
Nearly a third of blood samples taken in Chelsea show exposure to coronavirus - rpiguy
https://www.bostonglobe.com/2020/04/17/business/nearly-third-200-blood-samples-taken-chelsea-show-exposure-coronavirus/
======
acchow
Can we change the title?

Article title is "Nearly a third of 200 blood samples taken in Chelsea show
exposure to coronavirus"

The sample is clearly not random

~~~
arenaninja
Yep.. I saw a similar headline claiming LA infections are 55x the reported
rate. I think the math is likely simpler, and the infection is somewhere
between 3x-5x (10x tops). I've read a few reports saying about 50% of people
are asymptomatic, 30% show symptoms that aren't bad enough for hospitalization
and 20% require hospitalization. I can't find the source I got it from but I
believe it was a Chinese study

~~~
spion
Based on several studies using different methods, the death rate (infection
fatality rate, or IFR) is likely around 0.5%

So if you go back the average time from infection to death (2-3 weeks) you can
get a sense what the ratio was in your area back then.

[1]: [https://mothership.sg/2020/03/iceland-
covid-19/](https://mothership.sg/2020/03/iceland-covid-19/) (I did the work on
the side, extrapolated to about 0.2%-0.5%)

[2]:
[https://www.land.nrw/sites/default/files/asset/document/zwis...](https://www.land.nrw/sites/default/files/asset/document/zwischenergebnis_covid19_case_study_gangelt_0.pdf)
(Germany study, 0.37%, decent sampling method)

[3]:
[https://www.ncbi.nlm.nih.gov/pubmed/32234121](https://www.ncbi.nlm.nih.gov/pubmed/32234121)
(Diamond princess ship, 0.5%)

[4]:
[https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v...](https://www.medrxiv.org/content/10.1101/2020.04.14.20062463v1)
(Santa Clara county study, 0.15% - 0.3% if you do the math). Methodological
problems with the method of sampling that might increase the number of
positives and the test specificity issues might increase false positives, but
probably still in the ballpark

~~~
Retric
Dimond Princess had 14 deaths out of ~700 infected or a 2% fatality rate
though that number has slowly climbed over time. As they tested everyone
initially finding a large number of asymptotic cases that’s likely extremely
accurate.

The difference is the population was more heathy than the general population,
and while older on average, it had a lower percentage of people 95+ years old.

------
trav4225
Hmm, I wonder if they attempt to control for bias. It sounds as though they
were asking random passersby if they wanted a COVID-19 antibody test. If so,
it seems that the sample group could be biased toward including more people
who suspected that they may have been infected (previously or currently) than
a truly random sample.

~~~
jwegan
Also biased because they are sampling people who are out and about. People who
are out and about likely come into contact with more people than people who
are staying at home and rarely going out.

~~~
bitcurious
Out, about, and willing to speak to a stranger, which I think is the biggest
bias. Pretty much everyone I know goes on the occasional walk, but we all stay
the fuck away from other people.

~~~
hinkley
My Hollywood movie plot theory is that Covid affects inhibitions.

~~~
youareostriches
It’s certainly not a hollywood movie—for the flu, it’s already the case!

Change in Human Social Behavior in Response to a Common Vaccine

[https://pubmed.ncbi.nlm.nih.gov/20816312/](https://pubmed.ncbi.nlm.nih.gov/20816312/)

 _Conclusions: These results show that there is an immediate active behavioral
response to infection before the expected onset of symptoms or sickness
behavior._

------
Exmoor
>Nearly one third of 200 Chelsea residents who gave a drop of blood to
researchers on the street this week tested positive for antibodies linked to
COVID-19, a startling indication of how widespread infections have been in the
densely populated city.

>The doctors used a diagnostic device made by BioMedomics, of Morrisville,
N.C., to analyze drops of blood. It resembled an over-the-counter pregnancy
test and generated results on the street in about 10 minutes.

This is the only recent testing I've seen that appears to be anything remotely
resembling random. The sample size is also not incredibly small. That said,
this is the first I've heard that there's a handheld device which can deliver
an antibody test in 10 minutes and being completely out of that industry I
have to wonder how likely it is that it's delivering a high level of false-
positives?

