Although the study does not confirm a causal link between the two, it offers new evidence in response to the global public health emergency declared by the World Health Organization (WHO) in February.
In research published in medical journal The Lancet, France’s Institut Pasteur and its partners found a strong link between Zika and microcephaly during an outbreak of the virus in French Polynesia between October 2013 and March 2014.
“Out of eight cases of microcephaly that occurred over two years, seven occurred in the four months that followed the Zika epidemic,” says Dr. Simon Cauchemez, director of the mathematical modelling for infectious diseases unit at the Institut Pasteur and lead author of the study.
Based on analysis of data kept during the outbreak, Cauchemez explains his team was able to provide the most thorough quantification of the link between Zika and microcephaly disorders so far, and that the result tells mothers-to-be what exactly they risk if they come into contact with the virus during pregnancy.
“For the first time, we can provide a number to pregnant women and tell them, if you got infected by Zika in the first trimester of pregnancy, the risk that the foetus is going to be infected by microcephaly is about one per one hundred.”
Based on comparative data, the risk of microcephaly is fifty times greater than what has been recorded in cases where Zika infection is not present during the first three months of a pregnancy.
Zika microcephaly link
However, the study does not prove that Zika causes microcephaly, nor any of the other neurological disorders thought to be linked to the virus, which researchers have been trying to determine since the WHO declared a global public health emergency on 1 February.
“This new paper has quantified in more detail the likely relationship between Zika virus and one aspect of the neurological disorders, which is microcephaly. So it adds additional weight to the evidence, but on its own, it’s not conclusive,” says Dr. Christopher Dye, Director of Strategy with the WHO.
“Taken together with the other information that we already have, a picture of a causal link between this virus and these neurological conditions is now looking quite convincing.”
With the WHO warning the mosquito-borne virus could spread across Africa and Asia as well as South America, where the majority of microcephaly cases have been recorded up until now, one question arising from the Institut Pasteur study is whether the outbreak of Zika in French Polynesia applies to its appearance elsewhere.
“How well the virus can spread is going to depend on environmental factors, demographical factors, entomological factors,” says Dr. Cauchemez.
“There may also be differences in the risks of getting these complications in the population from French Polynesia compared to what it might be in South America, just because these populations might have different genetic backgrounds.”
Dr. Dye says the potentially global reach of Zika underlines the value of having the numerous studies underway in different regions the virus is present.
“We don’t yet know the consequences of Zika virus infection for neurological conditions across Africa and Asia,” he says, noting separate research is in progress into a current outbreak off the coast of West Africa, in Cape Verde.
“We eagerly await those results, which will be very revealing about how widespread these neurological disorders are likely to be around the world.”