Latest from Dr. Niman on The Agonist board:
Up until now, the vast majority of SARS can be traced to other SARS cases, which is why I said the tracing has thus far done a fairly good job of connecting SARS cases that meet the case definition. The related issue however, is the spread of the virus, which is what I believe was being detected in the 10-20% of patients who were PCR positive but failed to meet the case definition. Moreover, recent data connecting SARS I with SARS II suggest that actual SARS cases have also been missed because of the contact requirement, and this can have a snowball effect, particularly in mild cases. Thus, just as the SARS transmission chain can be halted with quarantine, the contact tracing chain can be halted by failing to follow-up mild cases.
Widespread screening with a reliable test can resolve the issue of viral spread. There are three tests in widespread use (not counting actual isolation of the virus). The PCR test used in Canada uses the same probes as tests of others. It is designed to be quite specific, but its sensitivity is reduced because the viral titers can fluctuate markedly and generally peak 10 days post initial symptoms. If a sample is collected too early or too late, or from the wrong anatomical site, the PCR test can generate a negative, even though the virus is present in the patient before or after the sample is collected.
Testing for antibody is more reliable, but again timing is a consideration. Testing prior to 21 days post initial symptoms can generate a false negative because samples collected less than 21 days post symptoms do not yet have a high enough titer. The CDC actually uses two tests for the antibody. The ELISA is for broad screenings and then positives are tested in an immunoflourescence assay. It is also possible that older or immunocompromised patients will have lower titers or develop antibodies later.
However, all of the above limitations address false negatives. False positives should be quite rare, especially if two positive results are obtained for the antibody and care is taken to avoid carry over in the PCR tests.
In the US, the number of positives has been exceedingly low. There have only been two cases positive by PCR (a women returning from Hong Kong in March and the NC index case who was reported as positive on Friday). There have been eight positives for SARS antibodies, including the two most well known cases from Toronto (the Pennsylvania resident after attending the religious retreat and the NC index case after visiting a hospitalized relative). All 8 positives in the US were probable cases which was 20% of the 41 tested. None of the 134 suspect case have been positive for antibody.
The US however, has had no reported deaths (although the NC-coworker status is still under investigation) and the suspect cases are largely patients with a history of travel to an affected area, not necessarily travel to impacted locations such as hospitals. Although details of the positives in Canada are lacking, it seems likely that some of the most suspicious cases are patients, health care workers, or relatives/contacts of the above. Of note was the first link connecting SARS I to SARS II in Toronto. Even though she had symptoms, as did her daughter, who was a health care worker, she was just declared positive last week even though she developed symptoms in mid-March. Thus, at best the data or data analysis are lagging (similarly the 96 year old index case of SARS II and 4 relatives were not declared to have SARS until associated health care workers began developing symptoms).
Thus, the percent positive will be greatly influenced by who is tested. Someone who returns from Toronto after visiting a relative is an unlikly victim of SARS CoV infection. However, someone attending a religious retreat with known SARS victimes or visiting a hospital caring for SARS patients seems to have a much greater risk.
Of course those most likely to have been infected with SARS would be those most closely linked to SARS cases. The transmission chain from SARS I to SARS II shows that even these patients are discovered long after the fact, and it is not clear how much tracing was or will be done on contacts. Until these contacts test negative with a reliable test, the significant SARS CoV transmission in mild cases will remain a very real possibility.
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Spotlight / SARS / Tracing Co-Worker Contacts
on: Today at 06:32:29am
The story below mentions monitoring of 40 employees of the Western Wake Medical Center who came in contact with James D Reed. The unanswered questions in the story below are the following:
Did James Reed go to work during the week preceding his hospitaliztion and if so, are those work related contacts being monitored?
What contacts did he have on the weekend just before he checked into the hospital and are they being monitored?
Are his family members being monitiored?
http://newsobserver.com/news/story/2619158p-2429801c.html