This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Which is not true about Polio vaccine? 
Select and explain. 
A. Difficult to maintain cold chain, B. Salk vaccine doesn't cause paralysis, C. Helps in increasing immunity, D. Immunity takes long time to develop
D: Immunity takes long time to develop
Answer: D: Immunity takes long time to develop 
Explanation: There is no long term carrier state for poliovirus in immunocompetent individuals, polioviruses have no non-primate reservoir in nature, and survival of the virus in the environment for an extended period of time appears to be remote. Therefore, interruption of person to person transmission of the virus by vaccination is the critical step in global polio eradication. The injected Salk vaccine confers IgG-mediated immunity in the bloodstream, which prevents polio infection from progressing to viremia and protects the motor neurons, thus eliminating the risk of bulbar polio and post-polio syndrome.