所謂bivalent是指第三針還是第四針booster?
https://www.fda.gov/emergency-preparedness-and-response/coronavirus-disease-2019-covid-19/covid-19-bivalent-vaccine-boosters#:~:text=The%20Pfizer%2DBioNTech%20COVID%2D19,dose%20with%20any%20authorized%20or