Cold, Flu, & Cough Health Center

No Bird Flu Pandemic -- Yet

Feb. 22, 2005 -- The CDC is "extremely concerned" over the "very ominous" threat of bird flu, CDC officials say.

It's not a new worry. Public health officials have been worried for years that a particularly nasty form of bird flu -- H5N1 influenza A virus -- will learn how to spread easily from human to human. That hasn't happened yet.

If it did, the impact on world health would dwarf the SARS outbreak, says CDC medical epidemiologist Tim Uyeki, MD.

"SARS is not that contagious, although there were super-spreading events," Uyeki tells WebMD. "Most people with SARS did not transmit the disease to many social contacts. In contrast, human influenza virus is very contagious. With a pandemic H5N1 influenza virus there would be no pre-existing immunity -- most people would be very susceptible -- so there would be efficient transmission to social contacts. … If it started where an infected person could get to Bangkok or Ho Chi Minh City and get on a plane, the spread could be very fast." Are you worried about bird flu? Take our poll.

How likely is it this really will happen? Uyeki is quick to note that nobody really knows. All kinds of relevant information are missing. But the little that is known is pretty scary.

So far, the World Health Organization (WHO) knows of 55 human infections with the H5N1 bird flu virus -- 37 in Vietnam, 17 in Thailand, and one in Cambodia. It killed 42 of these people. It's likely that there have been many more cases than this, many of them probably fatal. Most people probably get milder cases, but that's far from clear. Only people with severe disease show up in hospitals.

According to the CDC, the reported symptoms of avian influenza in humans have ranged from the typical flu-like symptoms of fever, cough, sore throat, and muscle aches to eye infections, pneumonia, acute respiratory distress, viral pneumonia, and other severe and life-threatening complications.

"Of the 11 cases since the end of December 2004, 10 were fatal. That is very alarming," Uyeki says. "It is quite likely that there are more severe and fatal cases that have occurred and this is an underestimate of impact among humans. The surveillance has not been done for milder illness or asymptomatic infection. We are simply picking up cases of severe disease, among which a high proportion is fatal."

The vast majority of these human infections came directly from chickens or ducks. There's been one case of possible human-to-human transmission (the tragic case of a young mother who caught it after holding her dying child overnight). There may have been others, but there's been no ongoing bird flu spread among humans.

"The good news is these viruses have not yet acquired the ability to transmit from person to person in a sustained manner. The key word is sustained," Uyeki says. "There have been a few instances of limited person-to-person transmission. This highlights the potential for a pandemic in the future."