Ron. Here's more on SARS, Super Spreaders, and Selection:
Although SARS predictions run the gamut from no SARS, to a few cases, to case controlled preparedness, to the Big One, I think it might be worthwhile to review the molecular data, because in the end, SARS is still a VERY molecular story.
Last season SARS was largely driven by super spreader events. The data that will be published in Lancet this week clearly shows that the vast majority of cases in Hong Kong trace back to the Metropole Hotel and sequence analysis of isolates around the world show that most of the significant International spread also traces back to the Metropole Hotel via super spreader events. The transmission at the Metropole Hotel led to outbreaks in Toronto, Hanoi, Singapore, and Hong Kong. The transmission in Hong Kong led to a super spreader event at the Prince of Wales Hospital, which then led to super spreader events at Amoy Gardens and China Air Flight CA-112. Amoy Gardens led to the spread of SARS in Taiwan and CA-112 brought SARS to Hohhot and back to Beijing in Mainland China.
Thus, if there was no transmission at the Metropole Hotel, most of the SARS cases outside of mainland China would not have happened last season. The optimistic forecast would say that with increased awareness, SARS may appear, but will not spread as it did at the Metropole Hotel. However, there were at least 4 SARS cases in Hong Kong which were imported from Guangzhou or Shenzhen before the Metropole Hotel, yet these four cases had little impact, even though there were no special SARS precautions. Thus, the spread of SARS in Hong Kong was controlled less by precautionary measures and more by the circumstances intrinsic to the Metropole Hotel transmission. One factor may have been timing, because the visit to the Hotel was about 10 days post initial symptoms, when viral loads tend to be highest. However, the Metropole index case was clearly infected with a number of variants, each readily distinguishable from other mainland isolates introduced into Hong Kong at the time.
Molecular analysis shows that from the number of mutations in the SARS CoV genome increased quite significantly between the national Guangzhou isolates and the Metropole Hotel explosion. Thus the real question centers around the likelihood of another Metropole Hotel incident. If there are none, then this year the number of fatal cases will be significantly reduced. If there is one, the greater awareness will also probably play a significant role. However, what would happen if there were 10 or 100?
The chances of 10 or 100 Metropole Hotels is almost certainly related to the molecular clock of the SARS CoV. The first two cases in Guangzhou have been relatively mild. Both patients were ill for about a week and then rapidly recovered. However, both cases were relatively young, and the sequence of the Spike gene of the first case is a more humanized version of civet isolates and in fact is an exact match of a "humanized" civet isolate. Last year, it looks like the molecular clock was set near zero at the end of 2002 and the number of mutations in the Metropole Hotel isolates in late February was close to 50.
The "humanized" civet isolate certainly seems to be well above 0, but of course all mutations are not equal, and their evolution is dependent on selection of a number of fairly random events. Thus, close watch of the sequences and the calendar may provide more information about this seasons outbreak, than press releases or preparedness lessons. |
