Medical Examiner

Fewer Than 100 Zika Cases Will Come From the Rio Olympics

In fact, it could be as few as six.

Aedes aegypti mosquitos are seen in a lab at the Fiocruz Institute on June 2, 2016 in Recife, Brazil.
Aedes aegypti mosquitos are seen in a lab at the Fiocruz Institute on June 2 in Recife, Brazil.

Mario Tama/Getty Images

Zika is frightening because we don’t know much about it. Uncertainty spreads fear, particularly when the rare risk is as catastrophic as microcephaly. But even as we can admit this truth, we must also learn to be calmed by emerging facts as they put the risks of illness in proper perspective.

Scientists are attempting to learn as much as they can about this disease. After much debate over whether Zika made hosting the Olympics in Rio too risky, new research from the Yale School of Public Health calculated the actual risk of the Olympics significantly adding to the global spread of the virus. That risk is extremely small, according to the study, which was published this week in the Annals of Internal Medicine. Since the projected attendance is between 350,000 to 500,000 visitors and the worst-case scenario probability of acquiring Zika in Rio de Janeiro over that period of time ranges from 1 case in 6,200 visitors to 1 in 56,300, the grand total of Zika cases that will result from the Olympics is projected to be somewhere between six and 80 infections. That is nowhere near the number needed to seed another region. In fact, of those six to 80 people projected to get Zika, the researchers concluded that in a worst-case scenario, around half, or between three and 37 people, will bring the infection back to their home country.  

Brazil has recorded close to 100,000 overall cases of Zika in 2016, with 7,000 among pregnant women. The total number of microcephaly cases in Brazil linked to Zika since the beginning of the outbreak is approximately 5,000. This is a reason for a travel advisory for pregnant or soon-to-be-pregnant women, but it is not a reason to panic or postpone or move the Olympics, especially as the number of cases is on the decline.

This is not to say that Zika should be taken lightly or the risks entirely dismissed. In fact, in the same journal, another Yale study looked at the effectiveness of pregnancy delay policies in Colombia and determined that by putting off pregnancy by nine to 24 months, women would decrease the cumulative incidence of prenatal Zika virus infections by 17 to 44 percent, respectively. That’s because Zika outbreaks tend to peak at eight months—so delaying until after that peak is the smartest way forward. This crucial information can be turned to practical guidance for women living in or traveling to Zika-infested regions. And science-backed guidelines are one of the best ways to fight fear.

Meanwhile, here in the U.S., Florida has seen the first two cases of Zika that appear to have been spread by local mosquitoes. That certainly sounds alarming, but these cases give no indication that Zika will take hold in our mosquito populations. Past experience with Dengue fever, which is far more common and involves the same mosquitoes, suggests that it won’t. We don’t have the conditions in our Southern states or the mosquito control problems Brazil or Colombia have to spark a major outbreak here. This is good news that is too frequently overlooked.

At the same time, since the start of the outbreak, 15 cases of Zika infection transmitted by sexual contact have been reported in the U.S. This includes all types of sex and transmission from man to woman as well as woman to man. Alarming, yes, but also fixable—the Centers for Disease Control and Prevention recommends wearing condoms or, in some cases, abstaining as a way to stop transmission. The predominant method of Zika transmission remains from mosquito to human. And though some common house mosquitoes have been found to carry Zika in Brazil, the vast majority of cases continues to involve the Aedes aegypti mosquito.

The problem is that each time a new wrinkle occurs—when we learn a new kind of mosquito carries Zika, realize another aspect of sexual transmission, or confirm a strong link to Guillain-Barré syndrome—the U.S. media broadcast the news as if the fundamental dangers of spreading the virus have changed. But the risk has not changed. It is just our perception of it that has. And more knowledge simply means higher likelihood that we will learn how to combat it.

Scientific discovery is essential to guiding public health. When it comes to Zika, to avoid public hysteria we have to continue to let new science contextualize the real versus imagined risk.

Read more in Slate about the Zika virus.