July 25, 2016

expert reaction to modelling study estimating number of childbearing women at risk of Zika infection in the Americas

The ongoing Zika outbreak in the Americas is the subject of a paper published in the journal Nature Microbiology which estimates the number of people, women of childbearing age, and pregnancies which may be affected by the epidemic.

All our previous output on this subject can be seen here. The SMC also produced a Factsheet on the Zika virus.

Prof. Jimmy Whitworth, Professor of International Public Health, London School of Hygiene & Tropical Medicine, said:

“The major concern with the Zika outbreak in Latin America is for the damage done to unborn babies when their mothers get infected. This paper uses thorough computer modelling techniques to conclude that about 1.65 million childbearing women will be infected during the current outbreak.

“They cautiously predict that tens of thousands of babies will be affected by the virus. They are probably correct, but there are a number of unknowns, such as whether the outbreak affects the birth rate by encouraging couples to delay pregnancy or seek abortions, and how frequently Zika infection negatively affects birth outcomes.

“The authors quote estimated rates of microcephaly but it is becoming increasingly clear that this is just the tip of the iceberg and that Zika can cause a wide range of damage to unborn babies, especially in the early stages of pregnancy.

“This paper outlines the scale of the challenge that the Zika outbreak presents to society and health services in Latin America. All of these affected babies and their families will need support and linkage to care, which needs to put in place as soon as possible. Over 1700 affected babies have been born in Brazil so far and the numbers are going to continue to increase in the months ahead.”

Dr Derek Gatherer, Lecturer in the Division of Biomedical and Life Sciences, Lancaster University, said:

“Calculating how many women may be at risk of delivering a microcephalic baby as a consequence of Zika virus infection is a complex process involving many variables. This latest paper uses the WorldPop database to map relevant predisposing factors across Latin America, and then combines that environmental prediction with human population density and data on how mosquito-borne viruses have spread in other populations to define how many women of child-bearing age might become infected.

“An estimate of 80-120 million people is given for the total number of people who are at risk of infection, of which perhaps as many as 2 million infections will be during pregnancy.

“Proceeding from this to a figure of the total number of microcephalic babies born, is a little more difficult as uncertainties still exist about the length of the risk period during pregnancy and the level of microcephaly risk for each pregnancy infected during that risk period.  The authors’ estimate of tens of thousands of negatively impacted pregnancies (whether by microcephaly or the other adverse complications of Zika) assumes about 5% risk per affected pregnancy.  By some standards this may be judged conservative.  If the 29% risk recently suggested in one study¹ is more accurate, then the final number of affected children born in Latin America may be over half a million.”

1: http://www.nejm.org/doi/full/10.1056/NEJMoa1602412#t=article

‘Model-based projections of Zika virus infections in childbearing women in the Americas’ by T. Alex Perkins et al. published in Nature Microbiology UK time on Monday 25 July 2016. 

Declared interests

Prof. Jimmy Whitworth: “Jimmy Whitworth directs the ERAES programme (Enhancing Research Activity in Epidemic Situations) at the London School of Hygiene & Tropical Medicine which is supported by the Wellcome Trust to provide funding for urgent research during outbreaks.”

Dr Derek Gatherer: “Derek Gatherer has no conflicting interests with regard to Zika virus.