A Malaysian Health Ministry worker sprays mosquito insecticide fog in the capital, Kuala Lumpur, on Sept. 14, a day after two new cases of Zika virus infection were detected in the country. (Joshua Paul/AP)

Disease experts trying to measure the risk of a mass Zika outbreak in Southeast Asia agree on one thing: It’s next to impossible to predict with any accuracy.

Last month, health ministries across the region wrestled with that uncertainty during a teleconference on ways to deal with the threat, a devastating reality in many Latin American countries.

On the face of it, Southeast Asia is vulnerable to Zika, which is spread by a type of mosquito common in the region. Singapore, Thailand and Malaysia have collectively confirmed hundreds of c­ases, including the births in Thailand of two babies with abnormally small heads — the region’s first experience of the Zika-linked microcephaly that has reached crisis proportions in Brazil. Late last month, the U.S. Centers for Disease Control and Prevention advised pregnant women not to travel to Southeast Asian countries affected by the virus.

At the same time, though, experts have found grounds for hope in the relatively small scale of the regional outbreaks, which suggests that local communities might be resistant to the virus in a way that Latin Americans are not.

A city worker fumigates around a temple in Bangkok last month in a bid to control the spread of mosquitoes carrying the Zika virus. Thailand has moved decisively to curtail the disease. (Chaiwat Subprasom/Reuters)

“One of the general pieces of wisdom is there is enough widespread immunity to Zika in Southeast Asia [that] the possibility of rapid spread, as in Brazil, seems much less likely,” said Julie Fischer, co-director of the Center for Global Health Science and Security at Georgetown University.

Unlike in Latin America, Zika has been present in parts of Southeast Asia for several decades, although that strain “has not been previously associated with brain defects,” said Marcia Castro, an associate professor of demography at Harvard University’s School of Public Health. Academic studies show, for example, that the virus has been causing fevers on the Indonesian island of Java since at least the 1980s, raising the possibility that some of the island’s 145 million people might have developed immunity to it.

But little is known about how prevalent Zika is in Indonesia today, which strain is circulating and the degree of immunity the population enjoys, making it hard to gauge the likelihood of a widespread outbreak. Indonesia is home to about 40 percent of Southeast Asia’s total population but lacks the financial and medical resources of wealthier neighbors such as Singapore and Thailand, which have moved decisively to contain the spread of the virus.

In a bid to learn more about Zika’s prevalence in the country, Indonesia’s National Institute of Health Research is collecting mosquito samples from every region to test for the presence of the virus and whether the strain found here is the same as Brazil’s. So far, the effort has not netted a single mosquito that carries Zika, said Siswanto, the institute’s director general, who uses one name. But he was cautious about interpreting what that meant.

“I’m not saying there’s no problem” with Zika, said Siswanto, who took part in last month’s teleconference. “I’m saying we haven’t discovered a problem yet.”

One difficulty is that Indonesia has only two centers where patients can be tested for the virus, and at $150 a test, most Indonesians — even those who live near a center — can’t afford one. Those who acquire Zika are therefore unlikely to know they have it, since in most cases people infected with the disease experience only flulike symptoms.

Erni Nelwan, a tropical disease specialist at the University of Indonesia’s medical school, worries that Indonesian health authorities wouldn’t be aware of a Zika outbreak unless it caused a significant number of microcephaly cases­ in densely populated areas, as happened in Brazil.

“Surveillance is very weak,” she said.

She was less confident than others that Southeast Asians might have developed immunity to Zika. “We can easily learn from Singapore’s situation,” she said. “New cases­ were being added every day until there were more than 300. There is the potential it might easily spread here also.”

In Singapore, the authorities usually succeed in isolating outbreaks as soon as they are discovered. Even so, on Thursday, a new disease hot spot was declared in Ubi Avenue 1, a part of town that hadn’t seen new cases­ for two weeks and had previously been declared free of Zika. Meanwhile, Thailand, whose 39 confirmed cases of Zika infection in pregnant women are the most in the region, has relaxed its usually stringent restrictions on abortion for women who have acquired the disease.

Given how much remains unknown about Zika, experts involved in the teleconference cautioned that basic tried-and-true approaches to controlling insect-borne diseases should be implemented across the region — including aerial spraying to kill mosquitoes and educating people to empty water containers to prevent breeding, as well as how best to avoid bites. Another major emphasis during the call, Siswanto said, was on ways to improve disease surveillance and coordinate responses regionwide.

“Nobody really knows what will happen in SE Asia,” Gonzalo Vazquez-Prokopec, a researcher in vector-borne and parasitic diseases at Emory University in Atlanta, said in an email. “It will be a wild guess to say that SE Asia will behave as the Americas, given all the unknowns. The best we can do is to keep trying to do good vector control and carefully monitor the situation.”