The news reports on the Guangzhou SARS case are getting more and more entertaining by the hour. The latest report indicates that the Guangzhou SARS index case was "slightly exposed" to the SARS coronavirus at some point.
It would be interesting to know how a "slightly exposed" determination was made, when the source of the infection is unknown. I suspect that the "slightly exposed" characterization is somehow related to a low titer, most likely of neutralizing antibodies. However, the level of neutralizing antibodies cannot be measured because the novel SARS CoV has not been isolated. The neutralizing antibodies are directed against the Spike protein and the entire S gene has been sequenced. As noted in several of the media reports, the novel coronavirus is 98.8 to 99.4% homologous to the SARS CoV S gene sequences at Genbank. That means that there are 22-44 nucleotide differences in the S gene when the novel coronavirus is compared to previously reported isolates.
Since the index case made neutralizing antibodies to the novel coronavirus, the low titer is not due to a "slight exposure", but instead is due to antigenic drift. This is well illustrated by recent data on antibody titers against the Fujian and Panama influenza viruses. Ferrets were exposed to the Panama strain and generated a robust titer of 1280. However, when that antisera was tested on the Fujian strain, the titer was only 40-80. The low titer was not due to a "slight exposure" of the ferrets to Fujian, but instead was due to the fact that most of the neutralizing antibodies against Panama failed to recognize Fujian. The number of nucleotide differences in the HA gene between Panama and Fujian is 13, so a difference of 22-44 nucleotides between the antibody SARS CoV test isolate and the novel Guangzhou isolate is significant. even after correcting for the fact that the HA of flu is about 1/3 of the Spike protein of SARS CoVs.
Thus, it seems that more effort should go into finding the source of the Guangzhou infection and less effort into coming up with inaccurate descriptions of the novel SARS CoV infection.
chinapost.com.tw
China patient may have been exposed to SARS
2004/1/3
BEIJING, Agencies
The World Health Organization (WHO) said Friday initial tests suggested a suspected Chinese SARS patient may have been "slightly exposed" to the SARS coronavirus at some point.
This was the preliminary conclusion from a virus neutralization test, which has been used only once so far, the WHO said in a statement, adding a final diagnosis might take days.
"The results from that neutralization test indicated that the patient may have been slightly exposed to the SARS coronavirus at some point, but it was impossible to determine exactly when," it said. |
