The study, led by University of Sao Paulo in Brazil, detected the presence of antibodies against dengue virus and SARS-CoV-2 in blood samples from 1,285 inhabitants of Mancio Lima, a small town in the state of Acre, part of Brazil's Amazon region.

"Our results show that the populations most exposed to dengue, possibly owing to socio-demographic factors, are precisely those that most risk falling very sick if they're infected by SARS-CoV-2. This is an example of what has been called a syndemic (synergic interaction between two epidemic diseases so that one exacerbates the effects of the other)," principal investigator Marcelo Urbano Ferreira, Professor at the varsity's Biomedical Sciences Institute (ICB-USP) told the Sao Paulo Research Foundation.

"On one hand, Covid-19 has hindered efforts to control dengue. On the other hand, the latter appears to increase the risk for those who contract the former," Ferreira added.

There may be a biological basis for the phenomenon, in the sense that antibodies against dengue virus somehow exacerbate Covid-19, or it may simply be due to socio-demographic factors that make certain population groups more vulnerable to both diseases for various reasons, the researcher reported in the journal Clinical Infectious Diseases.

"The results evidence the importance of reinforcing both the social distancing measures introduced to contain the spread of SARS-CoV-2 and efforts to control the dengue vector, as the two epidemics are occurring at the same time and affecting the same vulnerable population. This should be getting more attention from the federal government," Ferreira said.

The blood samples analysed for the study had been collected in November 2019 and November 2020. They were submitted to tests capable of detecting antibodies against all four dengue serotypes and against SARS-CoV-2.

The results showed that 37 per cent of the people had contracted dengue before November 2019 and 35 per cent had been infected by coronavirus before November 2020. Clinical data (symptoms and outcomes) of the volunteers diagnosed with Covid-19 were also analysed.