A 19 year old man from Indiana died of the new H1N1 virus on July 5, 2009 two weeks after first experiencing symptoms (WKRC-TV). Today, his 26 year old sister died of the same disease (WLWT) [hat-tip, howmanydays at PFI_Forum]. AFAIK, this is the first recorded fatal cluster involving the new H1N1 virus.
There are several possible explanations for this deadly cluster:
1. Co-incidence. Although all previous deaths have been scattered, at some point two people in close proximity might be expected to die at approximately the same time.
2. Similar risk factors. Some individuals are at greater risk of dying of the new H1N1 virus than others. It is possible that the brother and sister in Indiana shared one or more risk factors.
3. A more lethal virus. It is possible that the virus that infected these two people in Indiana has changed and now has a higher case fatality rate.
Given the implications of possibility 3, it is essential that isolates from both family members be sequenced, compared with each other and compared with isolates from other individuals in Indiana. In addition, the isolates should be tested in animal models to determine if they are more lethal than isolates from individuals with mild disease. As I have argued here and here, there are indications that the new H1N1 is unstable. Some isolates may be intrinsically more lethal. If so, determining whether lethal strains are becoming more common and/or spreading is of the highest priority.