Here's the link to CDCs site showing the numbers I was referring to:
https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/burden.html
Its from Feb 20 to Mar21, so some infections may not be recent enough to confer meaningful immunity, but the large 3rd wave in 4Q20-1Q21 would be.
CDC considers
full vaccination or recent infection and recovery within the last 3 mos to not require quarantine post travel. This is prob a starting timeframe for natural immunity (tho less effective than vaccination.)
"To better reflect the burden of COVID-19 – the full impact of the disease — CDC provides estimates of COVID-19 infections, symptomatic illnesses, and hospitalizations using a statistical model to adjust for cases that national surveillance networks are unable to capture for
a number of reasons. These estimates and the methodology used to calculate them are published in
Clinical Infectious Diseases and available
onlineexternal icon. These estimates will be updated periodically.
Estimated COVID-19 Infections, Symptomatic Illnesses, and Hospitalizations—United States
CDC estimates that from February 2020–March 2021:
1 in 1.8 (95% UI* 1.7 – 2.1) COVID–19 hospitalizations were reported
1 in 3.9 (95% UI* 3.5 – 4.4) COVID–19 symptomatic illnesses were reported
1 in 4.3 (95% UI* 3.7 – 5.0) total COVID–19 infections were reported
These estimates suggest that during that period, there were approximately:
114.6 Million
Estimated Total Infections
97.1 Million
Estimated Symptomatic Illnesses
5.6 Million
Estimated Hospitalizations"