Higher incidence of Zika in adult women than adult men in Rio de Janeiro suggests a significant contribution of sexual transmission from men to women.
OBJECTIVE
The recent emergence of Zika in Brazil and its association with an increased rate of congenital malformations has raised concerns over its impact on the birth rate in the country. Using data on the incidence of Zika in 2015-2016 and dengue in 2013 and 2015-2016 for the city of Rio de Janeiro (population 6.4 million), a massive increase of Zika in women compared to men was documented.


METHODS
The age-adjusted incidence was compared between men and women. A negative binomial Poisson generalized linear model was fitted to the Zika incidence data to determine the significance of sexual transmission statistically.


RESULTS
Even after correcting for the bias due to the systematic testing of pregnant women for Zika, there were found to be 90% more registered cases per 100000 women than men in the sexually active age group (15-65 years); this was not the case for age groups <15 years and >65 years. 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, the incidence of dengue fever was compared in men and women in 2015 and in 2013 (before Zika reached Rio de Janeiro): in both years, women were 30% more likely to be reported with dengue.


CONCLUSIONS
Women in the sexually active age group are far more likely to get Zika than men (+90% increase); sexual transmission is the most probable cause. Women in the 15-65 years 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.
Fundação Getulio Vargas, Praia de Botafogo, 190, Rio de Janeiro, Brazil. Electronic address: fccoelho@fgv.br.
Flavio Codeço
FC
Coelho
Secretaria Municipal de Saude, Rio de Janeiro, Brazil.
Betina
B
Durovni
Secretaria Municipal de Saude, Rio de Janeiro, Brazil.
Valeria
V
Saraceni
Secretaria Municipal de Saude, Rio de Janeiro, Brazil.
Cristina
C
Lemos
Programa de Computação Científica, Fundação Oswaldo Cruz, Rio de Janeiro, Brazil.
Claudia Torres
CT
Codeco
Fundação Getulio Vargas, Praia de Botafogo, 190, Rio de Janeiro, Brazil.
Sabrina
S
Camargo
Programa de Computação Científica, Fundação Oswaldo Cruz, Rio de Janeiro, Brazil; Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, UK.
Luiz Max
LM
de Carvalho
Programa de Computação Científica, Fundação Oswaldo Cruz, Rio de Janeiro, Brazil.
Leonardo
L
Bastos
Secretaria Municipal de Saude, Rio de Janeiro, Brazil.
Denise
D
Arduini
Programa de Computação Científica, Fundação Oswaldo Cruz, Rio de Janeiro, Brazil.
Daniel A M
DA
Villela
Fundação Getulio Vargas, Praia de Botafogo, 190, Rio de Janeiro, Brazil; MINES Paristech, PSL Research University, CERNA Centre for Industrial Economy, i3, CNRS UMR 9217, Paris, France.
Margaret
M
Armstrong
D000293:Adolescent; D000328:Adult; D000368:Aged; D001938:Brazil / Q000453:epidemiology; D005260:Female; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D011247:Pregnancy; D012737:Sex Factors; D015229:Sexually Transmitted Diseases, Viral / Q000453:epidemiology* / Q000635:transmission / Q000821:virology; D055815:Young Adult; D000071244:Zika Virus / Q000302:isolation & purification* / Q000502:physiology; D000071243:Zika Virus Infection / Q000453:epidemiology* / Q000635:transmission / Q000821:virology
Age-adjusted incidence; Dengue; Sexual transmission; Zika