In a new study published today, researchers have discovered that the Chinese bird flu strain, A(H7N9), has a mutation immunizing it to common anti-viral drugs. While this doesn’t necessarily herald cause for concern of a pandemic, the researchers recommend using anti-viral drugs with caution.

Many seasonal influenza strains become less transmissible after they become resistant to drugs such as Roche’s Tamiflu. But the scientists found that the avian influenza strain, doesn’t work in the same way. It maintains its ability to be transmitted, even after it develops an immunity to drugs. The researchers urge doctors to make wise choices when handing out anti-viral medications for H7N9 cases — they should avoid using common drugs like Tamiflu, which could help build resistance in the virus, the researchers suggested.

“Without baseline human immunity to the emergent avian influenza A(H7N9)… [anti-viral drugs] are vital for controlling viral replication in severe infections,” the study authors wrote in their abstract. Nicole Bouvier, a lead author of the study published in Nature Communications, told Reuters that it’s important to know that “these H7N9 viruses seem to transmit fairly inefficiently overall. But what was surprising about our study was that the drug-resistant virus was no less efficient than the drug-sensitive one. Usually what we see with influenza, is that resistance…also confers a fitness disadvantage on the virus.”

The researchers had received a sample of the H7N9 virus from a patient in China. They tested its response to Tamiflu, or oseltamivir, which is commonly used to treat the flu. Bouvier found that H7N9 was resistant to Tamiflu but that it could still spread to human cells in laboratory dishes.

The virus was first confirmed in a human earlier this year in China. Since March, there have been 143 people infected with H7N9 in China. Forty-five of them have died. Even back then, scientists worried about the extent of the damage H7N9 could produce, but so far it hasn’t caused a pandemic. Perhaps this is in part due to the strain’s difficulty in being transmitted from human to human. Today, the World Health Organization (WHO) reported two new bird flu patients in China, who are currently in critical condition. Reuters reports that another team of researchers in the U.S. has also been studying the H7N9 virus, and claims that it would need to undergo several mutations before becoming easily spreadable through humans. Currently, it is mostly transmittable through infected birds or other animals, or by handling contaminated poultry.