“The similarities are great between rubella and Zika. Both are fever-rash illnesses, and both have arthropathy, or arthritis, as manifestations,” said Susan Reef, who leads the rubella team in the CDC’s Global Immunization Division. “Also like rubella, there’s an asymptomatic component to Zika, so not everyone knows when they’re infected with rubella or with Zika.”
One of the keys to treating Zika, Reef told me, will be for doctors to develop a more complete understanding of the clinical manifestations of Zika among pregnant women. With rubella, it took the epidemic of 1964 for that to happen.
Researchers are also trying to compare the mechanisms by which Zika and rubella might affect fetuses. A New England Journal of Medicine study published in March found “many aspects” of the viruses that are similar. Both seem to cross the placenta and restrict organ development. But there are key differences, too—and some of them are alarming.
“A major difference of concern between ZIKV infections in Brazil in 2015–2016 and rubella virus infections in the U.S. pandemic of 1959–1965 is the level of population immunity,” the researchers wrote. “[Today], none of the population has antibodies to ZIKV. In contrast, in the United States during the rubella epidemic, there were 20,000 cases of the congenital rubella syndrome, but in 1959 only 17.5 percent of women of childbearing age lacked rubella antibodies.”
Another crucial difference: Rubella is spread by person-to-person contact; whereas Zika is spread by mosquito bites and sexual transmission. But researchers don’t know how long babies born with Zika remain infected—potentially posing a risk to those around them. “We need more data on the impact of Zika on a neonate,” Reef said. “For rubella, the infants are infectious; they have congenital rubella syndrome for up to a year. But we have no idea about infants born with Zika.”
There are also questions about how congenital Zika might affect babies as they develop. Children with congenital rubella are more prone to thyroid problems and diabetes, but researchers don’t yet know what health problems might be caused by Zika later in childhood. “There is so much we don’t know about Zika,” Reef said.
Another unknown is whether a vaccine for Zika will even be possible—and if it is, how long it will take to create. “There are tremendous challenges in developing vaccines for mosquito borne illnesses,” Honein said.
Beyond that, scientists still don’t know whether there’s any point in a woman’s pregnancy where the risks believed to be associated with a Zika infection are diminished. Early evidence suggests that Zika infections acquired later in pregnancy—including well into the third trimester—can be deadly for fetuses; whereas the risk to the fetus of a mother infected with rubella drops significantly after the first 16 weeks of pregnancy.
Both Honein and Reef, who have each worked in health care for decades, agree Zika represents a rare and serious public-health risk unlike anything the United States has experienced in generations.
“Identifying new causes of birth defects is very challenging,” Honein told me. “It’s a very unusual finding to have a virus that we didn’t previously know was linked to adverse fetal defects. We are learning more every day, but it feels very unusual to me.”