Returns an integer vector with years for which vaccination data are available.
Details
SI-PNI data come in two eras:
1994–2019: Aggregated data (doses applied and coverage).
2020+: Individual-level microdata (one row per vaccination dose), published monthly.
The returned vector is a static range extended with the microdata years
actually published on the healthbr-data R2 mirror once its manifest has
been read in the session (e.g. after calling
sipni_status()); this function itself never touches the
network. Use sipni_status() for month-level availability.