What if an H5N1 pandemic didn’t make anyone sick?

What if H5N1 adapted to humans and caused a pandemic that nobody noticed? Why, that would be quite a relief, most of us would agree. Perhaps it could happen. A few months back I posted about some South Korean poultry workers who had been infected asymptomatically with H5N1. They had antibodies to the virus but didn’t get sick. It seems this has happened again in South Korea.

South Korean officials said on 11 Jan 2007 that the bird flu virus had been transmitted to a human during a recent outbreak among poultry, but the person showed no symptoms of disease. The South’s Korean Centers for Disease Control and Prevention said a poultry farm worker, whom it didn’t identify, had been infected with the deadly H5N1 strain of the virus but developed natural immunity to the disease and wasn’t ill.

The poultry worker was said to have taken Tamiflu, and this was credited with him not getting sick. (I don’t know how solid an assumption that is.) ProMED-mail commenter Dr. Julian Tang has been thinking about these cases and offered his thoughts on the subject:

Surely, more research effort should be put into exploring this particular strain of H5N1, as this may be more typical of a human-adapted pandemic influenza virus. The more lethal, widespread H5N1 influenza virus (Z genotype) is less typical of previous pandemic influenza viruses, which infected millions but with an overall relatively low mortality of just a few percent or less, including the 1918 pandemic.

Do we know whether this South Korean V genotype virus is more transmissible than the more lethal Z genotype or whether it is more or less likely to develop neuraminidase inhibitor resistance? Could it be that the more dramatically lethal Z genotype is attracting so much attention because of its higher mortality in humans but that, ironically, the quieter, less lethal V genotype is the more likely pandemic influenza virus candidate, if one believes that H5N1 influenza is likely to be the next pandemic influenza strain at all?

Some researchers believe that a variant of the human influenza viruses (H3N2, H1N1, etc) already in existence are more likely to be the next pandemic influenza strain than a new zoonotic strain, which has to jump the species barrier and adapt to efficient infection and transmission between humans. If H5N1 was on the way to becoming the next pandemic influenza strain, I would be inclined to look for human cases like those in South Korea, whose mild or asymptomatic clinical picture may well indicate a successful adaptation to humans, suggesting the possibility of many humans having been infected but with only a small percentage of deaths — that is, similar to the previous pandemic human influenza viruses.

Excellent questions that I’m sure many H5N1 researchers are wondering about. I hope his line of thinking turns out to be the reality if H5N1 becomes transmissible among humans, but we probably won’t really have our answer until a pandemic is well underway.