Question

Some batches of polio vaccine used around 1960 were contaminated with SV40, a virus that in monkeys causes various cancers. Some researchers now claim that this contamination caused some cases of a certain cancer in humans, mesothelioma. This claim is not undercut by the fact that a very careful survey made in the 1960s of people who had received the contaminated vaccine found no elevated incidence of any cancer, since \(\underline{\hspace{3cm}}\)

• A. most cases of mesothelioma are caused by exposure to asbestos
• B. in some countries, there was no contamination of the vaccine
• C. SV40 is widely used in laboratories to produce cancers in animals
• D. mesotheliomas take several decades to develop
• E. mesothelioma was somewhat less common in 1960 than it is now

Hint:

In questions that present a puzzle or a discrepancy between two facts, look for an answer choice that introduces a new piece of information explaining why the two facts are not actually contradictory. Often, this involves a factor of timing, as seen here with the concept of a latency period.

Answer 1

The Correct Option is D

Step 1: Understanding the Apparent Contradiction
We have two seemingly conflicting pieces of information:
Claim: A contaminated vaccine from around 1960 caused a specific cancer (mesothelioma).
Evidence: A survey in the 1960s of people who got the vaccine found no increase in cancer.
The task is to find a reason why the 1960s survey evidence does not disprove the researchers' claim. We need to explain why the survey might have missed the effect that the researchers are now claiming to have found.

Step 2: Analyzing the Task
The core of the issue is timing. The vaccine was given around 1960, and the survey was also conducted in the 1960s. The researchers are making their claim "now" (at a much later date). To reconcile the two, we need a reason why an effect caused in 1960 would not be observable until much later.

Step 3: Evaluating the Options
(A) The fact that asbestos is the primary cause of mesothelioma does not mean the vaccine cannot be a secondary cause. However, it doesn't explain why the 1960s survey of the vaccinated group showed no elevated incidence compared to the general population at that time.
(B) The situation in other countries is irrelevant to the survey of people who specifically received the contaminated vaccine.
(C) The fact that SV40 causes cancer in lab animals would tend to support the researchers' claim, but it does nothing to explain the negative result of the 1960s human survey.
(D) This option provides a perfect explanation for the discrepancy. If mesothelioma has a long latency period and takes several decades to develop, then a survey conducted in the 1960s would be far too early to detect it. The cancers caused by the 1960 vaccine would not begin to appear until the 1980s, 1990s, or later. This means the 1960s survey results are completely consistent with the researchers' claim that the vaccine did, in fact, cause cancers that appeared many years later.
(E) The baseline commonness of the disease in 1960 is background information. It doesn't explain why the vaccinated group in particular failed to show an increase in incidence during the survey period.
Step 4: Final Answer
Option (D) is the correct answer because it resolves the apparent contradiction by introducing the concept of a long latency period for the cancer, rendering the early survey incapable of detecting the long-term effect of the vaccine.