Two Zika vaccine candidates have shown promising results in pre-clinical trials, scientists have announced. A single shot of either vaccines appeared effective in protecting mice from an infection, in the weeks that followed immunisation.

On 1 February 2016, the World Health Organisation declared Zika a public health emergency of international concern. They were particularly concerned about the link between the virus and a number of severe neurological conditions, such as microcephaly and Guillain Barre syndrome.

In the past few months, the development of an effective vaccine has therefore been a priority. The problem is that little is known about the virus' immunology.

In this recent research, published in the journal Nature, a DNA vaccine and a purified, inactivated virus vaccine were tested to assess the degree of protection they could offer, and if one was more effective than the other.

Optimism for a vaccine

The DNA vaccine candidate contained genetic snippets from a Zika virus strain that circulated recently in Brazil, with the aim of triggering immune responses. The purified, inactivated virus vaccine was based on a Zika virus that recently circulated in Puerto Rico and was inactivated in the lab by scientists.

The vaccines were developed by Harvard, the Beth Israel Deaconess Medical Centre and the Walter Reed Army Institute of Research. Different groups of mice received a single shot of one of the two vaccines and were then exposed four and eight weeks later to the virus.

Zika vaccine trial
WRAIR Pilot Bioproduction Facility.WRAIR

The findings were particularly encouraging: all the mice were protected from an infection. Both vaccine candidates were found to be safe and to elicit an antibody response to Zika virus that correlated with protection. Further work with the DNA vaccine demonstrated that protection was solely due to antibodies raised by vaccination.

"Protection was striking when the mice were challenged with the Zika virus. The level of antibodies were similar to that produced by vaccines for viruses of the same family – what we call flaviviruses", says study author Dan Barouch.

"Of course we need to be careful not to extrapolate too much results on mice to humans but the robustness of the protection provided, as well as the levels of antibodies produced in similar proportions to other vaccines that we know to be effective lead us to be optimism that we will get a vaccine".

Brazil Zika outbreat brain damange
Zika has been deemed a global health emergency in February Reuters

Clinical trials

The researchers say tests of the vaccines will be now be extended to other animal models – non human primates in particular. Clinical trials will then take place quite rapidly, probably before the end of the year.

In these clinical trials, the purified, inactivated virus vaccine could eventually be the preferred candidate, because up to now no DNA vaccine has been clinically approved for human use in the USA. In contrast, the purified inactivated virus vaccine builds on "a type of vaccine that has been licensed before," concludes Col Stephen Thomas, WRAIR Zika program lead.