It is strongly suspected that the Zika virus causes the foetal abnormality of microcephaly, but what is the actual risk of a child developing the devastating neurological condition if their mother is bitten by an infected mosquito?
A paper published this week in The Lancet is the first study to provide an answer to this and other questions. By retrospectively examining the Zika outbreak that occurred in French Polynesia from 2013 to 2015, the authors were able to identify 8 cases of microcephaly and also establish the likely number of women who were pregnant, infected with Zika, but whose babies did not develop microcephaly.
They calculated that the overall risk of a pregnant woman getting bitten and then going on to have a foetus with microcephaly was only 1% and that the most risky time to get infected would be in the first 3 months of pregnancy.
At face value this seems to be fairly good news, as infections with other viruses can produce a far higher chance of producing disease in developing babies: if a baby is infected with cytomegalovirus (or CMV), for example, there is a 13% chance of illness, whereas infection with rubella produces disease in a far higher proportion of foetuses. The scale of the problem with Zika becomes apparent when you factor in what percentage of the population can be infected with the virus during an epidemic, however. Although CMV may produce foetal disease 10 times more often than Zika, CMV is only usually active in 1-4% of the population; in French Polynesia 66% of the population became infected with Zika. The risk may be low, but many more pregnant women can be infected with Zika.
Can we develop immunity to Zika?
An interesting comment was made in a Nature article this week that as large percentages of Zika-affected populations are being infected with the virus, this could mean that women infected with Zika now, may develop natural immunity to the virus. The author went on to suggest that such an immunity may protect their future children from suffering the consequences of subsequent infection with the Zika: there would be fewer cases of microcephaly in the future, this current fear of foetal abnormality will naturally wane as general immunity increased.
This optimistic scenario is undoubtedly possible, but perhaps premature. As viruses evolve, differing strains invariably develop.
Take dengue fever for example. The same individual can be infected several times during their life with dengue, as they are bitten with mosquitos carrying differing strains of the virus. The same could well become possible for Zika in the future, meaning that any natural immunity gained now may not be protective for the future. The situation is a little more complex with dengue, however, and highlights some of the complexities of our immune system. Since the most severe consequences of dengue fever, the most severe illness, can actually occur when the same person is infected a second time with another sub-type of virus. Although unusual, our immune response to dengue can sometimes make the disease worse on subsequent infection. Our immune system doesn't always work in our favour. As can be seen, it's sometimes not as simple as once you are infected with a virus, you will be protected for life, and we don't yet know how the immune system deals with Zika.
Much is still unclear with the Zika virus, but the Lancet paper adds significantly to our knowledge. We not only know the potential risk of infection to a pregnant woman, but now, which stage of pregnancy is riskier.
Dr Simon Worrell, Head of Medical Communications, Healix International