Commentary: Zika doesn't need Summer Games to spread

By Amesh Adalja

In the runup to the Olympics, many people considered Rio de Janeiro, Brazil, the epicenter of the current Zika virus outbreak. Zika has already affected thousands of Americans because of ordinary travel from areas in which the main vector causing the outbreak, the Aedes aegypti mosquito, harbors the virus.

Some have warned that the Olympic Games should be avoided because the mass travel associated with the event will serve as a catalyst for Zika's proliferation in the United States. The truth is that the real turning point occurred more than a year ago - long before the recent local transmission of the disease by mosquitoes in Miami - when, for one of the first times in its decades-long history, Zika appeared in a region that serves as a global crossroads.

The headline-grabbing events in Miami come as no surprise. Experts knew it was just a matter of time before the right mosquito bit the right person within the United States, raising the threat level of the outbreak significantly. There was a high likelihood that Florida would be the home of this game-changing public health event. After all, a similar phenomenon occurred in Florida's recent past with two other viruses spread by Aedes mosquitoes: chikungunya and dengue.

The stakes with Zika are different from those of the other two viruses, however, because of the predilection for Zika to cause devastating fetal harm.

If a pregnant mother contracts the disease, there is a chance her baby could be born with a condition called microcephaly, in which the head is abnormally developed and severe cognitive disability is likely. As such, these local transmission events call for rapid dissemination of guidance to pregnant women and their partners as well as women of reproductive age. The bottom line, though, is that pregnant women are the only group that should completely avoid attending the Olympics.

The local transmission in Miami exposes an important truth about emerging infectious diseases in the modern era: The belief that Zika's spread will be massively accelerated through the Summer Games in Rio is not supported by facts. This virus spreads through routine travel and does not require mass gatherings to traverse new frontiers. Our modern, globalized economy and way of life spread diseases like Zika all on their own.

Holding on to the belief that Zika would get a major global upgrade from the Olympics is understandable. There is unequivocal evidence that certain infectious diseases - such as meningococcal meningitis, norovirus, and respiratory viruses like influenza - can use mass gatherings to hitch rides around the globe after emergence.

But this is not the case with Zika because of the characteristics of the microbe involved and the travel patterns associated with this specific outbreak.

First of all, modern travel is the perfect propellant for Zika. Before, when long-distance travel was much slower and less popular, infections were delimited by travel times that made it less likely (but clearly still possible) for someone with a highly infectious disease to seed another locale. But some paths of travel are now so well-trodden that a major international sporting event is not needed for widespread viral proliferation.

Such is the case with Zika from the Caribbean and South America. Travel to these destinations is extremely common, with multiple daily trips via air from almost every major American city, not to mention Caribbean cruise ships that dock at multiple destinations. In fact, it is estimated that just 0.25 percent of travel to active local Zika transmission areas will occur because of the Olympics, while 99.75 percent occurs for other reasons.

But it is not just frequent international travel that makes Zika such an efficient traveler. The virus has a magnified impact because the majority of those infected have no symptoms, and sexual transmission can occur, presenting ample opportunities for the disease to spread quietly. Contrast that with outbreaks in which an illness often ran its course or progressed far enough that symptoms limited opportunities for contagion: A person in bed with fever is less contagious than someone going about the activities of daily living.

We are a safer and stronger society when both individuals and governments make evidence-based decisions about our health. Advising individuals to skip the Olympics to combat Zika's proliferation is not necessary or wise. The Olympics were never going to be the linchpin of Zika's ascendency because our ordinary travel habits are enough to fuel the outbreak on their own.

The extremely significant but wholly unsurprising local transmission of Zika in Florida illustrates that the turning point for this outbreak has already occurred, before the Olympic torch ever reached Rio de Janeiro.

Amesh Adalja is a a board-certified infectious disease physician at the Center for Health Security at the University of Pittsburgh. @AmeshAA