One of the recurrent factoids about Zika that are almost always mentioned in reports on the virus is that only 20% of people who get infected ever develop symptoms. This little nugget of knowledge is now being questioned and the implications could affect how the virus’s spread is assessed.
The one-in-five figure stems from the 2007 outbreak in the Yap Islands, which are part of Micronesia. This was the first time Zika had been seen outside of Asia and Africa. Researchers studying the outbreak reported at the time that only 20% of patients with the virus appeared to show symptoms—and this finding is not being disputed. What is being questioned is how applicable the rate of symptomatic-to-asymptomatic cases in a singular outbreak on a small island could be to larger populations set against different climates or other factors that could affect how the virus progresses.
This view is echoed by Marc Fischer, a CDC epidemiologist and one of the authors of the original paper on the 2007 outbreak. He asserts that more research needs to be done on other settings and populations before a rate of symptoms can be generalized beyond Micronesia.
One of the reasons the Micronesia rate could be invalid in larger populations is because the study at the time only identified 49 confirmed and 59 probable Zika infections, out of an overall population of 7,000. Such a small number of cases and an even smaller number of confirmed infections mean that uncommon manifestations or traits of the virus might not be seen and that ratios could become skewed by pure chance.
Understanding the proper rate of asymptomatic Zika is especially important in the context of the global health alert it has triggered. Current laboratory tests for Zika are expensive, time-consuming, and have to be performed within a specific window of opportunity. Consequently, most diagnoses are made using visible symptoms and tests are only done in cases where neurological problems or pregnancy are involved.
The current assumption is that once a person is infected with Zika, their body produces antibodies that protect them against future cases. As a result, knowing how many people could be infected without realizing it is a key value in determining how many might still be at risk.
Additionally, a better understanding how environmental or population factors affect how Zika manifests could help explain some of the current mysteries surrounding the virus. Although Zika is the top suspect behind Brazil’s spike in microcephaly birth defects—508 confirmed and 3,900 still being investigated—researchers are unable to explain why the increased rate so far appears to be only appearing within Brazil and not in any of the other nations experiencing outbreaks.
Sources for Today’s Article:
Duffy, Mark R., et al, “Zika Virus Outbreak on Yap Island, Federated States of Micronesia,” The New England Journal of Medicine 2009, doi: 10.1056/NEJMoa0805715, last accessed February 20, 2016.