This third shot decision currently being made by some individual seems to be based on considerations other than just health. I'm not an expert but would postulate that any booster shot would probably be effective and a bit less of a risk for complications if it was a third jab of the same vaccine you have had two times previously. If one is getting a different vaccine that uses a completely different approach to stimulating the immune system I suspect this is being done to primarily address travel needs and not primarily to enhance Covid protection. Science hasn't yet announced that mixing traditional vaccines with a mRNA type is better, more effective or completely safe to do. Lots of opinions out there on the interweb but no health organization with credibility is advocating this booster practice on a large scale. It seems a booster shot will be needed at some point, but when and what type is best to use is still up-in-the-air.
It should be ok to mix & match vaccines for most people as individually most vaccines are safe for most people receiving them. However, there are no guarantees. If someone had no complications from the Sinovac shots, then one can probably assume a third shot of the same vaccine will offer a boost in protection without much risk of complications this time. However, for someone who has not had the mRNA type before and injects one on top of a traditional vaccine they cannot draw on past experiences to draw any conclusions in the absence of scientific data to use. There was much heated discussion about all of these vaccines when they first came out with respect to clinical trials, efficacy and it all being one big experiment with the net effects being unknown. This is pretty much where we are with the idea of booster shots and using more than one type of vaccine. Those who are committed to their goal will press forward regardless. Those without a pressing ulterior motive may just wait for the bodily of scientific data to catch up to the reality we currently face. Of course if you waited for something other than Sinovac to become available and just prefer the Pfizer over the other option then this booster issue doesn't apply to you at this point.
If one place won't do a third booster shot, why will another?My sister is a professor of geriatrics and practicing geriatrician at University of Texas Southwestern Medical School. She texted me recommending that I get the Pfizer booster. My husband, myself and one son have the Sinovac vaccine as most Dominicans. Another son waited out to get the Pfizer. I will check out to see if Multicentro Winston Churchill vaccination center has authorized the third dose. Good for Jumbo in Puerto Plata!