Scientists have edged closer to an effective Zika virus vaccine after demonstrating that three different formulations can protect monkeys from the disease, reports The Guardian.

The results suggest that the virus can be repelled by even low levels of immunity and have boosted confidence that a viable vaccine for humans is on the horizon.

Tests on 16 animals found that all three experimental vaccines offered complete protection against Zika infection one month later, though how long the protection could last for remains an urgent question for longer-term trials.

When vaccinated, the animals churned out antibodies that were more than sufficient to overwhelm the virus.

The Zika virus has swept through Latin America and left behind a trail of birth defects, such as microcephaly, which causes children to be born with small heads. This week, Florida reported the first US cases of local transmission of Zika virus. All previous cases were in people who had travelled to affected regions.

Despite the encouraging progress, the path to a viable vaccine in humans may not be straightforward. Recent work by Screaton’s group found that previous exposure to dengue virus could potentially make Zika infections more serious. If the opposite holds too, as some researchers suspect, a vaccine that floods the body with antibodies against Zika virus could make common dengue infections life-threatening.

The problem arises because Zika and dengue, which both belong to a group called flaviviruses, are so similar at the genetic level. This can confuse the immune system. Should a person catch dengue and later catch Zika virus, their body may attempt to fight off Zika with “old” antibodies raised against dengue. Rather than overwhelming the Zika virus, the antibodies might simply draw them into cells and cause the infection to take hold more quickly. Known as cross-reactivity, this raises a second potential hurdle: a person who has fought off dengue or similar flavivirus infections may have antibodies that destroy the Zika vaccine before it has had time to work.

Another issue scientists face comes from the natural immunity people will acquire to Zika as the infection spreads through the population. When people are already immune to a virus, it can be very hard to tell whether a vaccine on trial is helping to protect them.