The worst possible case is a virus that spreads easily, is highly lethal and is resistant to antivirals. This was a theoretical possibility.
From channelnewsasia.com, November 27, 2009:
Two patients who were infected by a H1N1 flu mutation that was also recently detected in Norway have died in France, health officials here said Friday.
“This mutation could increase the ability of the virus to affect the respiratory tracts and, in particular, the lung tissue,” said a statement from the government’s Health Surveillance Institute (InVS).
“For one of these patients, this mutation was accompanied by another mutation known to confer resistance to oseltamivir,” it added, referring to the main drug being used to treat H1N1 flu, under the brand name Tamiflu.
We do not know what the case fatality rate will be from this new, double-mutant strain of H1N1. Although the D225G mutation has reportedly been found in mild as well as severe cases, we do not know whether the “mild” cases had received Tamiflu. If so, it may have been that Tamiflu prevented a fatal outcome. Since the new mutant is resistant to Tamiflu, spread of this strain may result in a very high death rate.
Thus far, there is only one report of a patient with the more lethal, Tamiflu-resistant virus. However, it is entirely possible that there are more, especially in countries that are less than transparent about their cases and deaths.
I’m looking at you, China.