The research, led by Duke University professor Miguel Nicolelis, compared the geographic distribution of coronavirus cases with the spread of dengue over the past two years.
Places showing lower Covid-19 infection rates suffered intense dengue outbreaks in 2019 or 2020, Dr Nicolelis’s team found.
The analysis “raises the intriguing possibility” of an immunological cross-over between dengue and coronavirus antibodies, the study’s authors said.
This could mean that an effective and safe dengue vaccine could provide some level of immunity against Covid-19, they added.
Dr Nicolelis said the results were particularly interesting because previous studies have shown that people with dengue antibodies sometimes test falsely positive for coronavirus antibodies, even if they have never contracted Covid-19.
"This indicates that there is an immunological interaction between two viruses that nobody could have expected, because the two viruses are from completely different families," Dr Nicolelis said.
However, he acknowledged that further studies are needed to prove the connection.
Brazil has the world's third highest total of Covid-19 infections with more than 4.4 million cases – exceeded only by the US and India.
The team found a similar relationship between dengue outbreaks and a slower spread of coronavirus in other parts of Latin America, as well as Asia and islands in the Pacific and Indian Oceans.
Dr Nicolelis said his team came across the dengue discovery by accident.
They first made the connection during research on how highways played a major role in the distribution of cases across Brazil.
After identifying certain case-free spots on the map, the team went in search of possible explanations. A breakthrough came when the team compared the spread of dengue with that of the coronavirus.
"It was a shock. It was a total accident," Dr Nicolelis said.
"In science, that happens, you're shooting at one thing and you hit a target that you never imagined you would hit."
The study was published ahead of peer review, it will then be submitted to a scientific journal.