In the spring of 2016, there was a frenzy over the threat of Zika virus at Brazil's Olympic Games. As infections reached their peak, a group of scientists called for the games to be moved somewhere else. A number of athletes, worried about sexually transmitting the virus to pregnant partners, chose to stay home.
But a group of researchers with University of Utah and the United States Olympic Committee announced Saturday that they weren't able to find any evidence that U.S. Olympians, Paralympians or staff got Zika virus at all.
The group took blood samples from 457 athletes and staff before and after they traveled to the Olympics in Rio de Janeiro. They tested the blood for antibodies for Zika virus, along with other viruses carried by mosquitoes in the area: chikungunya, West Nile virus and dengue. Antibodies show that a person's immune system has fought off a virus.
Out of those 457 people tested, 32 people came back with antibodies to mosquito-borne viruses they hadn't had in their blood before: Three for chikungunya, two for dengue and 27 for West Nile virus.
"Because everyone was concentrating on Zika, we were all surprised that we detected other [mosquito-borne] viruses," says Dr. Krow Ampofo, a specialist in pediatric infectious diseases with the University of Utah who worked on the study. He presented the findings at a conference Saturday.
But when it comes to which viruses the athletes and staffers actually got, the results can sometimes be hazy. The viruses are so closely related that antibodies for one can look a lot like antibodies for another, yielding false positives and sometimes false negatives.
But, says Dr. Benjamin Pinsky, a clinical pathologist at Stanford University Medical Center, the results are "pretty good evidence" that the athletes and staff weren't exposed to Zika virus.
Usually, he says, the problem with the Zika virus antibody test is that it yields false positives, not false negatives. And it makes sense when you look at the chart of known Zika cases over time — by the time the Olympics started, the peak of the epidemic had passed.
For Pinsky, the takeaway was that despite all the precautions people took to protect themselves against Zika, people still got bitten by mosquitoes.
"Because of all the concern about Zika, most folks did use mosquito repellent. However, that was not good enough," he says. "That was my take-home. There was still exposure. And had Zika been more prevalent, there would have likely been many more infections."
Amir Attaran, a professor of public health and law at the University of Ottawa, is less convinced by the study's results. He was one of the scientists who wrote a letter calling on the Olympics to be moved somewhere else.
"Their results are conclusive for the following proposition: During the Olympics, people got exposed to new infections," he says. But because antibodies against the virus can be so similar, he says, "we're unable to say which ones."
But the fact that athletes and staff got viruses at all, he says, is something to think about.
After all, they were probably the most protected from mosquitoes of almost anyone at the Rio Games. They slept in air-conditioned rooms with screens on the windows, in areas that had been extensively sprayed with insecticides before the Olympics and Paralympics started. And they had been advised to wear long sleeves, to use repellent and to not spend time outside at dawn and dusk, when the mosquitoes that carry the viruses are most active.
"If you think about it, the athletes are likely the least affected. They and their support crew stayed in the Olympic village — and that's as good as mosquito control got," he says. "So, if you're picking up in this study 32 cases of some mosquito-borne illness among the most coddled group during the Olympics, what does that tell you about the rest?"
At the very least, it shows that mosquitoes are cunning little beasts.