A batch of new studies show the Zika virus is trickier than it appeared at first glance, lurking for months in pregnant females and interfering with the immune system’s response.

The findings help explain why the virus seems so mild in some people, yet causes devastating birth defects. And while the data suggests it is not going to be so easy to fight the epidemic, at least two studies offer some hope for a good, protective vaccine.

Perhaps the most sobering study found that Zika persists for weeks in pregnant females – in this case, in monkeys infected with the virus.

Zika was thought to cause a brief and often symptom-free infection in most people. But David O’Connor and colleagues at the University of Wisconsin in Madison found that while the virus was gone from most monkeys within 10 days, it stayed in the blood of pregnant monkeys for as long as two months.

Pregnancy suppresses the immune system, and it’s possible that pregnant women just cannot fight off infection as well, O’Connor and colleagues said in their report, published in the journal Nature Communications.

“The other, more provocative hypothesis is that it’s indicative of infection of the fetus, and what we’re observing in the maternal bloodstream is the shedding of virus by the fetus back into the mother’s bloodstream,” O’Connor said in a statement. “If that happens to be the case, it would suggest that there is a prolonged infection of the fetus that lasts much longer than the infection of the mother.”

Researchers found evidence of Zika in the blood, saliva, urine and vaginal fluid of the monkeys. The two monkeys are still pregnant, O’Connor said, so the researchers still don’t know if the babies are affected.

In a separate study, the team infected two monkeys very late in pregnancy and removed the infant monkeys surgically to observe them. “We collected more than 60 different tissues from each of these babies at C-section,” O’Connor told reporters. “So far we haven’t seen any evidence of viral infection at birth in these tissues.”

Several viruses can cause birth defects and doctors say the earlier in pregnancy a woman is infected, the more dangerous it is to the fetus. Other teams are studying pregnant women in Zika-affected areas to see what percentage of those who get infected have babies with birth defects, and to see whether fetuses infected later in pregnancy are less likely to suffer birth defects.

“We have good news for most people: If you are not pregnant and not at risk of becoming pregnant, you probably don’t need to be worried about Zika,” said O’Connor. “But my concern for Zika virus in pregnancy is much higher now than it was six months ago.”

Zika has spread quickly across Latin America and the Caribbean. The virus is carried by mosquitoes and can also spread through sexual contact. Most people don’t even know they are infected, but it can cause terrible birth defects if a woman is infected during pregnancy.

There’s no specific treatment and no vaccine. But trials have already started on one vaccine in human volunteers. Others are close to starting.

O’Connor said the study showed that monkeys infected once with Zika were immune to the virus. “But we currently do not know how long immunity lasts,” Dawn Dudley, who worked on the study, told reporters on a telephone briefing.

“This is good news for vaccine design,” O’Connor said. “It suggests the sort of immunity that occurs naturally is sufficient. If you can mimic that in a vaccine, you’ll likely have a very successful vaccine.”

A second study in mice had a similar result. Dan Barouch of Beth Israel Deaconess Medical Center and Harvard Medical School tested potential vaccines in mice.

“We showed that a single dose of either a DNA vaccine or purified live attenuated (weakened) vaccine provided protection against Zika challenge in susceptible mice,” Barouch told NBC News.

Barouch’s team tried two different sources of Zika - one taken from Brazil, the other from Puerto Rico. The vaccines protected against both, which suggests the virus isn’t mutating in a way that would affect a vaccine’s ability to protect against it.

“Taken together, our findings provide substantial optimism that the development of a safe and effective Zika virus vaccine for humans will likely be feasible,” Barouch and a colleague wrote in Nature Communications.

Zika is a close relative of yellow fever, chikungunya and dengue virus. Researchers say it’s very similar, genetically, to dengue, which is also circulating in the areas hardest-hit by Zika, such as Brazil and Puerto Rico.

Dengue comes in four strains and it affects the immune response in an odd way. People usually don’t get very sick when infected with one strain of dengue. But when people catch a second strain of the virus, they often show more severe symptoms.

Scientists had wondered whether people who had been infected with dengue might be more vulnerable to Zika’s rare health effects, which include birth defects and also paralyzing conditions such as Guillain-Barre syndrome.

Last week, an international team found evidence that people who had been infected with dengue in the past might be more susceptible to Zika infection. But they also found that human antibodies against dengue also protect against Zika, at least in lab dishes.

“There are plenty of unanswered questions, such as the extent to which immunity to other flaviviruses like dengue may or may not impact vaccine responses or protection,” Barouch said. “That is something that clearly needs to be explored.”

A team at Emory University reported similar findings Monday. Many of the Zika research findings are being shared freely online.

Another bit of good news - having monkeys to study may help predict what will happen among people down the road, said Matthew Aliota of the University of Wisconsin team.

“You may have to follow children for five years or longer to tell whether there is cognitive impairment in their development,” Aliota said. “But it’s something you can answer with macaques relatively quickly, and that speed is very important in the context of an epidemic.”