Birth Defects Potentially Related to Zika Infection Increased 4x During Early 2017

A recent report focused on the U.S. territories found widespread local Zika virus transmission increased 400 percent from January–March 2017, when compared with January–March 2016.

This US Centers for Disease Control and Prevention (CDC) report, published on January 24, 2020, found the peak occurrence of birth defects potentially related to Zika virus infection occurred in February 2017.

This was 6 months after the reported peak of the Zika virus outbreak in certain areas during August 2016. 

This conclusion is consistent with other findings regarding the time between the peak of a Zika virus outbreak and the recognition of an increase in potentially Zika-related birth defects.

Approximately 47 percent of Zika cases with laboratory test results available in areas with widespread local transmission had confirmed or possible laboratory evidence of infection. 

In areas with limited local transmission, the prevalence increased 20 percent from October–December 2016, although not significantly, no increase was observed in areas without local transmission.

Compared with the CDC's previous report, this analysis added 7 more jurisdictions and reported 18 months of data from monitoring births potentially affected by the outbreak. 

The previous report grouped widespread and limited local transmission areas together, reporting a 21 percent increase in prevalence for these areas combined.

Stratification by local transmission levels provides support that the significant increase in prevalence is exclusive to widespread local Zika virus transmission areas. 

Further, the baseline prevalence of birth defects potentially related to Zika virus infection during the reference period in the 22 jurisdictions is consistent with the baseline prevalence for 3 jurisdictions before the Zika virus was introduced in the Region of the Americas.

The findings in this report are subject to at least 4 limitations. 

• First, results might not be generalizable beyond the included jurisdictions because jurisdictions might differ in population demographics and case-finding methodology. 
• Second, heightened awareness can result in better identification of affected infants. For example, there might have been more extensive implementation of recommendations for eye exams in widespread local transmission areas. 
• Third, categorization of areas with limited local transmission included regions of Florida and Texas that were larger than the actual areas of local transmission, which might mask any increase in Zika-related birth defects in smaller geographic areas where transmission occurred. 
• Finally, the majority of cases did not have Zika virus testing reported. In widespread local transmission areas, approximately 75 percent of cases had at least one sample tested, although the relatively high prevalence of negative results could reflect that timing might not have been optimal for the detection of Zika virus in many cases. 

However, nearly half of those tested had laboratory evidence of Zika virus infection.

During the Zika virus outbreak, population-based birth defects surveillance programs were adapted to monitor birth defects potentially related to Zika virus infection during pregnancy. 

The use of population-based birth defects surveillance programs and the U.S. Zika Pregnancy and Infant Registry provide an example of a complementary approach in ascertaining both exposures and outcomes to better monitor new and emerging threats during pregnancy and impact on infants.

Birth defects surveillance was important for identifying infants with birth defects potentially related to Zika virus infection whose mothers were not tested during pregnancy or were not tested at a time when infection could be detected. 

The CDC says ‘health departments can use these data to inform referral services for affected infants and program planning.’ 

These findings underscore the important role of birth defects surveillance programs in preparing for emerging public health threats to pregnant women and infants.

During 2019, the CDC reported no locally-acquired Zika cases in US states. However, the leading US states to report travel-related Zika cases were California and Florida.

Unfortunately, the US Territories reported 57 Zika cases acquired through presumed local mosquito-borne transmission during 2019.

All study authors have completed and submitted the International Committee of Medical Journal Editors form for disclosure of potential conflicts of interest. No potential conflicts of interest were disclosed.

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