At present, there are four particular strains of influenza that are of concern to Americans. And although new strains evolve regularly, the Centers for Disease Control and Prevention (CDC) monitor for new outbreaks and prepare routinely whenever a new virus with pandemic potential is identified, including developing a candidate vaccine virus to make a vaccine if needed.
Avian Influenza A (H7N9) Virus
Human infections and continued poultry infections with a new strain of avian influenza (H7N9) continue to be reported in China. While there have been mild cases reported, most patients who contracted this strain of influenza developed severe respiratory illness, and some have died. To date, there has only been one case identified that took place outside of China, in Malaysia, and the strain has not yet been identified in people or poultry in the United States.
Chinese authorities are investigating transmission of this strain of influenza. Most of the infected have reported that they had direct contact with poultry; however, some cases reportedly had no such contact, so close contacts of patients are being monitored to determine whether human-to-human spread of H7N9 is possible.
Human infections of avian influenza (also called AI or bird flu) are rare but typically occur after direct exposure to infected poultry. Limited person-to-person spread has also occurred in the past, most notably with avian influenza A (strain H5N1). While it is possible that human-to-human spread of these viruses is possible, it is most important that these transmissions not be ongoing.
Because these viruses continue to mutate, it is possible that such a strain of flu virus could become able to be spread easily and sustainably among humans, which would trigger a flu pandemic. The CDC follows situations like these closely and coordinates with domestic and international partners, issuing guidance to medical facilities and public health authorities in the United States, as well as providing information for international travelers headed to China.