Passage Summary: We don't have natural polio anymore, but the current vaccine (OPV) actually gives 12 people polio every year. Switching to a different vaccine (IPV) would cut those cases in half, so we should definitely switch.

Conclusion: North American children should be switched from the live oral polio vaccine (OPV) to the inactivated polio vaccine (IPV).

Reasoning: Natural polio is gone in North America, but OPV causes 12 cases of polio annually; switching to IPV would reduce these vaccine-induced cases by half.

Analysis: The author assumes that the only relevant factor in choosing a vaccine is the number of vaccine-induced cases. To weaken this, we need to find a reason why OPV might still be the better choice despite those 12 cases. Perhaps OPV is better at preventing the 'naturally occurring' polio from returning, or perhaps IPV has other dangerous side effects that the author hasn't mentioned. Look for an answer that highlights a unique benefit of OPV or a hidden cost of IPV.