Clalit, Israel’s largest HMO, has found that a third shot of the Pfizer-BioNTech vaccine is 92 percent effective in preventing serious illness and 93 percent effective in preventing hospitalization, compared with waning protection from only two doses, …Clalit, Israel’s largest HMO, has found that a third shot of the Pfizer-BioNTech vaccine is 92 percent effective in preventing serious illness and 93 percent effective in preventing hospitalization, compared with waning protection from only two doses, …