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Zika virus (false colour transmission
electron microscope image) |
In a recent piece on mRNA vaccines[mRNA vaccine technology: industry is getting the message] , I mentioned Zika virus development as an indication for this emerging technology. only to realise my ignorance of this high profile pathogen (shameful, since I spent my post-doc years working in a school of tropical medicine)
First stop in filling
the knowledge gap was a visit to the ever-useful World Health Organisation website which
features a comprehensive “Zika timeline”. First discovered in 1947 in monkeys
living in a Ugandan forest (which gave its name to the virus), and shortly after
in species of Aedes mosquito,
epidemiological studies conducted in the 60s and 80s indicated widespread human
exposure to the Zika virus in Africa and Asia, with infection largely
associated with no, or only mild, symptoms.
The more
sinister nature of Zika infection emerged in 2007 and 2008, with the first confirmed
large scale outbreak on the Micronesian island of Yap, and evidence that
infection could be sexually transmitted. Further outbreaks occurred on various Pacific
Islands during 2013 and 2014 and pointed to a link between Zika infection and birth
defects and with Guillain–Barré syndrome, a rare autoimmune disease which
affects the nervous system. A year later, an epidemic characterized by a skin
rash bit otherwise mild symptoms was reported in north-eastern Brazil, but was not recognised as being due to Zika virus.
By October
2015, an increasing numbers of microcephaly cases (newborns with small heads,
indicative of abnormal brain development) were being reported. Further Zika outbreaks
occurred in several South American and Caribbean countries over the following
year, with the first cases (via sexual transmission) being identified in the
continental United States in 2016.
Being
generally asymptomatic, the prevalence of Zika infection is not easy to
quantify, but mosquito-transmitted infection has been reported in over 80
countries (with transmission ongoing in over 60 of these). Over 1.3 million
people are thought to have been infected in Brazil alone during the 2015
outbreak. In common with other vector-borne diseases, the spread of Zika owes
something to human mobility, although social factors, principally the inability
to afford protection against mosquitoes and high population density have been identified
as key drivers.
The Zika virus
has several properties that contribute to its ready transmission and to its
devastating effect on foetal development. The virus is highly persistent in
whole blood (up to 100 days) and in the male reproductive tract, allowing sexual
transmission. Zika has a preference for certain cell types that facilitate the passage
of infection through the placenta; animal studies suggest that viral preference extends
to neural progenitor cells essential for normal cortical development. Whether Zika infection
is a direct cause of Guillain–Barré syndrome
has not been established.
As with
malaria, Zika’s vulnerability lies in its dependence on mosquito vectors: and
targeted insecticide use, management of standing water and the conscientious use of
bed nets and repellents can significantly reduce transmission. Like malaria,
control initiatives are vulnerable to political and economic factors, including
climate change-related changes in mosquito distribution and abundance. Unlike
malaria, Zika has a simple lifecycle; as with other flaviviruses (including
the causative agents of yellow fever and Japanese encephalitis), infection
should, in theory, be preventable through vaccination.
Vaccine
development efforts are almost contemporary with the Zika outbreak itself, beginning in
the second half of 2015 with the genetic analysis of Brazilian Zika isolates,
with the first clinical study of a Zika vaccine being reported in late 2017.
Around 45 candidate vaccines have been developed through academic, governmental
and industrial efforts, with nine of these reaching the clinic, representing both established (inactivated virus) and experimental approaches (DNA and mRNA
vaccines). The WHO’s initial requirement is for a vaccine that can be deployed
in response to outbreaks with the primary goal of preventing congenital Zika
syndrome through minimizing virus carriage in the immediate population.
Early clinical
studies have established that vaccination can elicit aneutralizing antibody responses,
although, while an accepted hallmark of flavivirus vaccine efficacy, the importance
of neutralising antibodies, and the minimum levels needed to establish
protection have still to be established in the context of Zika infection. Other
important unknowns include the duration of effective vaccine-induced immunity and whether the
reproductive tract can be protected from infection.
Perversely,
given the untold misery arising from Zika outbreaks around the globe, the virus’s
propensity for neural progenitor cells may offer a new means of treating the
most aggressive and intractable form of brain tumour, glioblastoma.
Chinese researchers
have found that an experimental live attenuated Zika vaccine functions as an
oncolytic virotherapy [Going viral] in an animal model of glioblastoma, specifically
infecting and destroying glioma stem cells thought to be responsible for the
inevitable recurrence of the tumour. Elimination was also observed using glioma
stem cells isolated from individual patients.
Photo credit: Credit:
NIH/NIAID
Recruitment of healthy volunteers for initial evaluation of ChAdOx1 nCoV-19, a candidate non-replicating adenovirus vaccine developed at the Oxford Vaccine Centre in underway. 
