I was originally a bit skeptical of the estimate of 53 million by the study by CDC scientists, but I looked further into it. It makes sense that the younger and working population has been infected at a much higher rate than the older population. So, in the first wave, SARS Cov 2 infections probably were distributed fairly evenly among the population demographics, but then the second wave was driven by mainly younger and middle-aged people. You can see many more infections in that demographics without seeing as large of an increase in mortality. This study estimates that infections in the 18-49 cohort were double those of 65+.
https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1780/6000389