How Zika Can Spread

With the threat of Zika looming over the 2016 Olympics, athletes have to make a choice to either risk getting the disease or stay home. But how big is that risk, and how can you measure it? It may not be perfect, but scientists have come up with a rough estimate on how the Zika virus can spread in Brazil.

The basic reproduction number or R-naught is a mathematical term that can be used to calculate how likely a contagious disease will spread among a certain population. Basically, if one person is infected, the R-naught number will determine how many other people will catch the disease from that one sick person. For example, an R-naught of three means three people are likely to be infected from one person. The goal is for an infectious disease to reach an R-naught of less than one— meaning the infected person will not spread the disease to anyone.

Public health researchers have used outbreaks of similar diseases like Dengue and prevalence of the Ae. aegypti mosquitoes which can carry Zika to calculate a rough estimate of the Zika R-naught for different parts of Brazil. The resulting R-naughts are found in the graphic below. Click “Infect!” on the interactive below to see how Zika can spread with different R-naughts in different Brazilian cities.

Infect!

However, visiting any of the cities with high R-naught does not necessarily mean you will get Zika. The numbers are largely based on context. If you are in an area with a lot of mosquitoes, no screens on your house and very little access to health services, the R-naught will most likely increase regardless of where you are in Brazil. Similarly, if you’re Natal and everyone in your party is quarantined in a sterile environment, including the infected person, the R-naught can go down to less than 1. These models are just estimates but they do explain why in Brazilian cities with high R-naught have had more cases of microcephaly.

It’s also important to remember that because Zika is spread through mosquitoes and sexual contact. This keeps the R-naught for Zika much lower than a disease like the measles, which is airborne and much easier to catch.