(CNN)Prince Harry and Meghan, the Duchess of Sussex, are expecting their first child, the UK's royal family announced Monday as the couple arrived in Australia for their first tour overseas. The tour will include visits to Tonga and Fiji, which both carry risk of the Zika virus, a cause of birth defects.
Kensington Palace considered medical issues before Meghan left for her trip, a royal source said Monday, but the couple did not make any changes to their schedule.
The World Health Organization and the US Centers for Disease Control and Prevention advise against travel to both islands while pregnant. The UK Foreign Office suggests that women consider postponing nonessential travel until after pregnancy.
The CDC classifies Fiji and Tonga as being "areas with risk of Zika infection." WHO places Tonga in risk category 1, an area with new introduction or ongoing transmission, and Fiji in category 2, an area of evidence of the virus circulating before 2015 or ongoing transmission.
The WHO Zika virus country classification scheme is under review and revision.
In 2018, Fiji has not reported any cases of Zika, and one case was confirmed in 2017, according to its Ministry of Health. No cases of complications linked to Zika have been reported in either 2017 or 2018.
"In these countries there is some risk but not high risk," said Adam Kucharski, associate professor and epidemiologist at the London School of Hygiene & Tropical Medicine.
Kurcharski explained that both Tonga and Fiji saw most of their cases of Zika reported in 2016 and that the current level of risk is lower than it was then. "There's still potential, but it's lower than it was," he said, adding that with any risk, it's good to seek medical and travel advise.
Infection with Zika during pregnancy has been linked to miscarriage and stillbirth, and can cause babies to be born with microcephaly (a smaller than usual head size indicating incomplete brain development), vision problems or neurological defects. The CDC estimates that one in seven babies exposed to Zika has health problems.
The virus is transmitted primarily through the bite of infected Aedes mosquitoes. It can also spread through unprotected sex with an infected person, even if that person is not showing symptoms. It could also spread through blood transfusions.
Zika virus can be transmitted by a pregnant woman to her fetus, but the likelihood of an infection affecting pregnancy or causing birth defects is unknown, according to the CDC. The full range of health effects during pregnancy is also unknown, the agency says.
If pregnant women find themselves in a region affected by Zika, experts advise they take steps to prevent mosquito bites -- such as wearing covered clothing and using insect repellants and mosquito nets -- and use condoms or abstain from intercourse during the entire pregnancy.
When going to or in affected regions, it's "good to still be vigilant," Kurcharski said, by evaluating the risks, seeking advise and taking precautions.
Most people infected with the Zika virus won't have symptoms. Symptoms typically appear a few days to a week after exposure, although the precise incubation period is not known, according to the CDC.
Fever, rash, joint pain and conjunctivitis (pink eye) are the most common symptoms. Some patients may also experience muscle pain or headaches. The major concern remains among pregnant women.
As of March, the CDC estimated that at least 90 countries and territories had active Zika virus transmissions.
People in Brazil and other parts of South and Central America became infected when Zika first appeared in the Western Hemisphere in 2015.
This has provided a natural "herd immunity" -- infected once, people are not likely to get infected a second time -- and so a major outbreak in these regions any time soon is not likely, said Dr. Albert Ko, a professor of infectious diseases at the Yale School of Public Health in a previous report.
Kurcharski agreed that most areas saw outbreaks in 2016 to 2017 and seeing fewer cases now.
Globally, Zika virus transmission peaked in 2015-16 and has continued to decline throughout 2017 and 2018, according to WHO. Many areas in the Pacific have evidence of interrupted transmission or report rare or sporadic cases. The UN agency also stresses the importance of vigilance in monitoring the emergence, re-emergence and global spread of Zika virus infection to protect women and infants worldwide.
No vaccine for the virus is available, but clinical trials are being conducted.
A recent report predicted that changes in climate are aiding the spread of mosquito-borne diseases by affecting not just the distribution of insects like mosquitoes but also how quickly viruses replicate within them and how long the bugs live. All of that might have contributed to recent Zika virus outbreaks, according to the CDC.
