I read the following statement in this article:
Vaccines are designed to protect against invaders that are encountered rarely - not all the time
Is it true? If yes, why?
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Not true. Vaccines were initially made for the highly contagious diseases that used to cause epidemics (which obviously means they were not rare).
The efficacy of a vaccine depends on multiple factors which includes adaptability of the pathogen.
The all-the-time encountered pathogen that the article is talking about is HIV; the reason for why no effective vaccine exists for it is because it has high mutation rate.
The source article had several major errors in it. I wouldn't trust any of it.
Vaccines are designed to protect against invaders that are encountered rarely - not all the time.
This is one of the major errors. There is a vaccine for chicken pox now - an infection which affected approximately 4,000,000 cases per year in the US before the vaccine. That is a very high number of children.
The overall HPV prevalence was 26.8% among US females aged 14 to 59 years before the vaccine. (This vaccine refutes two major errors in the paper.)
Measles, mumps, German Measles, Influenza, Haemophilus influenzae, and Pneumoccocal pneumonia were all common before vaccines for them were developed. Those are just the ones I can think of off the top of my head.
Factors that influence the ability to make a vaccine against a pathogen include the mechanism of pathogenicity, the ability to render the vaccine less harmful than the wild-type illness (such as polio), the ability to mass produce easily, reliably, etc.
Theoretically, a vaccine could be constructed for any pathogen, but the value of doing so if different. In business, this is called 'Return on Investment'. So, vaccines against serious common diseases are more valuable than ones against minor rare ones.
Polio, smallpox, whooping cough, rubella and typhoid are not rare. They are rare now.
A vaccine is not made because a pathogen is rare. It's made because a pathogen is so vicious that you're body does not have enough time to counteract it. Your body is always attacked by invaders, even now.
But usually the Innate immune responses are enough to rid you of the threat. And, when it isn't, it acts as a barrier until the adaptive responses fire up. This time gap is crucial, because the adaptive system needs antigenic presentation from the dendritic cells to differentiate B-cells into plasma and memory cells, then create antibodies and finally these antibodies will opsonize the pathogenic agent and then let the complement system or the CTLs take over.
This is where vaccines come in, Vaccines generally but not always stimulate the Adaptive responses, allowing the creation of Plasma B cells and memory cells. So next time when an invader arrives, the Adaptive response responds as soon as an antigen is encountered. Which is why, the diseases mentioned above have become such a rarity, but rest assured that if individuals choose not vaccinate then these diseases will re-emerge once again as they are doing now in the US.
I cannot post more than 2 links, so if you're interested read up. and If you're even more interested, pick up a copy of Kuby's Immunology, it helped me when I was in college and I'm sure it will help you out too.
http:// en.wikipedia.org/wiki/Antigen_presentation http:// en.wikipedia.org/wiki/Dendritic_cell http:// en.wikipedia.org/wiki/B_cell http:// en.wikipedia.org/wiki/Opsonin