Note: Written June 26th
I can state pretty conclusively using data from all over the world that at least 2 people will die from every 100 confirmed cases of COVID-19. I showed that in my past blog. This calculation, we are told, is the Case Fatality Rate (CFR). The Big Kahuna we need to know is the Infection Fatality Rate (IFR).
The IFR is the number of deaths divided by the number of people who contacted COVID-19, This includes those that are symptomatic and those that are asymptomatic. The conventional wisdom appears to be that more people are symptomatic -and therefore - not tested.
There seems to be supporting evidence for this which I want to look at.
Arons et al. now report in the Journal an outbreak of Covid-19 in a skilled nursing facility in Washington State where a health care provider who was working while symptomatic tested positive for infection with SARS-CoV-2 on March 1, 2020. [New England Journal of Medicine]
Here we have a 'ground zero' situation. The assumption here is that this one worker passed on the infection to those in the nursing home.
Residents of the facility were then offered two facility-wide point-prevalence screenings for SARS-CoV-2 by real-time reverse-transcriptase polymerase chain reaction (rRT-PCR) of nasopharyngeal swabs on March 13 and March 19–20.
12 days later the residents were tested with the nasal swab looking for the virus - not the antibodies.
Among 76 residents in the point-prevalence surveys, 48 (63%) had positive rRT-PCR results, with 27 (56%) essentially asymptomatic, although symptoms subsequently developed in 24 of these residents (within a median of 4 days) and they were reclassified as presymptomatic.
This is for the residence - which being in a nursing home - would most likely be impacted by a viral infection like COVID-19.
An important finding of this report is that more than half the residents of this skilled nursing facility (27 of 48) who had positive tests were asymptomatic at testing.
Yet, four days later 24 of those 27 became symptomatic. The reason this is deemed important in this study is they are looking at transmission. I am looking at how many asymptomatic folks would likely never get tested and should be added to the denominator when we do the IFR calculation.
Looking at this data, it would appear that only three out of 76 would never have bothered to get tested as they remained asymptomatic. But these are old people in a nursing home so they are possible much more susceptible to the virus. Still, we can add this to the list in trying to understand what is actually happening in the real world.
Let's look at older people that are not in nursing homes. The Diamond Princess is, I think, the best example of what we might see in the rest of the world. The only exception here is that the passengers were older than the general population and therefore possibly more likely to show symptoms.
On 5 February 2020, in Yokohama, Japan, a cruise ship hosting 3,711 people underwent a 2-week quarantine after a former passenger was found with COVID-19 post-disembarking. As at 20 February, 634 persons on board tested positive for the causative virus. We conducted statistical modelling to derive the delay adjusted asymptomatic proportion of infections, along with the infections’ timeline. The estimated asymptomatic proportion was 17.9% (95% credible interval (CrI): 15.5–20.2%). [Stanford]
Let's assume that for every confirmed case there are at least 17.9% that we do not know about. What we would now see as the IFR - confirmed cases and asymptomatic never tested - would look like this as of today - June 26:
This assumes that the deaths reported as of June 26 came from the reported cases as of June 4 (see previous post on why 22 days). Based on the reported cases we can add 17.9% more cases we suspect are asymptomatic and were never tested.
Still...this is old people data. Let's look at something a bit more representative.
Dig deeper! The rabbit is in there somewhere!
Part 6
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