THREAD: There’s uncertainty how many Americans were already exposed to #COVID19 and developed immunity. Until we do widespread serological testing to see who has antibodies, we won’t know for sure. Experts, models put number in single digits, perhaps 2-5% of U.S. population. 1/n
The excellent Trevor Bedford of the Hutch had a good thread on this yesterday. I’ll recap - and build on - some of his characteristically insightful points. 2/n
Serology (testing for antibodies people produce after being exposed to #COVID19) tells you where you’ve been. It doesn’t tell you where you’re going. In other words, it reveals where infection has spread. But doesn’t diagnose active infection or tell you where it’s spreading. 3/n
Serology surveys can have value. For example they inform public health decisions. If local population has high incidence of prior exposure (and some immunity to) #COVID19 then measures you take in setting of outbreak may be more austere vs area where prior exposure is minimal 4/n
Serology surveys can also inform workforce decisions. In some professions where there’s a lot of exposure to people (healthcare workers, food handlers, TSA agents) there could be higher prior exposure and rates of some immunity. That can help some people return to work sooner 5/n
But serology shouldn’t be used to try and diagnose active infection. For that, given the current technology we have, we need PCR based tests that screen for active virus. Serology should only be relied upon for telling you who has already been infected. 6/n
So how many people have been exposed to #COVID19, and have antibodies? This report estimates total number of infections in different European countries based on observed cases and deaths, generally finding between 1% and 15% of population was infected. 8/n imperial.ac.uk/medicine/depar… 
In U.S., experts suggest to me rate is lower. My informal survey of thought leaders in this area puts estimate anywhere from 1-5%. Ultimately we must confirm this with serology studies (which are underway). But it falls short of the 50-70% needed to achieve “herd” immunity 8/n
Serology tests are hitting the U.S. market. Ortho is mass producing serologic tests to run on its current analyzers in more than 1,000 U.S. hospital and reference labs. Each can process 150 tests an hour. By May, it’ll be making one million tests/week. 9/n axios.com/serological-te…
Ultimately, a question is how presence of antibodies correlate with immunity; and how long immunity lasts. Some studies suggest antibody levels can vary in different populations, with younger people producing less. Does that mean they have less immunity? scmp.com/news/china/sci…
Nonetheless, sero-surveys can inform clinical and public health decisions and will be important part of understanding and addressing spread of #COVID19. The @CDCgov has begun large sero-surveys. We should have insight into U.S. covid incidence soon. 11/n statnews.com/2020/04/04/cdc…
