The risk of a vaccinated young person getting severely ill from COVID is incredibly low, though not zero.

Judged as of 16 June 2021. True if the public-health data and studies available then (e.g. CDC breakthrough-infection surveillance, vaccine trial and real-world effectiveness studies) show that vaccinated young people (roughly under 30–40) had a very low absolute risk of severe COVID illness (hospitalisation or death), e.g. well under 1 in 10,000 per infection or per period, and much lower than for unvaccinated people of the same age. Mostly true if the risk was low but 'incredibly low' overstates it somewhat (e.g. low mainly in relative terms, or for some subgroups such as the immunocompromised it was not low). Mixed if the evidence then was too thin to say how low the risk was. Mostly false if severe illness among vaccinated young people was uncommon but not markedly rare. False if the data showed a substantial risk.

Set before the research began.

1. E1 Why CDC changed tracking of ‘breakthrough’ COVID-19 cases PolitiFact · 20 May 2021

   “The latest publicly available CDC data showed that out of 95 million people who were fully vaccinated as of April 26, there were at least 9,245 recorded breakthrough COVID-19 cases. Among them, there were 832 hospitalizations and 132 deaths, though not all of them were related to COVID-19.”

2. E2 Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data The Lancet (Haas et al., hosted by Israel Government site gov.il) · 5 May 2021

   “96∙7% (96∙0–97∙3; 0·6 vs 0·1 per 100 000 person-days) against COVID-19-related death.”

3. E3 Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data The Lancet (Haas et al., hosted by Israel Government site gov.il) · 5 May 2021

   “97∙2% (96∙8–97∙5; 4∙6 vs 0∙3 per 100 000 person-days) against COVID-19-related hospitalisation”

4. E4 Impact and effectiveness of mRNA BNT162b2 vaccine against SARS-CoV-2 infections and COVID-19 cases, hospitalisations, and deaths following a nationwide vaccination campaign in Israel: an observational study using national surveillance data The Lancet (Haas et al., hosted by Israel Government site gov.il) · 5 May 2021

   “Two doses of BNT162b2 are highly effective across all age groups (≥16 years, including older adults aged ≥85 years)”

E1 reports CDC data for 95 million fully vaccinated Americans as of late April 2021: 832 hospitalisations and 132 deaths among recorded breakthrough cases, and not all of them were related to COVID-19. That is a very low absolute rate across the whole vaccinated population. In the Israeli national study, vaccinated people's rates of COVID-19 hospitalisation and death were far below those of unvaccinated people (0.3 vs 4.6 and 0.1 vs 0.6 per 100,000 person-days) (E2, E3), and E4 says protection was high in every age group from 16 up. None of the evidence gives risk figures for young vaccinated people specifically, so the claim's direction is well supported, but how low the risk was for young people (against the 1-in-10,000 benchmark) has to be inferred from all-age figures rather than read off directly.