According to this report by Public Health England (p.35):

whilst vaccination may reduce an individual’s overall risk of becoming infected, once they are infected there is limited difference in viral load (and Ct values) between those who are vaccinated and unvaccinated. Given they have similar Ct values, this suggests limited difference in infectiousness.

If this is true, how would vaccination help stop the virus? Logic would suggest just the opposite: since vaccination does not decrease significantly infectiousness, chances for new (possibly deadlier) mutations to arise should increase. Could someone explain which piece of data I'm missing?

  • @Armand thanks for your response. Could you provide the data to back up your comment? (vaccination reduces risk of becoming infected) Aug 9, 2021 at 12:15
  • Armand already did, and the quote in your question says it too.
    – Bryan Krause
    Aug 9, 2021 at 14:17
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    @BryanKrause the quote is "it may reduce ... the overall risk of becoming infected". Just because something may do something, doesn't mean it actually does. That's why I was asking for the data that would back up such statement. Aug 9, 2021 at 17:01
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    The "may reduce" is tied to "an individual". The "while...may" construct here is not suggesting that it's possible vaccines don't reduce infection risk in a population, but in a given individual. Are you seriously asking questions about vaccination without being aware of all the efficacy trials of vaccines?
    – Bryan Krause
    Aug 9, 2021 at 17:05
  • Well, vaccination does reduce total infectiousness. Because less people get infected. They say that if you are infected, you're just as infectious as an unvaccinated person who gets infected, but if you are not infected then your infectiousness is zero.
    – user253751
    Aug 31, 2021 at 9:20

1 Answer 1


As you note in your quote "vaccination may reduce [data shows it does, substantially] an individual's overall risk of becoming infected". If an individual is not infected, they are not infectious and do not spread the virus. New mutations arise in proportion to the number of people infected, so if fewer people become infected (due to the vaccine), fewer new variants are generated.

Here's one study that found "CDC COVID-19 Study Shows mRNA Vaccines Reduce Risk of Infection by 91 Percent for Fully Vaccinated People" CDC press release at https://www.cdc.gov/media/releases/2021/p0607-mrna-reduce-risks.html

Study is "Prevention and Attenuation of COVID-19 by BNT162b2 and mRNA-1273 Vaccines" doi: https://doi.org/10.1101/2021.06.01.21257987

Exposure to COVID would be whatever mix of variants were circulating where and when they did the tests. The test population of health workers is described in the scientific paper as being from 8 locations (roughly half in Arizona). Data was collected from mid-December 2020 through mid-April 2021. "Of 93 genetically sequenced viruses, 10 were variants of concern (8 B.1.429, 1 B.1.1.7, 1 B.1.427) and 1 was a variant of interest (P.2) (Table_S3)"

This is of course just one study, but illustrates the concept of protection against infection.

  • Note that all links state that vaccination lowers the risk of becoming infected. It doesn't stop infection completely, but it does lower the viral load and speed of viral spread. Aug 9, 2021 at 14:40
  • Thank you for your answer. I am not able however to find out which covid-19 VOC this data refers to (alpha, beta, gamma, or delta). Would you know where it is mentioned? Aug 9, 2021 at 17:03
  • Updated answer with variant info.
    – Armand
    Aug 9, 2021 at 17:50
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    @ChrisRogers Not sure about what exactly you intended to say with your wording,This and other studies clearly show that most/many of those vaccinated are indeed protected against becoming infected at all. I think it is still unclear whether in the group of vaccinated people anyone might catch it if exposed enough times, or whether only certain individuals among the vaccinated are less protected and so they are susceptible to the "breakthrough" infections.
    – Armand
    Aug 9, 2021 at 17:57
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    @ChrisRogers That still seems like a non sequitur. Nothing in Armand's answer states or assumes that the risk is reduced by 100%. It's quite rare for anything in medicine or biology to be 100%.
    – Bryan Krause
    Aug 9, 2021 at 21:58

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