vespignani computer

Underreporting of Zika is rife; researchers project epidemic’s spread

by Thea Singer

With the report from Florida Gov. Rick Scott on Monday that 14 people in the state have been infected with the Zika virus most likely through mos­quito trans­mis­sion, the con­cern about out­breaks in the U.S. has intensified.

The news comes on the heels of new research by North­eastern pro­fessor Alessandro Vespig­nani that can help coun­tries in the Amer­icas plan a response.

The new study, along with inter­ac­tive maps, pro­vides cur­rent num­bers as well pro­jec­tions for the number of Zika cases in the Amer­icas through Jan­uary 2017. It also pro­vides pro­jec­tions for the number of micro­cephaly cases asso­ci­ated with the dis­ease through October 2017, a date chosen to allow for the nine months of preg­nancy. Micro­cephaly is a serious neu­ro­log­ical birth defect char­ac­ter­ized by a smaller than normal head.

The research is a col­lab­o­ra­tion over­seen by the Center for Infer­ence and Dynamics of Infec­tious Dis­eases, a Models of Infec­tious Dis­ease Agent Study Center of Excel­lence funded by the National Insti­tutes of Health.

Tack­ling Zika has been “a call to arms,” says Vespig­nani. “We’ve been working on the mod­eling around the clock since Jan­uary,” adds Matteo Chi­nazzi a post­doc­toral research asso­ciate in Vespignani’s lab­o­ra­tory for the Mod­eling of Bio­log­ical and Socio-​​Technical Sys­tems, or MOBS, and a coau­thor of the study.

The team of 14 researchers uses large-​​scale com­pu­ta­tional epi­demic models that inte­grate socio-​​demographic and travel data of target pop­u­la­tions along with sim­u­la­tions of infec­tion trans­mis­sion among mil­lions of indi­vid­uals to recon­struct dis­ease spread in the past and project it into the future.

Number of reported cases may be just the tip of the iceberg

Under-​​reporting is rife in affected coun­tries because up to 80 per­cent of people with the dis­ease are asymp­to­matic, says Vespig­nani, Stern­berg Dis­tin­guished Pro­fessor of Physics and director of the Net­work Sci­ence Insti­tute at North­eastern. “Even of those with symp­toms, prob­ably only one-​​third will go to the doctor and get diag­nosed,” he says.

Indeed, the number of travel-​​associated cases of Zika in the U.S. reported by the Cen­ters for Dis­ease Con­trol and Pre­ven­tion may be just the tip of the ice­berg, according to the research.

The team, half of which is at North­eastern, pro­jected that as of June 15 there were close to 30,000 cases of travel-​​related Zika in the U.S., a number 25 times greater than that reported by the CDC on the same date.

The dis­crep­ancy results from the dif­fer­ence between reported cases of the mosquito-​​borne virus—those actu­ally diag­nosed and reported to the CDC’s sur­veil­lance system—and those that fly under the radar but that the researchers’ mod­eling algo­rithms can project.

We don’t project very large out­breaks in the con­ti­nental U.S.,” says Vespig­nani, whose lab has been run­ning the sim­u­la­tions of infec­tion transmission—a job that requires using some 30,000 proces­sors at once. “But there is a cer­tain set of coun­tries in the Amer­icas that has the right mos­qui­toes, the right weather, and the right socioe­co­nomic con­di­tions for major out­breaks.” Those con­di­tions include lack of air con­di­tioning, poor san­i­ta­tion, and little access to edu­ca­tion, for example, instruc­tion on pre­ven­ta­tive mea­sures such as removal of standing water, which attracts mosquitoes.

Among those coun­tries are Brazil, with 15% of the pop­u­la­tion affected by the virus; Colombia, with 8%, and Puerto Rico with 10%. Puerto Rico is being par­tic­u­larly hard hit right now. “That’s because Puerto Rico is entering mos­quito season,” says Qian Zhang, a post­doc­toral research asso­ciate in MOBS and a coau­thor of the study. “The weather con­di­tions, including tem­per­a­ture and humidity, are now favor­able for the Zika spread.”

Impact of Olympics-​​related travel on Zika spread “extremely small”

Still, the risk of con­tracting Zika as a result of the 2016 Olympics in Rio de Janeiro is extremely small, says Vespig­nani. That’s because the increase in air travel from Zika-​​affected areas will be minimal—less than 1 per­cent. The number of cases in Brazil, where the virus sur­faced between August 2013 and April 2014, reached its peak in the first half of 2015 and has been declining since, affecting close to 10 to 15 per­cent of the population.

The number of people trav­eling with Zika all over the world has already been huge,” says Vespig­nani. “And Rio is not very much affected at the moment. So the half-​​million people who will travel there for the Games are just a small per­tur­ba­tion in the entire pic­ture of the virus’ spread.”

Dif­fi­culty of the endeavor

Pro­jecting the spread of Zika has been much more dif­fi­cult than doing so for Ebola or the flu, says Vespig­nani, who has mapped both. That’s because the dis­ease is pri­marily trans­mitted not from person to person but from mos­qui­toes to people, most often the species Aedes aegypti but also Aedes albopictus,both of which carry the dengue and yellow fever viruses as well.

Thus data on human mobility, socio-​​demographics, and tem­per­a­ture changes—the bread and butter of epi­demic modeling—must be com­pounded with data on the mos­qui­toes, much of which is uncer­tain, such as their travel pat­terns, abun­dance, and life­cycle depending on tem­per­a­ture. “Unfor­tu­nately, mos­qui­toes do not have a GPS attached to them,” says Ana Pastore-​​Piontti, also a post­doc­toral research asso­ciate on the MOBS team who has also worked with Vespig­nani on past dis­ease threats such as the Ebola epidemic.

In addi­tion, rel­a­tively little is known about Zika itself, for example, pre­cisely when and where the virus arrived in Brazil, the length of the incu­ba­tion period in humans and mos­qui­toes, and whether humans can develop immu­nity to the virus.

Indeed, with no data avail­able specif­i­cally on the rela­tion­ship between Zika and its host mos­qui­toes, the researchers had to rely on the his­tor­ical lit­er­a­ture on other mosquito-​​borne dis­eases including dengue, malaria, and chikun­gunya. “But that means that we are making a lot of assump­tions that Zika is close to dengue, for example,” says Kaiyuan Sun, PhD’19. He and Dina Mistry, PhD’18, are also coau­thors of the Zika study.

Given all the uncer­tain­ties, the researchers cau­tion that their find­ings are “pro­jec­tions,” rather than “fore­casts.” “We use ‘fore­cast’ when we have a level of con­fi­dence in past data—such as the origin of the dis­ease and the pro­gres­sion of outbreaks—that allows us, even with some fluc­tu­a­tions, to project into the future,” says Vespig­nani. “With Zika we are saying, ‘These are the sce­narios based on a number of assump­tions and an attempt to get some plau­sible path for the future.’”

This study, for the first time, reveals that path. “The mod­eling results should be inter­preted cau­tiously but the frame­work emerging from them is cru­cial for the inter­pre­ta­tion of the data that we will con­tinue to col­lect,” says Vespig­nani. “They pro­vide a base­line for the under­standing of the mag­ni­tude and timing of the epi­demic and can help us plan a response to out­breaks in the Americas.”

Originally published in [email protected] on August 2, 2016.

College of Science