The new method for estimating Zika risk suggests up to 1.65 million childbearing women could be infected with the virus.
The researchers - who come from the University of Southampton and the University of Oxford in the United Kingdom, as well as the University of Notre Dame in Indiana - publish their findings in Nature Microbiology.
In February of this year, the World Health Organization (WHO) labeled the Zika virus epidemic a Public Health Emergency of International Concern (PHEIC), acknowledging the high potential for the virus to travel across the Americas.
Zika virus is spread by the bite of an infected Aedes aegypti mosquito, which the Centers for Disease Control and Prevention (CDC) describe as "aggressive daytime biters" that can also bite at night.
Because the virus has been associated with a range of fetal developmental conditions - including microcephaly - and there is no vaccine available, the epidemic poses major concerns, particularly for the nearly 15 million children born in the Americas each year.
Over 90 million infections are likely
Prof. Andrew Tatem, study author from the University of Southampton, notes that predicting how many childbearing women are at risk from Zika is very challenging, given that many cases do not show symptoms. An estimated 80 percent of Zika infections do not display signs of illness.
- A pregnant woman can pass Zika to her fetus
- Infection during pregnancy can cause certain birth defects, including microcephaly
- Currently in the U.S., Zika is not being spread by mosquitoes.
"This largely invalidates methods based on case data and presents a formidable challenge for scientists trying to understand the likely impact of the disease on populations," he adds.
The lack of symptoms, along with inconsistent case reporting, make case-based data unreliable, according to the researchers.
As such, the team designed an improved method for estimating the risk that Zika presents, by taking into account ecological theories such as herd immunity and basic reproduction number, which is the estimated number of new people that one infected person can infect.
They examined the impact of the virus at local levels, at a scale of 5 kilometers squared, and they also considered disease patterns shown in similar epidemics - such as those seen with dengue and chikungunya viruses.
Additionally, the researchers factored in how the virus is transmitted, climate conditions, and virus incubation periods.
By using existing data on population, fertility, pregnancies, births, and socioeconomic conditions, the researchers were able to model the potential scale of the spread of Zika.
Along with their projection that up to 1.65 million childbearing women could be infected by the Zika virus, the researchers also found that over 90 million infections could result from the initial spread of Zika in Latin America and the Caribbean.
Furthermore, the team found that Brazil is expected to have the largest total number of infections, by more than threefold, given its size and conditions for transmission.
'Important early contribution to global Zika efforts'
The researchers say that the virus may not reach every corner of Central and South America, so the total figure of 1.65 million is an upper limit for the epidemic's first wave.
Still, their figures provide a better understanding of areas that will likely be most affected, shining a light on those areas that will need the most support.
"These projections are an important early contribution to global efforts to understand the scale of the Zika epidemic, and provide information about its possible magnitude to help allow for better planning for surveillance and outbreak response, both internationally and locally."
Prof. Andrew Tatem
Although the links with Zika infection and babies born with microcephaly have caused alarm for pregnant women around the world, the CDC note that Zika infection in a woman who is not pregnant should not introduce a risk of birth defects in future pregnancies after the virus has cleared.