When researchers looked at 43 estimates of under-reporting (less than 100% capture rate) of passive surveillance, the middle 50% of the estimates ran from 2% capture of underlying adverse events up to 18% capture.
If reported events are only 2% of actual events, you multiply by 50 to get the true number of events which occurred. Error bars were used in order to show both ends of the continuum (2% capture up to 18% capture) for deaths reported to VAERS:
The low estimate for 2020 was 950 actual deaths, and the high estimate was 8,550 actual deaths. But for 2021, the low estimate was 58,672 actual deaths (approximate death in the Vietnam War) and the high estimate was 528,050 actual deaths.
Reference
[VAERS reports of death] — https://openvaers.com/covid-data
[Capture rates characteristic of passive surveillance span from 2% up to 18%] — Hazell L, Shakir SA. Under-reporting of adverse drug reactions : a systematic review. Drug Saf. 2006;29(5):385-96. doi: 10.2165/00002018-200629050-00003. PMID: 16689555. https://pubmed.ncbi.nlm.nih.gov/16689555/