Former dengue patients who contract Covid-19 could experience more severe symptoms, a small study has suggested, raising concerns that countries in Asia, where mosquito-borne diseases are endemic, could face a "double burden" at a time health systems are stretched thin by the coronavirus pandemic.
The study, published in May by the University of Oxford's peer-reviewed journal Clinical Infectious Diseases, analysed blood samples from 1,285 people in the small town of MAncio Lima in Brazil 's Amazon region between 2018 and 2020. It found that coronavirus patients who previously had dengue were twice as likely to display Covid-19 symptoms.
Lead researcher, Dr Marcelo Urbano Ferreira, said the research team found no association between a prior dengue infection and the risk of getting infected with the novel coronavirus during the first Covid-19 wave in Brazil.
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While those who had dengue before were at increased risk of presenting Covid-19 related symptoms, they also showed more severe symptoms on average, Ferreira said. But the latter hypothesis was not directly tested in the study.
Still, the result suggested that "sequential dengue and Covid-19 epidemics may impose an extra burden of disease to affected communities in the tropical and subtropical world", the study said.
Dr Tikki Pangestu, an infectious diseases expert at the National University of Singapore (NUS), said while he had not seen any evidence of a link between dengue and Covid-19 symptoms as suggested in the Brazil study, the findings could be significant "if it's proven to be so in other countries endemic for dengue, not just in Brazil".
"Asian countries should do similar studies to see if this association also exists in Asia," Pangestu said.
Dengue thrives in crowded tropical areas and is spread by mosquitoes that breed in stagnant pools of water. Rapid urbanisation in parts of Asia and Latin America - coupled with poor sewer and waste infrastructure systems - have worsened its spread, and it is estimated that there are 390 million dengue virus infections annually.
According to the World Health Organization, about 129 countries face the risk of dengue infection, with those in Southeast Asia accounting for more than half of global cases, especially Malaysia, the Philippines and Vietnam.
In 2019, the worst dengue outbreak on record, the region recorded 658,301 cases, figures from the WHO showed. Of these, the Philippines recorded 359,605 cases, Malaysia reported 106,660 cases, while Vietnam saw 108,927 cases.
Data for 2020 is still being compiled. As of October last year, the Philippines had 69,185 dengue cases, Malaysia had 80,590 cases, and Vietnam had 70,585 cases.
Scott O'Neill, founder and director at the World Mosquito Program, a global not-for-profit initiative, said the Brazil study's results were "intriguing, but not sufficient at present" to suggest a causal relationship between dengue and Covid-19.
"I doubt there is a biological or mechanistic cause, but I wonder if there is something in the social or economic conditions," he said, suggesting poverty might increase exposure to both dengue and Covid-19.
"Most dengue transmission occurs around the house. With Covid-19 lockdowns keeping (people) confined at home, you might expect to see more dengue."
Dr Raman Velayudhan, a scientist at the WHO's Department of Control of Neglected Tropical Diseases, said people with dengue or Covid-19 could display similar symptoms, such as fever, headache and muscle pains, which could complicate diagnosis and case management.
He encouraged Asean countries to double down on efforts to cut the population of mosquitoes before the rainy season, which typically starts in June and ends around October.
Pangestu, from the National University of Singapore, said countries should also focus on dengue vaccination to prevent "future crises, as they can be faced with a 'double burden' of both dengue and Sars-CoV2 outbreaks".
But dengue vaccination in the region might not go smoothly. The Philippines has battled vaccine hesitancy since a vaccine scare a few years ago involving the French-made Dengvaxia shot.
In mid-2016, the Philippines rolled out a school-based dengue immunisation programme that targeted some 1 million nine-year-old public school children.
About two years after that, Dengvaxia's manufacturer Sanofi Pasteur revealed that its vaccine could expose inoculated people to a higher risk of more severe dengue and hospitalisation, as compared with those who were unvaccinated. At this point, more than 800,000 children had been inoculated.
That incident prompted mass panic and contributed to high levels of distrust in public vaccinations. To date, less than 4 per cent of the Philippines' 108 million people has received at least one dose of a Covid-19 vaccine shot, even though it has the second-highest number of coronavirus cases in Southeast Asia.
But a 2019 article published in the peer-reviewed Nature journal, which looked at global immunisation, showed there was a gradual increase in Dengvaxia confidence after the scandal died down in the Philippines, suggesting vaccine hesitancy could be reversed.
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Dr Harapan Harapan, an infectious disease expert at Universitas Syiah Kuala's School of Medicine in Indonesia, said at the moment, the country had no national effort for dengue inoculation.
The available vaccine options included Japan 's Takeda, which is still in phase 3 trials, and the Indonesian government would review the shots before including them in the national list of medicine recommendations, Harapan said.
"In Indonesia, it is not compulsory to get dengue vaccines and the government has little funding or subsidies for these vaccines," he said. "Getting the Covid-19 vaccine is not mandatory either, but there is a push at different (governmental) levels to get civil servants to get vaccinated."
He added that there was no available data yet on the effect that Covid-19 and dengue vaccine doses have on each other.
This article originally appeared on the South China Morning Post (www.scmp.com), the leading news media reporting on China and Asia.