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I assume I know the answer to this already but wanted to confirm before I respond to someone that appears to be arguing that vaccines don't make you immune (they only stop your symptoms?) - but you remain infectious?:

She also doesn’t realise that if a vaccine works like she believes then the vaccinated would have less symptoms which in result offers more chance of spreading it. literally how covid spreads so fast now. “no or mild symptoms“ to most. Thats the sole reason it spreads so fast. So technically getting vaccinated is more dangerous in spreading disease than someone that gets sick and stays at home.

Edit: This question is about vaccines in a more general scope as that was the discussion at hand, but if anything it would be more closely targeted at COVID-19, which I'm aware there is currently no vaccine for.

My understanding of vaccines in general is that they don't technically make you "immune", but they make you immune to the effects of the virus, because your body will already know how to beat it.

But in saying that, my assumption has also been that because you have somewhat of a immunity to it then if you get "infected" with the disease you aren't contagious to others.

Additionally, I assume getting "infected" with the disease and being protected against it's effects is different to being asymptomatic - both have no symptoms, but you are still truly infected with it if asymptomatic and can still pass it to others, it's just not affecting your body enough to give you symptoms.

Just thinking now, but if your body knows how to fight it off already, then maybe you are truly immune to it - just because it enters your body doesn't mean it infects you right? Your body knows how to fight it off before it does. Like, an attacker can enter your grounds and knock at the door, but your guards know how to fight them off before they enter the area of your grounds where they 'cause the trouble'.

...but on the case you CAN get infected with it, but it doesn't present any symptoms, are you still contagious to others?

Additionally, I'm not talking bout virus shredding here from getting a vaccine, I know all about that.

One would think the proof is in the pudding in that many diseases have all but been eradicated from the world because of some vaccines, but I argue with myself that if vaccines don't make you immune then perhaps these diseases have only largely seemingly been eradicated because most everyone has been vaccinated (with vaccination preventing you from the effects only, not getting infected?) - thus only appearing on the surface they have been eradicated, but they are still around, but just don't pose a health risk to those that are vaccinated?

So, primary question is, what is meant exactly by "immunity", can you still contract the disease and if so would you still be contagious to others?

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  • $\begingroup$ It would really depend on what kind of vaccine/disease you are referring to, as it there is a substantial amount of variation in the immunity they offer. Could you clarify your question? $\endgroup$ – user438383 Aug 6 at 18:42
  • $\begingroup$ @4galaxy7 I have edited my post. It was more in a general nature of how vaccines work, but I have clarified a little. $\endgroup$ – Brett Aug 6 at 18:45
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I tried to break down your question into smaller parts and answer them individually:

Q. What is immunity?

A. Simply put, immunity is the ability to react to an infectious agent and stop it from causing disease.

Q. Do vaccines make you immune to infectious diseases?

A. Yes, by definition. If it doesn't make you immune, it's not a vaccine.

Q. Can we get disease even after being vaccinated?

A. Yes, because no vaccine is 100% efficacious. For example, the MMR vaccine has about 97% efficacy in preventing measles. This means that 3% of vaccinated people can still get measles on exposure.

Q. Do vaccines prevent infection?

A. It depends on how the vaccine works. Some vaccines enable the immune system to actually reduce survival and multiplication of the infectious agent. The polio vaccine is an example$^1$. It leads to generation of antibodies that neutralise the virus$^2$.

Other vaccines prevent disease (or reduce its severity) but do not prevent infection. The diphtheria toxoid is a good example of this$^3$, because the antibodies generated by vaccination neutralise the toxin while leaving the bacterium unscathed. (I believe all toxoids behave like this.)

Q. Can we spread infection even after getting vaccinated?

A. Yes. This is easy to see for those vaccines (like diphtheria, mentioned above) that prevent disease but not infection.

Vaccinated individuals can spread infection with the other, multiplication-stopping kind of vaccines too. Again, this happens because no vaccine is 100% efficacious. Note that chances of spread are reduced (as compared to the unvaccinated scenario), even if they are not zero. For instance, see the work of Seward et al$^4$ on breakthrough varicella.

Q. The big question: is getting vaccinated more dangerous than not getting vaccinated?

A. Sorry, but this is not the place to discuss it. It requires a careful look at the risks and benefits of vaccination, not just for the individual but also for the community at large. That would be a discussion more appropriate for Medical Sciences Stack Exchange.

To summarise: yes, we can catch a disease and spread it even after being vaccinated. The chances of this happening vary with the kind of vaccine.


Journal articles cited

  1. Howe HA. Am J Public Health Nations Health. 1957 Jul;47(7):871–5. https://doi.org/10.2105/ajph.47.7.871
  2. Faden H, Modlin JF, Thoms ML, et al. J Infect Dis. 1990 Dec 1;162(6):1291–7. https://doi.org/10.1093/infdis/162.6.1291
  3. Miller LW, Older JJ, Drake J, et al. Am J Dis Child. 1972 Mar;123(3):197–9. https://doi.org/10.1001/archpedi.1972.02110090067004
  4. Seward JF, Zhang JX, Maupin TJ, et al. JAMA. 2004 Aug 11;292(6):704–8. https://doi.org/10.1001/jama.292.6.704

Suggested reading

  1. https://www.who.int/health-topics/vaccines-and-immunization
  2. https://www.ncbi.nlm.nih.gov/books/NBK27131/
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    $\begingroup$ Another factor is diseases like influenza, where the virus has a very large number of variants, and vaccines are effective against only some of them. (Which is why there's a new influenza vaccine, or vaccines, every year.) $\endgroup$ – jamesqf Aug 8 at 16:37
  • $\begingroup$ Thank you for your information answer. Couple questions - So with vaccines such as the diphtheria one, is it safe to say that you can spread it in much the same way as if you had the disease? Secondly, you state that the other kind of vaccines you are still able to spread it because no vaccine is 100% efficacious; so basically you're saying you CAN'T spread it if the vaccine has been effective (as you won't get infected), but in the small percentage that it isn't effective then THAT'S when you are still able to spread it - because you will then thus be infected? $\endgroup$ – Brett Aug 8 at 19:09
  • $\begingroup$ @Brett 1. Yes, vaccinated persons with adequate immune response can spread the diphtheria bacillus (as described in journal article 3 above). 2. Yes, in the other case it's the people who do not respond to the vaccine that can spread the infection. But many other factors are involved, like organism load, herd immunity, etc. $\endgroup$ – Adhish Aug 9 at 5:02
  • $\begingroup$ Thanks again. With regards to vaccines like the Diphtheria one, as you state you can still spread it as it only prevent against disease, not infection; but a secondary question on that, when getting the vaccine is there any worry about spreading it FROM the vaccine itself, as in do these types of vaccines contain the live virus or would it just be similar in fashion to "virus shedding" from vaccines where some contain weakened versions of a live virus and the only concern to others around you is generally only immunocompromised people? $\endgroup$ – Brett Aug 9 at 12:34
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    $\begingroup$ The WHO vaccine safety course is a good place to learn about the different types of vaccines and their safety profile: vaccine-safety-training.org. $\endgroup$ – Adhish Aug 10 at 9:26

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