~~~
epmaybe
Because you quoted the article using BioMedomics' antibody diagnostic test, I
will quote the accuracy from their site[0]:

> In order to test the detection sensitivity and specificity of the COVID-19
> IgG-IgM combined antibody test, blood samples were collected from COVID-19
> patients from multiple hospitals and Chinese CDC laboratories. The tests
> were done separately at each site. A total of 525 cases were tested: 397
> (positive) clinically confirmed (including PCR test) SARS-CoV-2-infected
> patients and 128 non- SARS-CoV-2-infected patients (128 negative). The
> testing results of vein blood without viral inactivation were summarized in
> the Table 1. Of the 397 blood sample from SARS-CoV-2-infected patients, 352
> tested positive, resulting in a sensitivity of 88.66%. Twelve of the blood
> samples from the 128 non-SARS-CoV-2 infection patients tested positive,
> generating a specificity of 90.63%.

[0]: [https://www.biomedomics.com/products/infectious-
disease/covi...](https://www.biomedomics.com/products/infectious-
disease/covid-19-rt/)

~~~
skybrian
Specificity of 90% seems really low? Does that mean almost 1 in 10 would test
positive even if nobody had it?

~~~
DenisM
Yes, it's 10% false positive. But it's not "really low" in the sense that it's
typical for antibody tests. Which is one reason not to do wide-spread testing.

~~~
subroutine
Or you could flip that around and make testing so plentiful everyone is tested
3x (perhaps by independent labs), so the FP rate is more like...

1/10 * 1/10 * 1/10 (1/1000)

~~~
ejstronge
This would require that false positives are truly random and not influenced by
sample-specific factors. Is this the case? In antibody testing, I would
imagine that false positives result from cross-reactivity instead of random
chance.

~~~
subroutine
Sure, there are certainly protocols that could be made that would almost
ensure FP is due to sample factors.... or even test related factors that would
maximize 3x FP (same lab, same lab tech, same machine, same day, back to back,
same reagent stock).

We shouldn't adopt those protocols.

------
rogerkirkness
If the R0 is truly near six, we have probably climbed near the top of the S
curve already all over the place. We will know in a month.

------
tanilama
> The 200 participants generally appeared healthy, but about half told the
> doctors they had had at least one symptom of COVID-19 in the past four
> weeks.

The total sample size is small and this doesn't seem random enough?

~~~
Hermel
You should be much more worried about bias than about sample size. See also
this calculation, where it is discussed that a sample of 400 is more reliable
than a slightly biased sample of a million participants:

[https://marginalrevolution.com/marginalrevolution/2020/01/bi...](https://marginalrevolution.com/marginalrevolution/2020/01/big-
datasmall-bias.html)

------
ltbarcly3
This isn't random at all, lots of people are staying inside and going out to
the store once a week or less, they will be dramatically under tested with
this methodology.

Meanwhile other people are going out every day, or they know they previously
had covid so they consider themselves exempt from shelter in place, or they
have a job that requires them to continue heading out to work despite shelter
in place (and therefore they have had far more exposure to covid than
average).

------
scythe
A third of a random sample taken in _Chelsea, Massachusetts_ :

[https://en.wikipedia.org/wiki/Chelsea,_Massachusetts](https://en.wikipedia.org/wiki/Chelsea,_Massachusetts)

>Chelsea is [...] directly across the Mystic River from the city of Boston.
[...] It is also the second most densely populated city in Massachusetts
behind Somerville.

A random sample in a highly affected and extremely dense area is not a model
for the whole state.

~~~
lubujackson
Chelsea is literally the hardest hit town in MA:
[https://www.nbcboston.com/news/local/the-20-communities-
with...](https://www.nbcboston.com/news/local/the-20-communities-with-the-
highest-rate-of-coronavirus-cases-in-mass/2108780/)

I have some natural doubts over all these antibody tests that indicate herd
immunity while covering up major flaws...

------
neonate
[http://archive.md/UDrho](http://archive.md/UDrho)

------
point78
So halfway to herd immunity?

~~~
Exmoor
It was discussed in another thread that I'll try to dig up, but the percentage
of the population required to reach herd immunity goes up the more contagious
a virus is. If way more people are infected then we think, then it points to
some of the higher R0 estimates being accurate and would therefore require a
much higher portion of the population to be immune before we reach herd
immunity.

