One of the things you need to keep in mind about all public health surveillance is that the numbers presented are the best estimate of what is really going on. In some instances, the number will be an overestimate of the true number of cases because the system is too sensitive and is picking up too many false positives. This is what you want when you don’t want to miss any cases. On the other hand, if you want to minimize the “chasing around” of false positive cases, then you want the system to be more specific, leading to an underestimate of the true number of cases. Where on the sensitivity-specificity spectrum a system falls depends on what your needs are.
With that in mind, let’s look at a significant event that was reported from Puerto Rico this week. A child was born with Zika-associated microcephaly, according to NBC:
“The baby was born in the past two weeks with severe brain defects as well as hearing and vision problems and remains hospitalized, according to Health Secretary Ana Rius. She said authorities are investigating why the microcephaly case was identified so late although the mother had Zika symptoms between her second and third month of pregnancy. She added that the mother did not receive continuous prenatal care until late in her second trimester.”
That part about not knowing why the case was identified late is a normal artifact of any public health surveillance system. Unless you have 100% recording/reporting of cases at 100% of possible locations where 100% of pregnant women are receiving any kind of care — including home deliveries — then you are not going to catch all the cases. Of course, this is troubling to those who would like to know the full impact of the Zika outbreak, and to those in the general population who want to know what the true burden of the disease is as well as their risk of contracting Zika and suffering consequences.
Another big unknown about Zika is how many total cases will be found when it is all said and done, and whether or not Zika will find a permanent home on the island. Either way, the expectations are dire. Again, from the NBC article:
“Puerto Rico has a Zika epidemic with nearly 31,500 cases reported, including more than 2,400 in pregnant women. A total of 233 people have been hospitalized and six deaths have been reported, four of them linked to Zika and other medical conditions and the remaining two to a paralyzing condition called Guillain-Barre syndrome known to be caused by Zika and other infections.
The U.S. Centers for Disease Control and Prevention expects a surge of babies born with severe deformities in Puerto Rico in upcoming months. A recent study estimates that up to 10,300 pregnant women on the island could be infected with Zika and that between 100 to 270 babies could be born with microcephaly.”
Those are 100 to 270 children with microcephaly, costing all of us (everyone),”about $4 million per child.” And that’s just the children we’ll know about because of the surveillance system. There are likely to be plenty who are not picked up by the system, are not born full-term (or miscarried), or are born outside Puerto Rico but originated in that outbreak. How policy drives the epidemiology, and how the epidemiology drives policy are all questions still up in the air.