Why mystifiy the issue when one does not understand that Covid vaccine protection has an expiry date. They found that people with 2 Pfizer no longer have enough antibodies after 270 days and therefore, they need a booster. As for Sinovac and another similar kind, it is 5 months or 150 days after completing 3 doses. Those who switch from 2 doses of Pfizer to Sinovac as booster has no recommendation and therefore, it would then be an arbitrary decision by the Private Physicians who might say about the need for additional 3 doses. Maybe when more data are available, there might be a recommendation. If one is not sure whether to take additional 3 doses of Sinovac, just do an antibody (serology) test before completing and taking all the 3 additional Sinovac shots as recommended by the Physicians.