This question assesses the understanding of vaccination coverage and its implications for public health. The key point is that vaccination coverage must meet a certain threshold to ensure herd immunity, which protects even those who are not vaccinated. If coverage falls below this threshold, the likelihood of outbreaks increases significantly. Option A is incorrect because eradication requires sustained high coverage. Option C is misleading as vaccines do not become ineffective simply due to low coverage. Option D is incorrect because natural immunity does not develop without exposure to the disease, which can be dangerous. Therefore, the correct answer is B.