To understand the outbreak of a disease like Zika, and ultimately to fight it, researchers must work on multiple levels. There are questions of molecules and chemical processes: how does the virus infect a cell, and what components provoke an immune response that can be harnessed for treatments and vaccines? On a larger scale, there are questions about populations. How does a pathogen discovered in Uganda spread from French Polynesia to Brazil and the Caribbean? How do families and individuals cope with infants born with complications from the disease, like microcephaly (an abnormally small head size) and neurological damage, whose lasting effects still have not been fully determined? Harvard researchers have already made strides at all of these levels, and, as North America prepares for another outbreak this summer, now seek to answer further questions.
A disease like no other
“Zika, for a mosquito-borne disease, is really like no other,” said Marcia Castro, an associate professor of demography at the T.H. Chan School of Public Health who studies vector-borne tropical diseases like Zika. Although it is a flavivirus, characterized by a single strand of RNA within a protein envelope, it appears distinct from many of its relatives, such as dengue, yellow fever, West Nile virus, and chikungunya. For one, it has been linked to higher incidence of Guillain-Barre syndrome, in which the body’s immune system attacks nerves, causing paralysis. It can also be sexually transmitted and efficiently transferred from a pregnant woman to a fetus, causing birth defects.
Furthermore, according to a team of researchers at Harvard Medical School’s Center for Virology and Vaccine Research (CVVR) at Beth Israel Deaconess Medical Center, the disease stays in the body for longer than had been previously realized. In a new paper published in the journal Cell with collaborators from Los Alamos National Laboratory, the Walter Reed Army Institute of Research, and the research services firm Bioqual, the Harvard investigators demonstrated that although the virus left most bodily fluids of rhesus monkeys within two weeks of infection, it remained in cerebrospinal fluid for up to 42 days, and in lymph nodes for up to 72 days. In other words, the virus lingers long after it has disappeared from the bloodstream.
But Zika is difficult to diagnose: in about 80 percent of cases, affected individuals show no symptoms even in the first two weeks after infection. In others, patients may report a rash, fever, or joint pain—symptoms similar to other flaviviruses like dengue or chikungunya. New genetic testing methods developed by James Collins and Feng Zhang, at the Broad Institute of MIT and Harvard, have dramatically improved Zika diagnosing accuracy by building on the gene-editing technology CRISPR to detect miniscule variations within the Zika genome. But at the beginning of an outbreak and in many clinical contexts, such methods remain unavailable.
This half-hidden spread of the disease, Castro thought, may have profound impacts on fertility and the health of mothers and families. For instance, it could push birth rates down as women decide to postpone their pregnancies in order to avoid the risk of passing Zika to a child. It could also increase fetal death due to complications from the virus, or it could affect women’s decisions to have abortions.
Castro, a native of Brazil (one of the countries most affected by the virus), looked at public health data covering millions of births over several years. What she found was surprising. In the period covering the height of the outbreak in Brazil—roughly 2015 through early 2016—fertility rates in Brazil barely budged, hovering around 1.9 children per woman, and although there was a slight uptick in fetal deaths, it was not statistically significant. “It’s not something that raises a big red flag,” she explained. Women in Zika-infected areas of Brazil were still getting pregnant at the same, admittedly low, rates as before, and contraceptive use, although high, remained at similar levels.
Something seemed to be changing with abortions, however. After the beginning of the Zika outbreak, the cross-correlation between abortions and births shifted in several Brazilian states, meaning that women were choosing to abort at different periods in their pregnancy than before the virus appeared. Although it’s still to early to tell whether there is a causal relationship, it does mean that there was a statistically relevant relationship between the onset of Zika and women’s choices around their pregnancies.
Abortions are illegal in Brazil in all but a few cases (like rape and danger to the life of the mother). Therefore, Castro is developing an estimate of abortions using a proxy: the number of patients who needed inpatient care in hospitals due to complications from abortions. As more women seek unsafe abortions outside of the legal healthcare system, she explained, the number of such complications goes up, and this result appears in public health surveys even when abortions are not recorded. Using methods like this will help clarify the link between abortions and Zika.
The molecular level
Currently, though, “vector control is pretty much the only thing we can do,” she said. A vector is a carrier of a disease; in the case of Zika, the mosquito species Aedes aegypti is responsible for the bulk of the recent outbreak. In Brazil and other areas where the disease has spread, the insect has taken advantage of poor infrastructure in fast-growing cities. Castro explained that in 2015, for example, 85 percent of Aedes-breeding habitats in Northeast Brazil, the hardest-hit region in the country, were home water containers. “Cities became really the safe haven for Aedes aegypti.”
But at a molecular level, the Center for Virology and Vaccine Research is working to develop a vaccine that will be able to stem the disease. The center added a Zika operation in 2016 as the epidemic became apparent, applying the vaccine development infrastructure it had employed for HIV to the fight against the flavivirus. “One of the interesting features about [this vaccine’s development] is its speed,” said Dan Barouch, a professor at Harvard Medical School and director of the CVVR. “It’s probably a record.” Zika was a promising candidate for a vaccine: vaccines have already been developed for other flaviviruses like yellow fever and Japanese encephalitis; various Zika strains showed little variation; and there was evidence that people exposed to the disease could eventually develop immunity.
Within six months of the start of the outbreak that began around January 2016, the journal Nature had published an article authored by the CVVR team, along with researchers from the University of São Paulo and Walter Reed, demonstrating models for a vaccine that worked in mice by introducing DNA that cells use to create proteins present in the Zika virus. Those proteins, in turn, provoked immune responses. Two other types of vaccines—a purified inactivated virus (PIV) vaccine, consisting of the entire virus that has been modified to prevent it functioning, or an adenovirus vector vaccine, which used other viruses that had been modified to express certain proteins of Zika—also provoked an immune response. A little more than a month after that, another article appeared, this time in Science, showing that rhesus monkeys were protected after receiving immunizations from each type of vaccine.
Within 10 months of starting work on a Zika vaccine, the CVVR team and its collaborators had moved to clinical trials. According to an article in Immunity co-authored by Barouch, ongoing Phase I trials are testing the safety and appropriate dosages of the vaccine for both DNA and inactivated virus versions. Along with partners’ trials at Walter Reed and St. Louis University, CVVR’s trial is focusing on the latter, which provoked a stronger immune response than the DNA alternative. Furthermore, no DNA vaccine has yet become a licensed product; using inactivated virus is generally a more traditional, dependable option.
There are still questions to be answered. Barouch seeks to understand the potential vaccine’s interactions with dengue antibodies: there is evidence that those who have been exposed to dengue may have more severe Zika symptoms, and this relationship could affect the safety and effectiveness of any treatment. Marcia Castro hopes to refine her results even further in terms of time and geography. She is also beginning research on populations beyond those with the disease, to groups such as mothers of infants with Zika. She plans to focus on the Brazilian state of Ceará to conduct interviews and research on how the outbreak has created public health problems that extend beyond the virus itself. She believes there are “a huge amount of mental health problems.”
As summer approaches, American health authorities remain concerned. Texas is expanding test recommendations for pregnant women, and Florida is testing a method of controlling Aedes aegypti populations by releasing males infected with bacteria that prevent successful mating. Areas at risk of Aedes aegypti spreading the virus are investing heavily in mosquito control, and in April a Senate panel approved $100 million more to fight Zika, although the measure still faces a vote in the full chamber. For the moment, as Castro explained in the case of Brazil, vector control may be the most effective method to fight the disease.