The last few months of 2005 saw extensive media coverage of a strain of avian influenza, H5N1, which has been documented as infecting a relatively small number of people in Asian countries. Unfortunately, this coverage has sparked unnecessary fear about the possible spread of this flu subtype among people and/or among poultry. Unfounded fears can result in inappropriate actions that do more harm than good. Therefore, it is important for everyone to understand some basic facts about avian influenza H5N1. These facts should help individuals avoid inappropriate actions and to decide when, if ever, they might need to alter their behavior due to this illness. Most of the following explanations are from public information released by the federal Centers for Disease Control (cdc.gov), the American Veterinary Medical Association (avma.org), and the World Health Organization (who.int/en/). A wealth of additional information is available from those sites and they are continuously updated to reflect any emerging news about H5N1.

First, what does the designation "H5N1" mean, and what is the relationship of this subtype to other forms of avian flu? There are two groups of proteins found on the surface of avian influenza viruses, hemagglutinin ("H") proteins and neuraminidase ("N") proteins. Each of these groups of proteins has many different types-there are 16 different types of hemagglutinin proteins, that is, H1, H2, H3, etc.; and there are nine different types of neuraminidase proteins, that is, types N1, N2, N3, etc. As can be seen from multiplying the numbers of the two types, there are 144 possible combinations of the different H and N proteins, and thus, 144 different subtypes of the avian influenza. As a practical matter, the important thing to remember is that H5N1 is just one of the 144 different subtypes of avian influenza. Thus, just because you may hear of an occurrence of avian influenza, that does not necessarily mean anything especially serious is occurring. In fact, most of the types of avian influenza are low-pathogenicity strains ("LPAI" or "low-pathogenicity avian influenza"), which do not cause any serious illness in either birds or people. For example, an LPAI strain was discovered at a Pennsylvania live bird market in late 2005, and while the market was temporarily suspended and its premises disinfected, authorities did not feel any danger was presented by this strain and the market was soon reopened. (Lancaster Farming, Dec. 3, 2005, p. A21; Dec. 24, 2005, p. A42.)

But what dangers may be presented by the H5N1 strain? It is important to note that, as of the end of 2005, the most significant danger, namely, the spread of H5N1 widely among the human population, remained theoretical. That is, virologists believe that perhaps a strain like H5N1 could mutate into a form allowing human-to-human transmission, which might cause widespread illness because humans would have no previous exposure to this strain. However, there is as yet no evidence of human-to-human transmission. Nearly all of the people who have thus far contracted H5N1, all residing in the Asian nations of China, Thailand, Vietnam, Cambodia and Indonesia, most likely contracted the illness from close contact with domesticated birds.

Ed. note: Just before we went to press-and after this was written-deaths have also been reported in Turkey.

But isn't the H5N1 strain especially dangerous to people? Although some of the press coverage may imply such dangerousness, at present, noone has any way of knowing if that is true. If anything, the documented human H5N1 cases to date may suggest that it is not especially easy for humans to contract this virus. The first known appearance of this virus was in Hong Kong in 1997, when 18 cases were documented, resulting in six deaths. Then, in 2003, the strain was documented in three people in Vietnam, all of whom died. In 2004 human cases occurred in Thailand and in Vietnam, and in 2005 human cases appeared in those two nations and also in Cambodia, China and Indonesia. Adding together the 2003-2005 cases, there have been 142 documented human cases during those three years, and 74 documented human deaths. (For information on cumulative numbers of documented cases of H5N1 in humans, see who.int/csr/disease/avian_influenza/en/). These occurrences come from a very populous part of the world-the combined population as of 2005 of the five countries where H5N1 has appeared in humans is nearly one and three-quarter billion people-nearly six times the population of the United States and almost one-third of total global population. (Source: census.gov/cgi-bin/ipc/idbrank.pl). Moreover, poultry, particularly poultry purchased at live-bird markets, are a very common food source in the nations where the human cases have occurred. Therefore, one would have to say that the number of confirmed cases is comparatively limited-only 142 known cases in the past three years, and when one includes the 1997 Hong Kong cases, only 160 documented cases over a period of nearly a decade.

But isn't the apparently high mortality rate a cause for great concern? The fact is, no one can have any clear idea of a "mortality rate" for human H5N1 infections overall from the very small number of documented cases. (See "Who's Counting: Flu Deaths...," abcnews.go.com/Technology/print?id=1432589). True, of the documented cases, about half the infected people have died. But there is really no way whatsoever to know how many people may have had H5N1. The 160 documented cases since 1997 are people who ended up in sophisticated urban-area hospitals because they were very seriously ill. We do not know if there may have been many (or any) other cases of people who have contracted the H5N1 strain and who had only moderate or mild illness, or perhaps even no noticeable symptoms at all. By comparison, consider that when West Nile virus first came to the United States several years ago, health authorities were concerned that it might cause considerable serious human illness. However, it has turned out that 80% of people exposed to West Nile virus have no symptoms at all. Serious illness from West Nile occurs only in less than 1% of infected people. Similarly, in relation to avian flu H5N1, it is too early to say anything definitive about an overall mortality rate or about how serious the disease is likely to be in humans who contract it, because the number of documented cases to date is quite small and because only the severest cases thus far have been documented.

Will antiviral medication help against H5N1? There has been much discussion of the antiviral drug oseltamivir, marketed under the brand name Tamiflu by Roche Laboratories Inc., as a possible treatment for H5N1; in fact, the U.S. government plans to stockpile enough Tamiflu to treat 25% of the population in the event of an H5N1 outbreak. The scientific data available concerning Tamiflu indicates it might be at best of limited help against H5N1. Tamiflu was approved by the FDA on the basis of clinical studies of patients having unspecified influenza A (not H5N1) infections. Based on the patients' self-assessments of the severity of their flu symptoms as "none," "mild," "moderate" or "severe," Tamiflu was found to shorten the duration of flu symptoms by about one day to perhaps 1-1/2 days. Five Vietnamese patients who contracted H5N1 in December 2003/January 2004 were treated with oseltamivir; four of these patients died and one recovered. (New England Journal of Medicine, December 22, 2005, vol. 350: 1179-1188). However, the usefulness of the oseltamivir treatment for these Vietnamese patients may have been compromised by the fact that the drug might have been administered too late. (Id.) Generally, patients should be given oseltamivir as soon as possible after the appearance of flu symptoms. In addition, a study has indicated that some H5N1 viruses will mutate to a form resistant to oseltamivir. (New England Journal of Medicine, December 22, 2005, vol. 353: 2667-2672). At best, therefore, it is uncertain how effective oseltamivir/Tamiflu would be against H5N1. A U.S. stockpile of Tamiflu may nonetheless prove helpful against some other strain of influenza. Experts advise against individuals stockpiling their own Tamiflu because of potential problems with incorrect doses or possible side effects from the drug.