Sexual transmission causes a marked increase in the incidence of Zika in women in Rio de Janeiro, Brazil.
The recent emergence of Zika in Brazil and its association with increased congenital malformation rates has raised concerns over its impact on the birth rates in the country. Using data on the incidence of Zika in 2015-2016 and dengue in 2013 and 2015-16 for the city of Rio de Janeiro (pop: 6.4 million), we document a massive increase of Zika in women compared to men. Even after correcting for the bias due to the systematic testing of pregnant women for Zika, there are 90% more registered cases per 100,000 women in the sexually active age group (15-65 years) than for men but not before 15 or after 65. Assuming that infected men transmit the disease to women in their semen but that the converse is not true, some extra incidence in women is to be expected. An alternate hypothesis would be that women visit doctors more often than men. To test this, we compared the incidence of dengue fever in men and women in 2015 and in 2013 (before Zika reached Rio de Janeiro): in both years, women are 30% more likely to be reported with dengue.\n\nSumming up, women in the sexually active age bracket are far more likely to get Zika than men (+90% increase); sexual transmission is the most probable cause. Women in the 15-65 age group are also 30% more likely to be reported with dengue than men, which is probably due to women being more careful with their health.