<s>[INST] context: Measles Both measles and tetanus toxoid are highly efficacious vaccines (>95% efficacy). But if we compare the doses required and duration of immunity achieved, measles vaccine is clearly ahead of tetanus toxoid. - A single dose of measles vaccine if given at 12 months can achieve 95% efficacy, and if given at 15 months can achieve 98% efficacy. A 2" dose of measles vaccine achieves >99% efficacy. "This immunity appears to be of long duration probably for life."-Park 20/e p138 - Tetanus toxoid achieves the same efficacy (>95%) after 3 doses and boosters are required every 5 to 10 yrs to maintain the immunity. (Whereas measles requires no boosters) BCG and OPV can be easily ruled out. BCG has variable efficacy of 0-80%. OPV has >90% efficacy in industrialised countries and 72-98% in hot climates when cold chain is properly maintained. (Lower protection against type 3 which can cause rare vaccine associated paralytic polio.) Ghai writes-"Primary vaccine failure occasionally occurs after three doses of oral polio vaccine." [/INST] question: Vaccine with maximum efficacy is | options: A: OPV B: Measles C: BCG D: TT | answer: B </s>