CDC's March 3, 2017 baseline report counted 747 infants and fetuses with specified birth defects in Massachusetts, North Carolina and metropolitan Atlanta during earlier years. The reported prevalence was 2.86 per 1,000 live births. The numerator therefore included fetal losses; it was not simply a fraction of liveborn children with a diagnosis. [mm6608a4][mm6503e3]

The report compared that baseline with a registry of pregnancies involving laboratory evidence of possible Zika infection. Its approximately twentyfold difference was descriptive, not a controlled estimate of what infection would do to an individual pregnancy. Selection into the registry and closer follow-up could change the comparison independently of the underlying frequency of abnormalities. [mm6608a4][mm6503e3]

Other limits matter too: the three areas might not represent the country, and earlier travel-associated infections were possible. Congenital deafness was incompletely captured. Comparable registry figures for other defects were unavailable. Unspecified flavivirus infections could enlarge the registry denominator and dilute its proportion, so the limitations cannot be reduced to a single upward bias. [mm6608a4][mm6503e3]

What counted as possible infection also needs explanation. CDC's January 2016 guidance described antibody cross-reactivity, potential false positives and neutralization testing that could help discriminate between related viruses without always resolving the result. This is historical diagnostic context, not current advice. The two CDC documents contribute different pieces of one institutional evidence chain: how defects were counted and why laboratory categories should not be treated as interchangeable. [mm6608a4][mm6503e3]

Source documents

  1. Baseline Prevalence of Birth Defects Associated with Congenital Zika Virus Infection — Massachusetts, North Carolina, and Atlanta, Georgia, 2013–2014https://www.cdc.gov/mmwr/volumes/66/wr/mm6608a4.htm
  2. Interim Guidelines for the Evaluation and Testing of Infants with Possible Congenital Zika Virus Infection — United States, 2016https://www.cdc.gov/mmwr/volumes/65/wr/mm6503e3.htm