In this study they evaluated whether prior contact with endemic infectious diseases such as dengue could modify the prognosis of COVID-19. They studied 2,830 subjects with laboratory-confirmed symptomatic and asymptomatic SARS-CoV-2 infection in Rio Branco, Brazil. Of 2351 participants (mean age 40.4 ± 12.7 years, 49% men), 1,177 (50%) reported a history of dengue. There was a decrease in mortality of up to 60 days among COVID-19 patients who had a history of symptomatic dengue infection. These findings raise the possibility that dengue may induce some degree of immune protection against SARS VOC-2 infection. Subjects with a history of dengue generally had more symptoms and were more likely to have a chest CT scan and receive corticosteroids despite lower mortality. It remains to be determined whether detrimental effects of immune enhancement will occur upon reinfection with COVID-19 or after vaccination.