Edit: Here it is:
[https://news.ycombinator.com/item?id=22819057](https://news.ycombinator.com/item?id=22819057)

~~~
james_s_tayler
My understanding is it's (R0 - 1)/R0

So if R0 is 4 then 3/4 of the population is required to be immune before herd
immunity kicks in.

~~~
retsibsi
Yeah, I'm not an epidemiologist but I think the principle is simple: it's just
a matter of getting the actual transmission rate below 1, so that any outbreak
will naturally die out. If in the absence of any immunity the average infected
person infects 3 others, but now 2/3 of people are immune, then the average
infected person will actually only infect one other. Ditto for a transmission
rate of four and 3/4 immunity, and so on.

It's worth noting, though, that the natural tendency during an epidemic is for
the total number of infections to exceed the herd immunity threshold; I think
the phrase to google is 'herd immunity overshoot'.

(It's also worth noting the serious problems with this study, as pointed out
by other commenters.)

~~~
Gibbon1
> herd immunity overshoot

I saw that too when mucking with simple models. The percent infected
overshoots (R0 - 1)/R0. So that is a threshold for herd immunity not the
ultimate infection ratio. They're only equivalent under steady state
conditions.

Also saw an study released by the CDC that estimated that the initial r0 in
Wuhan was 5.8. Explains what happened in Northern Italy and New York. The
epidemic achieved break out while most infected were still mildly ill or
asymptomatic.

------
ssivark
Does this account for the rate of false positives, compounded by the low base
rate of disease incidence? When designing such binary estimators, we have a
trade off between precision & recall. If the virus affects 1% of the
population, and we have a 5% rate of false positives, and close to 100%
recall, then ~85% (5:1 odds) of tested positives would be spurious.

------
duxup
It would be nice if it was more widespread than we think. Presumably that
would mean we would be closer to the end of the tunnel?

------
ScottFree
Does anybody how these antibodies are being identified and how distinct they
are from other, similar viruses (like the flu)?

------
Rantenki
Alright. So what's the rate of false positives for that test?

In other words, if this test (and similarly the stanford study in LA) have
even a small rate of false positives, combined with any sampling bias, it's
going to lead to some misleading conclusions.

------
cameldrv
Has anyone been able to figure out what the number of COVID deaths are in
Chelsea specifically?

~~~
contravariant
The article claims 712 detected cases resulting in (at least) 39 deaths.

Chelsea has a population of about 40k, this isn't mentioned in the article but
matches the article's figure of around 1900 cases per 100,000.

Assuming 30% of the population is infected this puts the current number of
deaths at around 0.3% of the total infected. There is some bias due to the
rate of false positives of the test and the lag in the number of deaths, so
the actual fatality rate may well be higher, but the order of magnitude seems
consistent with the numbers discussed in previous threads [1].

[1]:
[https://news.ycombinator.com/item?id=22901311](https://news.ycombinator.com/item?id=22901311)

------
kanzenryu2
There have been reports of tests giving up to 30% false positives

------
DougN7
This sucks, but also means we’re inching closer to the 70-80% necessary for
herd immunity so maybe a silver lining?

------
wtvanhest
This is another irresponsible headline.

I cannot read the article, so correct me if I am wrong, but if what others
said is true...

It was not random because they didn't randomly select people, they let people
self-select!

If they gave the total number of people who took the test, and total number of
people asked, you could at least get a range.

Of course, that range would still be biased to people who are walking around
during a time when they should be sheltering in place.

At best, 30% of people who were offered a covid antibody test in public had
antibodies.

I think its safe to say that journalists have a responsibility to get the
headlines right on this topic or they risk causing a lot of people to make bad
decisions after hearing that 30% of people already have this disease. There is
a real human cost of this click-bait, and it will be deaths.

~~~
fortran77
This is a strong response from someone who claims not to have read the
article.

~~~
retsibsi
Is it wrong, though? I did read the article, and I don't think so.

One relevant thing GP may have missed is that the researchers "excluded anyone
who had tested positive for the virus in the standard nasal swab test". So
they weren't quite trying to directly estimate the proportion of people with
antibodies, but the proportion of _people without a positive test_ with
antibodies. But I suspect this didn't change the results much: how many people
in the area have tested positive _and_ recovered sufficiently to be out in
public?

