I don't believe it's a matter of 100% guarantee of immunity. The current covid vaccines are designed to prevent severe infection, NOT prevent infection. My understanding is that the current vaccines ~95% efficacy is against severe infection, but we don't actually know what the efficacy against infection is.

If you compare this to say smallpox vaccine, it is 95% effective at preventing infections. https://www.health.ny.gov/publications/7022/