SARS-2 antibody confusion - IgG and IgM

[PeterZ]

 

Recent antibody testing in NY state & CA (LA and Santa Clara) suggest that at least 10 times the known infected have already contracted and resolved the virus  

My question - for positive SARS-2 antibody test, in conjunction with neg PCR test, how does one know elevated antibodies are due to SARS-2 infection vs flu, or some other infection, as IgM persists many months after a flu infection resolves?  Are there subtypes of IgG and IgM specific to SARS-2?

[CharonY]

Yes the immunoglobulins have different binding specificity. For these types of assays you use an antigen that is specific to what you want to detect (i.e. specific to SARS-CoV-2) and thereby  only detect those immonoglobulins that are (at least in theory) indicative of a COVID-19 infection. Of course these tests have a limited specificity, which usually needs to be validated in trials. Current assays went through a fast approval process and can be a bit less specific than for other types of tests.

[J.C.MacSwell]

I keep hearing the death rate may be much closer to common influenza based on antibody tests. But wouldn't that be true (lower death rate) of flu also, if they tested for antibodies to the flu?

Also they talk about the denominator for the death rate being likely much greater than known cases, but seem to want to ignore the fact that the numerator also, though to a lesser extent, will be greater than current deaths for those currently infected.

I'm not against loosening some restrictions in a controlled manner where reasonable to do so, but this is far from a common flu, even with all the steps we are taking to mitigate it.

Edited April 25, 2020 by J.C.MacSwell

[CharonY]

That is a bit more complicated and I lack time to elaborate it right now. But one issue is definition, in clinical one often uses fatalities after detection of symptoms for practical reasons. Assessing likelihood after infection is another one which has other uses but due to the ongoing situation reports are a bit of a weird mix of both.

Another aspect is that if true infections are higher, it means it is less lethal, but spreads more than anticipated. While implications are different, clinically it still means an increased burden to the health care system, as you mentioned.

And again, I thing comparison with flu are problematic, as many think it is harmless. Yet they result I'm significant annual seat tolls (even with vaccine).