Covid-19 Antibody Seroprevalence in Santa Clara County, California

Not concluding prior immunity but concluding that they now have immunity after having been infected. That is in progress towards herd immunity this number of people actually infected being revealed by these antibody studies is what is interesting. And based on my hacky heuristic described above that is currently around 2.6%. So we are still a long way off herd immunity. But at the same time closer than we thought we were.

Saw this comment [1] on Reddit that outlines the limitations of the study well:

This is the most poorly-designed serosurvey we've seen yet, frankly. It advertised on Facebook asking for people who wanted antibody testing. This has an enormous potential effect on the sample - I'm so much more likely to take the time to get tested if I think it will benefit me, and It's most likely to benefit me if I'm more likely to have had COVID. An opt-in design with a low response rate has huge potential to bias results.

Sample bias (in the other direction) is the reason that the NIH has not yet released serosurvey results from Washington:

We’re cautious because blood donors are not a representative sample. They are asymptomatic, afebrile people [without a fever]. We have a “healthy donor effect.” The donor-based incidence data could lag behind population incidence by a month or 2 because of this bias.

Presumably, they rightly fear that, with such a high level of uncertainty, bias could lead to bad policy and would negatively impact public health. I'm certain that these data are informing policy decisions at the national level, but they haven't released them out of an abundance of caution. Those conducting this study would have done well to adopt that same caution.

If you read closely on the validation of the test, the study did barely any independent validation to determine specificity/sensitivity - only 30! pre-covid samples tested independently of the manufacturer. Given the performance of other commercial tests and the dependence of specificity on cross-reactivity + antibody prevalence in the population, this strikes me as extremely irresponsible.

This paper elides the fact that other rigorous serosurveys are neither consistent with this level of underascertainment nor the IFR this paper proposes. Many of you are familiar with the Gangelt study, which I have criticized. Nevertheless, it is an order of magnitude more trustworthy than this paper (both insofar as it sampled a larger slice of the population and had a much much higher response rate). It also inferred a much higher fatality rate of 0.37%. IFR will, of course, vary from population to population, and so will ascertainment rate. Nevertheless, the range proposed here strains credibility, considering the study's flaws. 0.13% of NYC's population has already died, and the paths of other countries suggest a slow decline in daily deaths, not a quick one. Considering that herd immunity predicts transmission to stop at 50-70% prevalence, this is baldly inconsistent with this study's findings.

For all of the above reasons, I hope people making personal and public health decisions wait for rigorous results from the NIH and other organizations and understand that skepticism of this result is warranted. I also hope that the media reports responsibly on this study and its limitations and speaks with other experts before doing so.

[1] https://www.reddit.com/r/COVID19/comments/g32wjh/covid19_ant...

Lots of reasons I don't think you can extrapolate meaningful death rates from this yet. One big one is that time from infection to death covers a very wide range: in Wuhan time from hospitalization to death had both a mean and a standard deviation of about 2 weeks, and time from infection to hospitalization isn't well known yet, but is likely at least a week. Add to that the fact that some cases are diagnosed post-mortem and you might not know about all of the fatalities for early April cases until well into May.

it sounds like there was. Look at the 2019/20 flu season from the cdc. it was about 3rd place in last 10 years despite the country basically shutting down.

Anecdotally, I remember in December / January reading about this being the worst flu season in years (time). it would have been interesting to know if the flu was effecting the normal cohort or ignoring children.

I myself got the 2nd worst sick I have ever been at the end of November visiting San Francisco. (really bad dry cough, fever). It was strange in the sense that I had mild symptoms for a bout two weeks and then 'took my breath away'. went to the hospital, no pneumonia, but low oxygen levels. my friends kid got the same thing, mild cough for a few days and fine. didn't really recover until mid January. Was it covid19? probably not, was probably the flu.

That doesn't discount the possibility this was running around the globe even in December. I don't stick my head in the sand and wait for people to tell me so to consider that's at least a very possible scenario.

https://www.cdc.gov/flu/about/burden/preliminary-in-season-e...

https://time.com/5758953/flu-season-2019-2020/

And last, but definitely not least: you can't draw the conclusion that presence of antibodies equates immunity. It will correlate but there is no guarantee.