Some Class 2 lenses beat the standard: Avaira, for example, block at least 75% of UV-A radiation and 99% of UV-B radiation, according to the package insert.
Only a limited number meet Food and Drug Administration standards for "Class 1" protection, in which 90% of UV-A radiation is blocked and 99% of UV-B radiation is blocked.
Vitamin D is incorporated in the body when skin is exposed to sunlight of a certain wavelength but in the UK, during winter months, UV-B radiation can be insufficient to properly support vitamin D synthesis in the body.