The ultrastructural details exhibited by the new influenza A (H7N9) virus. CDC/Cynthia S. Goldsmith and Thomas Rowe

A new strain of the bird flu has been identified by researchers that remains powerful after becoming resistant to antiviral drugs.

The findings, published in Nature Communications, reveals how an avian strain of influenza A has emerged that can develop a resistance to antiviral medications. Labeled as H7N9, the mutation that originated in China, has no vaccine to protect patients from the potentially deadly virus.

"Many of the people infected with H7N9 during the outbreak in China were elderly or had other conditions that predisposed them to severe influenza illness," lead investigator Dr. Nicole Bouvier from the Icahn School of Medicine at Mount Sinai, said in a press release. "Nevertheless, our study suggests that flu viruses can indeed develop drug-resistant mutations without suffering a penalty in terms of their own fitness."

Doctors studied an outbreak of the H7N9 last spring. The virus infected at least 135 people and caused 44 deaths. Without a vaccine, the only defense is antiviral medication. But, scientists found H7N9 was able to develop a mutation to combat the antiviral medication Tamiflu (oseltamivir), commonly used to treat the virus.

While the virus has a limited ability to spread between humans, doctors should be wary of what kinds of antiviral medications to use to treat patients with H7N9.

"It's important to emphasize that these H7N9 viruses seem to transmit fairly inefficiently overall," Dr. Bouvier told the NY Daily News. "But surprisingly, transmission of the drug-resistant virus was no less efficient than that of the drug-sensitive version."

While older treatments like amantadine are no longer effective, newer antiviral medications like Tamiflu, a pill, and Relenza (zanamivir), a powder that is inhaled, can block an enzyme that helps the virus replicate. In both cases, patients can develop a resistance to the drugs and not every country has the drugs in intravenous form for people with severe infections.

"Our study underscores the need to develop a bigger arsenal of antiviral drugs and vaccines, which will allow us to outsmart the influenza virus," Dr. Bouvier said.

And the virus continues to spread. On Dec. 11, three samples collected from two live poultry markets in southern China tested positive for the H7N9 avian influenza virus. The health authority that conducted the examination concluded that the risk of human infection was “very high,” BusinessWeek reports.

In response to two new cases of the H7N9 virus in Hong Kong, the city has stepped up its preventative measures – watching for travelers with fever, limiting hospital visiting hours and enforcing cleaning rules at live chicken stalls in city markets.

“The Serious Response Level under the government’s preparedness plan for influenza pandemic has been activated while the Center for Health and Protection’s epidemiological investigation and follow-up actions are currently in full swing,” a spokesman for the health center said Friday in a statement.