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Comment by acchow | Hacker News Reader

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

“Public health experts already knew Chelsea had the state’s highest rate of confirmed COVID-19 cases and that the actual rate was probably higher”

Should be 1/3 + in town with highest confirmed case rate. It may be a random sample within that town but it’s explicitly not representative of MA

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

If anyone has good data on the asymptomatic proportion, I'd appreciate a link. I keep seeing 50% (or sometimes 'up to 50%'), but to the extent that I've looked at actual sources, it seems that people are conflating presymptomatic positive tests with lastingly asymptomatic infections.

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/ (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... (Germany study, 0.37%, decent sampling method)

[3]: 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... (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

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.

This also came out today. http://publichealth.lacounty.gov/phcommon/public/media/media... We really have no idea what the IFR is, just that is probably in the .1% to 1% range. Also, it takes around 2 weeks to develop antibodies, so the tests are showing how many people had covid 2 weeks before testing.

That antibody timeline was an estimate derived from data obtained from the "original" SARS virus, but it isn't accurate with the SARS-CoV-2 virus. IgM antibodies are present within hours of the onset of symptoms, and IgG antibodies after a few days.

Source?

Sure, find any manufacturer of rapid antibody test kits and look at their documentation for a nice trailhead into plenty of literature about antibody response times with this virus.

Ok, changed now. Submitted title was '30% of random sample test positive for Covid antibodies in Massachusetts'

Your never going to have a completely random test. Your randomness comes from you picking for an area and the people who chose to do the test. Saying otherwise is being pedantic to your detriment.

It’s normal to randomly sample multiple locations. Random samples from even 10 random locations are much more useful.

Not really... more data can't fix sampling bias.

It’s not a question of ‘more data’ as I could take 50 or 50,000 samples from a single person and I gain nothing about the overall population.

Similarly if I sample 50 or 50,000 people from a single city I learn nothing about people outside of that city. But, by sampling N random locations I at worst have N samples to work with.

PS: At the extreme, if I sample every single member of a population then ‘more data’ solved any bias problems. Smaller samples are subject to a wider range of biases.

Not if you are trying to figure out the best decision for a specific city/state

You can choose to offer people some incentive to take the test. If you offered $50 per person you would probably eliminate most sampling bias.

You're more likely to attract people of lower incomes, which we already suspect are more vulnerable to Covid given the kinds of jobs they have.

> Your never going to have a completely random test.

You are not, but on the full spectrum of somewhat random selection methods, talking to people on the street is about as biased as you can get.

In addition, the wording "test positive for Covid antibodies" may lead people to think that 30% of the population in MA is immune to Covid19. It obfuscates the fact that the mere presence of antibodies does not imply immunity. There has to be a sufficient amount of antibodies in order to successfully fight off reinfection (which could be worse than the initial infection) ...

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