Just a month after announcing that two candidate vaccines showed promising results in mice trials, scientists have discovered that both have now provided rhesus macaque monkeys with complete protection against Zika virus. The vaccines also appear safe, as no side effects were reported, paving the way for human clinical trials.

On 1 February, 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 or the Guillain Barre syndrome.

The development of an effective vaccine to stop the epidemics and protect pregnant women and their foetuses, as well of the rest of the population, has since then been regarded as a priority by the scientific community.

The two vaccine candidates that were tested in monkeys consisted of a DNA vaccine and a purified, inactivated virus vaccine.

While the first is constituted of genetic snippets from a Zika virus strain that circulated recently in Brazil, the second was based on a Zika virus that recently circulated in Puerto Rico and was inactivated in the lab by scientists.

These vaccines are developed by Harvard, the Beth Israel Deaconess Medical Centre and the Walter Reed Army Institute of Research.

From mice to monkey

In mice, the DNA vaccine and the inactivated virus vaccine had been shown to provide full protection against the virus and to elicit an antibody response to Zika that correlated with such protection.

In this latest study on rhesus macaques, published in the journal Science, these two vaccines were again tested, as well as a third candidate – an adenovirus vector-based vaccine. All three were compared with a placebo vaccine injected into the bodies of 'control' monkeys.

They were found to initiate a robust protection against infection, when the animals were exposed to Zika virus strains. No detectable Zika virus was identified in the blood or other bodily secretions. Finally, no side-effects were observed in the animals.

rhesus macaques
Rhesus macaques are often used in scientific experiments, as non-human primatesGeoff Gallice/Flickr

The adenovirus vector-based vaccine which is based on the same principles DNA vaccine, with only a fragment of Zika virus DNA being introduced into the body to trigger an immune response, proved particularly interesting to scientists.

They noted that it provoked a broader and more potent antibody response than the DNA vaccine, suggesting it might be slightly more effective.

The researchers say these findings should prompt human clinical trials to start in the near future, testing all three vaccine candidates.