 all of the studies considered were observational. "As there are many differences between people who are and are not vaccinated, there is a risk in all these studies that factors other than vaccination status may have influenced the results (in any direction)," it pointed out.
Additionally, the studies varied widely in their definition of long COVID, and the rapid review precluded pooled or meta-analysis of the findings together.
U.K. Health Security Agency
Source Reference: UKHSA COVID-19 Evidence Team "The effectiveness of vaccination against long COVID: A rapid evidence briefing" U.K. Health Security Agency 2022.