Brazil does not share enough samples and disease data needed to analyse whether the Zika virus is linked to the increased number of babies born with abnormally small heads in the country, UN and US health officials told AP.
Laboratories in the US and Europe are forced to work with samples from previous outbreaks and insufficient data is frustrating efforts to develop diagnostic tests, drugs and vaccines.
Scientists say having so little to work with is hampering their ability to track the evolution of the virus.
Four officials at the World Health Organisation told AP that the Brazilians do not provide up-to-date information to international partners.
"WHO has gotten zero from them, no clinical or lab findings," one of the officials said.
Brazilian law is one of the main problems behind the issue, since it is illegal for Brazilian researchers and institutes to share genetic material, including blood samples containing Zika and other viruses.
As of January 30, Brazilian Health Ministry reported a total number of 4,783 suspected cases of microcephaly since October, while 709 were found to be negative.
The spike in cases prompted WHO to declare an international emergency on Monday.
Dr. Marcos Espinal, director of communicable diseases in the World Health Organisation's regional office in Washington said he hoped the issue might be resolved after discussions between the US and Brazilian presidents.
He said WHO's role was mainly to be a broker to encourage countries to share. He confirmed the estimate of other scientists, Brazil had provided fewer than 20 samples.
"There is no way this should not be solved in the foreseeable future," he said. "Waiting is always risky during an emergency."
Last May, after the cases of Zika emerged for the first time, Brazilian President Dilma Roussef took a step to regulate how researchers use the country's genetic resources. But the regulatory framework hasn't yet been drafted leaving scientists in legal limbo.
Paulo Gadelha, president of the Oswaldo Cruz Foundation, Brazil's premier state-run research institute for tropical diseases said that sending samples abroad is considered a crime.
"Until the law is implemented, we're legally prohibited from sending samples abroad," he said. "Even if we wanted to send this material abroad, we can't because it's considered a crime."
Despite the ban, foreign researchers are able to access samples.Tissue samples from two newborns who died and two fetuses recently examined by the Centres for Disease Control and Prevention was shared with the US.
However, a US official said that wasn't enough to develop accurate tests for the virus or help determine whether Zika is in fact behind the recent jump in the number of congenital defects.
Due to a lack of samples, public health officials across the world rely on older viruses, or discretely take samples from private patients.
Speaking on the condition of anonymity, the US official said the CDC was relying on a strain taken from a 2013 outbreak in French Polynesia to perfect its Zika tests.
US researchers who are trying to sequence Zika's genetic code have been forced to work with virus samples from Puerto Rico for the same reason, he said.
In England, researchers use samples drawn from Micronesia, the site of an outbreak in 2007. The French use samples from Polynesia and Martinique. In Spain, scientists have a Ugandan strain of Zika supplied by the United States. Even Portugal, Brazil's former colonial master, doesn't have the Brazilian strain; the National Health Institute in Lisbon said its tests relied on a US sample from the 1980s.
Some researchers are bypassing Brazil's bureaucracy by getting samples sent to them for testing by a private lab, said Dr. Jonas Schmidt-Chanasit, an expert on mosquito-borne diseases at the Bernhard Nocht Institute for Tropical Medicine in Hamburg.
"It's almost impossible to get samples from the country," Schmidt-Chanasit told AP, referring to Brazil. "It's not going via official government channels. Our source is simply the rich people who want a diagnosis."
A similar problem occurred a decade ago, when Indonesia refused to share bird flu samples with WHO. They claimed that Western scientists would use them to make drugs and vaccines the country couldn't afford.
Lawrence Gostin, director of WHO's Collaborating Centre on Public Health Law and Human Rights at Georgetown University, said there are no rules to enforce government to share information of viruses and samples.
"If countries don't share, the only repercussions they face are public condemnation," he said.