The results from this Canadian healthcare worker study may be confusing, because it suggests lower long Covid risks with reinfections.
The per infection risk in this study was lower for reinfections, but those with more infections had more long Covid.
Other risk factors for long Covid were infections with pre-Omicron variants and more severe acute infections.
https://www.sciencedirect.com/science/article/pii/S120197122500236X
This US study has made the headlines. Omicron reinfections in children and adolescents doubled the risk of long Covid.
https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(25)00476-1/fulltext
This study from Luxembourg had a very small sample (N = 224), but they found that more infections lead to more long Covid symptoms and more vaccinations led to fewer of these symptoms.
https://link.springer.com/article/10.1186/s12982-025-00787-0
This study among Spanish healthcare workers found more long Covid in females, those with reinfections, and those who had multiple types of vaccines (not just Pfizer).
https://www.sciencedirect.com/science/article/abs/pii/S0025775326001417
This study from Romania seems to suggest that the main protective effect form vaccines against long Covid is fewer reinfections (and reinfections increase the risk).
This study from China found more persistent symptoms in older people and with reinfections.
https://bmjopen.bmj.com/content/bmjopen/16/4/e113446.full.pdf
Another study from China finds more lingering symptoms for more severe acute symptoms, older age, and multiple infections.
This study from the US estimates that, at present, around 1 long Covid case arises in the US every minute.