Until now only injectable vaccines against influenza have been availab

Your stimulus says the following:
Until now, only injections of vaccine were available.
These were primarily used by older adults.
Now nasal sprays are available that prevent flu in children.
Children are more likely to contract and spread flu.

Conclusion: If nasal sprays are made available for children, it will greatly reduce the spread of flu across the population.

Does something come to mind when you read this conclusion? What came to my mind was that if children are most likely to contract and spread flu, they should have been given flu injections and that would have prevented spread of flu across the population. Why is it that availability of nasal spray will prevent the spread the flu but injections did not?

Now, I need to strengthen the argument so I should focus on my conclusion and see what will strengthen it the most.
Let's read the options:
1. If a person has already been given an injection, he is immune to flu. Nasal spray on top will not help in anyway.
2. This says that nasal sprays work in the same was as do injections. It does not tell me why injections could not prevent the spread of flu while the nasal spray will.
3. This tells us that the subsidies have kept injections affordable for all older adults. But it doesn't say that otherwise their cost is prohibitive. Also, it doesn't say anything about the cost of nasal spray. If this option said, "Injections are very expensive otherwise but nasal spray is a cheap alternative", it might have made a stronger contender. Still, we do not know whether cost is a factor that parents consider at all when getting vaccines for their children. To make this option the answer, we might even have to add something like "Parents are not willing to get their kids immunized if the vaccine is very expensive"
4. Few older adults catch flu from children doesn't strengthen my conclusion that making nasal sprays available for children will reduce the spread of flu across the population.
5. Now that is what I was looking for! A reason why parents don't give flu shots to their kids but will be willing to give nasal sprays. Parents don't like to give shots to their kids (could be because of pain associated with a shot or whatever) Now, with the availability of the nasal spray, they will be more inclined to have their kids vaccinated. This probably will help in preventing the spread of flu across the population.
Answer (E).
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Hi Rock750,

Conclusion: making the new vaccine widely available for children will greatly reduce the spread of influenza across the population.

D. Of the older adults who contract influenza, relatively few contract it from children with influenza.

This choice will have no effect on the conclusion. If we make the vaccine widely available then it will not reduce the spread of influenza across the population because, as per D, older adults do not contract influenza from children. Making the new vaccine widely available does not imply that the children will be willing to take the vaccine

E. Many parents would be more inclined to have their children vaccinated against influenza if it did not involve an injection.

This choice does in face strengthens the conclusion. If parents are more inclined to have their children vaccinated then a larger number of children will be protected from influenza, thus, making the vaccine widely available will reduce the spread of influenza.

This answer choice takes its form from an assumption that is required for the conclusion to be true.

Assumption: Children will be willing (or parents inclined) to take the the influenza medicine in the newly administrated form. Negating this will make the conclusion to fall apart.

Hope this helps,

Vercules
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Isn't D a kind of assumption here ?

Note that the question is between nasal sprays and injections. Injections were available but why is it that availability of nasal sprays will bring about a change in the situation?
(D) Few older adults catch flu from children

Few means hardly any. If anything, it is against our conclusion "If nasal sprays are made available for children, it will greatly reduce the spread of flu across the population."
Why will nasal spray made available to children reduce flu across the population?
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Can anyone explain why Option C is not a strengthener?

While i do believe option E to be a much better strengthener.

Here is my reasoning for Option C ( Please correct me if i am wrong) :

In the passage its given that Injection is primarily taken by Adults.
Now , nasal spay comes and its effective for children. So ,if new vaccine is widely available for children, Influenza spread will greatly reduce across POPULATION.

Option C: So, if the Injection is still affordable for Adults it means that there will be no decrease in the number of adults taking vaccines. And we know that nasal spray is now widely available so, number of children getting vaccinated is expected to Increase . So if there is NO DECREASE IN ADULTS GETTING VACCINATED and an INCREASE IN CHILDREN GETTING VACCINATED, Influenza spread will surely decrease across POPULATION ( adults + children) .

Notice that the conclusion is not that influenza spread will decrease.

The conclusion is that making the new vaccine widely available for children will greatly reduce the spread of influenza across the population.

So, the conclusion is specifically about the effects of the making the new vaccine available.

Thus, the correct answer must support the conclusion that making the new vaccine available for children will have those effects.

(C) does not provide any support for the conclusion that making the new vaccine available for children will greatly reduce the spread of influenza.

(E) supports the conclusion of the argument presented.
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Pre-thinking:

Conclusion: Making the new (nasal) vaccine widely available for children will greatly reduce the spread of influenza across the population.

Premise on which it is based:
i) A new vaccine administered in a nasal spray form has proven effective in preventing influenza in children.
ii) Children are significantly more likely than adults to contract and spread influenza.
iii) Until now only injectable vaccines against influenza have been available. They have been primarily used by older adults.

Notice the subtle language shift between the premises and the conclusion - the conclusion talks about "reduce" and "availability" whereas the premises talk about "administered". If the new vaccines must reduce the spread of influenza across the population, then we are looking for something that bridges the gap between the "availability" of the vaccine for children and its administration in children (since we know that, if administered, it is effective in preventing influenza in children). Let us examine the answer options.

A. If a person receives both the nasal spray and the injectable vaccine, they do not interfere with each other. Does not bridge the gap between availability and administration. Eliminate.

B. The new vaccine uses the same mechanism to ward off influenza as injectable vaccines do. Mechanism of functioning is irrelevant to the conclusion. Eliminate.

C. Government subsidies have kept the injectable vaccines affordable for adults. This does not address the new vaccine at all, which is central to the conclusion. Eliminate.

D. Of the older adults who contract influenza, relatively few contract it from children with influenza. If anything, this acts as a weakener, since preventing influenza in children will not help in preventing influenza in adults. Eliminate.

E. Many parents would be more inclined to have their children vaccinated against influenza if it did not involve an injection. Correct answer as it bridges a gap between availability and usage, as discussed in our pre-thinking.

Hope this helps.
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Understanding the argument -
Until now only injectable vaccines against influenza have been available. - Fact
They have been primarily used by older adults who are at risk for complications from influenza. - Fact.
A new vaccine administered in a nasal spray form has proven effective in preventing influenza in children. - Fact
Since children are significantly more likely than adults to contract and spread influenza, making the new vaccine widely available for children will greatly reduce the spread of influenza across the population. - Conclusion.

Just "availability" will not be enough for something to reduce the spread. Right? Yes, we need to put it into use as well. So, the assumption here is that it'll be administered as well. And we are looking for a strengthener. Right? So, assumptions are a 100% strengthener.

Option Elimination -

A. If a person receives both the nasal spray and the injectable vaccine, they do not interfere with each other. - The scope of our argument is to strengthen the idea that "making the new vaccine widely available for children will greatly reduce the spread of influenza." This choice is out of scope.

B. The new vaccine uses the same mechanism to ward off influenza as injectable vaccines do. - It's still the same issue. If something is available, it'll have no effect until it's given to children. We still don't know whether it's given or not given to the children; it's out of scope.

C. Government subsidies have kept the injectable vaccines affordable for adults. - It doesn't even talk about vaccines; it's out of scope.

D. Of the older adults who contract influenza, relatively few contract it from children with influenza. - Weakener.

E. Many parents would be more inclined to have their children vaccinated against influenza if it did not involve an injection. - This is conditional. If there is no injection, parents would be inclined to have their children vaccinated. So, it intends towards that the children can get vaccinated, a critical step towards the reduction os cases. Ok.