A recently published journal article , documented that prior Zika infections significantly increases risk of both symptomatic and more severe forms of dengue disease. This is the first study ever of its kind, and it relied on data from two cohorts of Nicaraguan children who lived through a Zika epidemic in 2016 and a dengue epidemic in 2019.
Since Zika is a flavivirus and a cousin of dengue, there were suspicious that antibody-dependent enhancement could be occurring, whereby antibodies to Zika can interact with dengue viruses and make the infection worse. Unfortunately, this finding impacts the ability to design an effective vaccine that protects against Zika without also increasing the risk of dengue.
Important to this is the fact that the human body produces antibodies to help the immune system fight an infection, and the antibodies’ chemical shape sticks to the pathogen of concern and flags the invader to be broken down by immune cells. In this instance, the antibody is designed to stick to Zika, but also tries to stick to dengue.
Therefore, antibodies fighting Zika virus can attach to dengue viruses, but still not be able to neutralize them because when a passing immune cell senses the antibody “flag” and tries to break down the dengue virus, it can get infected. The findings of this work can help hospitals treat people with dengue disease, with prior Zika exposure.
 Katzelnick et al., Zika virus infection enhances future risk of severe dengue disease Science 369, 1123–1128 (2020) 28 August 2020  “People with Zika Antibodies More Vulnerable to Severe Dengue Disease,” 27 August 2020, https://globalbiodefense.com/2020/08/27/people-with-zikaantibodies-more-vulnerable-to-severe-dengue-disease/ , Global Biodefense Lab