On Friday, Singapore's case count had soared to 189, health authorities reported.
The developments came as a new study released Thursday identified eight countries in Asia and Africa that researchers say are at the greatest risk of Zika virus transmission.
The countries — India, China, the Philippines, Indonesia, Nigeria, Vietnam, Pakistan and Bangladesh — all have hot and humid climate conditions; the primary type of mosquitoes that spreads the virus; a high volume of travelers returning from Zika-affected areas in the Americas; and large, dense populations. And several have limited health resources.
Although China has more people, India has the largest population at risk, because of the combination of climate and mosquitoes, one researcher noted. In China, the area of highest risk is centered around the southeastern part of the country, which includes populous Guangdong province.
Until the latest outbreak in Singapore, most of the attention has been where Zika's global spread has hit hardest, in the Americas and the Caribbean. According to the World Health Organization, 55 countries have reported evidence of the virus since 2015; all but eight are in the Americas and the Caribbean.
But health officials are worried about the impact that Zika could have in the vulnerable regions of Africa and Asia, where an estimated 2.6 billion people live.
The study in the Lancet Infectious Diseases used mathematical modeling and analyzed monthly flows of airline travel, 50 years of climate data, population data and health-care expenditures. It is the first to estimate the risk of Zika outbreaks in Africa and the Asia-Pacific region. Researchers included scientists from the London School of Hygiene & Tropical Medicine, Oxford University and the University of Toronto.
"Our broader purpose was to give a look ahead to some of the next frontiers" where there hasn't been as much attention to Zika, said Kamran Khan, one of the study authors and an infectious-disease physician and scientist at St. Michael's Hospital in Toronto. "If you're thinking about how to use resources, here are the places and times where you would want to use resources in the most efficient ways possible."
The analysis was completed before the outbreak began recently in Singapore. If the situation there continues to unfold significantly, that could increase the likelihood of transmission in Asia, Khan said. Because 4 of 5 infected people typically don't show symptoms, the true number of infections in Singapore is likely to be much higher than the 151 cases authorities confirmed as of mid-Thursday.
"We do know that Singapore is a pretty significant transportation hub, and there is a lot of movement into parts of South Asia from Singapore," Khan said.
Researchers modeled three scenarios for Zika transmission. One was based on the suitability for spread of dengue virus, which closely resembles Zika. The second scenario included areas with Aedes aegypti mosquitoes, the primary vector for transmitting both diseases. The third scenario included areas with Aedes aegypti as well as Aedes albopictus mosquitoes. The latter species is also a vector, although a less efficient one.
A big unknown is the extent to which communities in Asia and Africa might have immunity to Zika, because the virus has sporadically been reported on both continents. It's unclear whether the strain circulating in the Americas will affect people differently if they previously have been infected with a different strain.
In an accompanying comment, two researchers not involved in the study wrote that "an important prerequisite for the occurrence of large-scale epidemics is the presence of an immunologically naive population. This is a big unknown in the high-risk countries identified."
The best-case scenario "would be the existence of previously unknown widespread Zika virus immunity in Asia and Africa," wrote Abraham Goorhuis and Martin Grobusch of the Center of Tropical Medicine and Travel Medicine at the University of Amsterdam.
This post has been updated.