Early-stage laboratory research, part-funded by Wellcome, suggests that the recent outbreak of Zika in South America may have been driven in part by the high prevalence of dengue in the region.
"We now need further studies to confirm these findings, and to progress towards a vaccine," said senior author Professor Gavin Screaton from Imperial College London.
Researchers from the UK, France and Thailand found that antibodies the body makes against dengue can also react to the Zika virus.
But instead of clearing the Zika virus, the antibodies instead allow it to enter cells where it can quickly replicate. Higher levels of Zika in the blood may in turn lead to increased transmission of the virus.
This phenomenon, known as antibody-dependent enhancement, is seen also seen in dengue and is thought to explain why a second dengue infection is often more serious than the first.
Dengue and Zika are very similar viruses. They are members of the same virus 'family' – called flaviviridae – and both are transmitted by the Aedes mosquito.
The research results appear in two new studies published today in Nature and Nature Immunology.