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I'm not sure if it's just a movies thing but I always notice that the original strain of a virus is required to cure a virus, why is this? is this even true?

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Nope. This is a total movie fallacy/exaggeration.

How worthwhile it would be to isolate the original strain is debatable, but certainly not necessary for vaccinology purposes.

On one hand, the inital strain might be interesting to study if its an aetiological agent which has never caused human disease before. It would be the first 'snapshot' of the genomics/transcriptomics of an organism/agent which has been able to jump a species barrier perhaps, or has evolved to survive at our body temperature, evade our immunity and so on.

However, as the infection progresses through a population, in order for it to stay infectious, it must by definition have maintained the molecular patterns that permitted it in the first place, so isolation of the 'zero-th', 10th or even 10,000th infection shouldn't make any real difference from a therapeutic standpoint. It may even be the case that as subsequent infection has in effect, passaged, the organism, that the later strains may even be more dangerous than the initial one.

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Oddly enough, we were discussing this at a tea break the other day (I work in a research institute that does a lot of vaccine development).

Firstly (as the other answer notes) this is a complete fallacy, but useful for dramatic purposes. It's basically a "MacGuffin" or "plot coupon".

Secondly, disease-causing agents evolve as they spread. This actually occurs pretty quickly, both because pathogens generally have short lifespans and because our immune systems are adaptive, and "learn" how to fight pathogens they encounter, so they need to adapt to evade that response (continually). This results in what is often called a coevolutionary "arms race" and is why for instance you need to get a flu shot every year.

Mutations may also affect other traits, such as how a pathogen spreads - a well-known example is the appearance of the E1-A226V mutation in chikungunya virus which allowed it to be spread by a different mosquito species.

Lastly, there can be changes that do not affect the ability of a pathogen to spread or to fight the host immune system; for example every time a population passes through a "bottleneck" (for instance if the agent is introduced to a new continent by a single infected air traveller) the new region is populated exclusively by descendants of that small population, and will reflect its genetic makeup.

Now, going right back up to the top you might have already spotted the problem here. Vaccines expose your immune system to a less dangerous (chemically inactivated, thermally inactivated, or otherwise attenuated) form of a pathogen or parts of the pathogen (such as its outer coat) to allow your immune system to recognise it while not actively being under attack. If the vaccine is made from an old strain of the pathogen - let's say, last year's flu - your immune system might learn how to fight last year's flu but won't be equipped to fight this year's.

As the other answer notes there could be research value in having a sample of the original strain (for instance, it might be easier to prove where it came from), but it's much less useful for vaccine development than the latest version.

(We also discussed the frequent fallacies that vaccines are effective within seconds, or work if you already have a disease, but those are separate questions...)

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