk of dengue transmission. This indicates that places such as schools and commonly visited public areas can be hotspots for dengue transmission and play a key role in the spread of the disease.
There is a need for further research into how human locomotor behavior (the places people visit, how long they spend there and with whom) affects the risk of dengue transmission. This can help decision makers decide if measures such as contact tracing, testing or quarantine can help control the spread of the disease.
Dr. Oliver Brady, associate professor and MRC fellow at LSHTM and senior author of the study, said: “Currently, efforts to combat dengue fever are focused on or around the households of people who become ill. We now know that in some countries we should also focus “measures in the places they have recently visited to reduce dengue transmission. Despite all the damage it has caused, this pandemic has allowed us to inform about new interventions and targeted strategies to prevent dengue.”
Dengue is a viral infection that is transmitted by Aedes species of mosquitoes that produce flu-like symptoms. It is found in tropical and subtropical climates worldwide and is most common in urban areas.
It is one of the only infectious diseases that shows a sustained increase in cases each year, and the WHO now estimates that about half of the world’s population is at risk of getting dengue.
Transmission is closely linked to the weather, the surrounding environment and human mobility. It is also closely related to the climate, with hot and humid tropical climate ideal for transmission. Dengue season in many countries takes place around June to September, where peaks in cases can cause overcrowding in hospitals – just like with COVID-19.
Dengue only spreads from mosquitoes to humans and vice versa, and is not transmitted from humans to other people. However, changes in people’s movements and behavior can have an effect on infection, for example through reduced exposure to mosquitoes or fewer opportunities for infected people to go out and pass on the virus to uninfected mosquitoes present there. COVID-19 and the restrictions on human movement imposed during the pandemic therefore provide a unique opportunity to explore how human movement and behavior contribute to dengue transmission.
The number of cases of dengue fever suddenly began to decline in April 2020 in many countries following the introduction of public health and social measures aimed at the spread of COVID-19 and the resulting change in human movement and shift to more time spent in residential areas. In 2020, dengue cases fell by 40.2% in Latin America and 58.4% in Southeast Asia, with just over two million cases recorded in America and Southeast Asia in 2020.
However, it is complex to elucidate the effects of COVID-19 disruption as 2019 saw the largest global dengue outbreak in history, with more than 5.2 million cases recorded in North and South America and Southeast Asia. This led to high levels of immunity, which is also expected to reduce cases of dengue by 2020.
Dr. Brady added: “Prior to this study, we did not know whether COVID-19 disorders could increase or decrease the global burden of dengue. Although we could assume that reduction in human movement would reduce virus transmission, it would also interfere with mosquito control measures already in place. “This disruption could result in long-term consequences for dengue cases, which may not be clear until the next epidemic.”
The research team examined two different targets for COVID-19-related disorders – public health and social measures, such as school closure and public transportation, home stay requirements, collection restrictions; and human movement behavior over time spent in homes and public places. They also outlined the different strengths of restrictions in shutdowns in different countries around the world.
By combining all these data and analyzing trends, they were able to show that reduced time spent in public areas was closely linked to reduced dengue risk.
Nine out of 11 countries in Central America, the Caribbean and the Philippines experienced a complete repression of their dengue season by 2020, while other countries experienced a greatly reduced season. In countries where COVID-19 restriction measures began at the peak of the dengue season, there was a sharper-than-expected decline in cases, despite an above-average incidence being recorded earlier in the year.
This decrease in cases can also be attributed to reduced rates in people seeking treatment, a higher potential for misdiagnosis and reduced availability of laboratory tests for dengue may result in misdiagnosis. However, some countries like Sri Lanka predicted that this could be a problem at the beginning of the pandemic, so they undertook outreach work to encourage people to be diagnosed and seek treatment. Despite this, there was no change in the number of reported serious cases and deaths, suggesting that the reduction in treatment search was not the reason for the reduction in cases.
There is a need for a better understanding of how treatment-seeking behaviors change during an epidemic, as access to care and rapid diagnostics change in order to fully assess and interpret the change in case numbers. The team emphasizes the need for longer-term, more routine measurement of the incidence of dengue in each population.
Although overall cases decreased in 2020, Peru and Singapore reported a lower incidence of dengue fever than the average in 2020. This could have been due to the unpredictable natural year-to-year variation in the incidence of dengue fever, such as. occurs due to the occurrence of various dengue fever. virus variants, or could suggest the greater role of being bitten by mosquitoes at home plays in the spread of dengue in these countries.
When the recorded climate in 2020 corresponded to the average climate of the last six years, the researchers did not find a correlation between climate and the reduction in dengue risk in 2020.
Researchers note that it remains to be seen how many of the estimated 0.72 million cases were actually averted or simply delayed in recent years, as human movement returns to pre-COVID levels, saying it is the key to continuing to monitor dengue trends in 2022 and beyond.
The researchers recognize the limitations of this study, including the lack of data on the different types of dengue that can cause outbreaks and the potential changes in dengue reporting due to COVID-19 disorders.