"If it was first, "one dose", then "two doses is fully...

"If it was first, "one dose", then "two doses is fully vaccinated" then, "boost", and now "four doses", how do you explain this shift?"

Yes it works.
Look at the covid death rate in Australia one of the lowest in the world,in other countries where the vaccine rollout was slower or low vaccinations the death rates are much higher.

cases Deaths

	Spain	13,412,263		114,084		13,218,892		79,287	339	287,083	2,442	471,036,328	10,082,298	46,719,142
1	13	Vietnam	11,477,886		43,148		10,590,917		843,821	106	115,993	436	85,826,548	867,342	98,953,541
2	14	Australia	10,239,574	+5,559	15,207	+39	10,172,662	+4,659	51,705	43	392,791	583	78,835,048	3,024,116	26,068,792
3	15	Argentina	9,708,420		129,897		9,563,348	+1,128	15,175	294	211,006	2,823	35,716,069	776,264	46,010,234
4	16	Netherlands	8,422,336		22,643		8,362,464	+1,286	37,229	35	489,345	1,316	21,107,399	1,226,358	17,211,447
5	17	Iran	7,548,320		144,421		7,325,370		78,529	157	87,748	1,679	54,095,556	628,851	86,022,837
6	18	Mexico	7,087,812