Why vaccines DO prevent transmission.

So, I had a discussion this weekend where I was told that getting a vaccine doesn't prevent transmission.

And my immediate thought was "that does not compute".

So I went and I did some research, and I found a lot of material which supported this stance. However the terminology was vague. But, what I ultimately walked away with was this; whoever is saying this is wrong. Or at least, the way it is being broadly interpreted is incorrect. And that is just as bad.

As far as I can tell, the argument is that the Delta strain is virulent enough that there isn't much difference between vaccinated and un-vaccinated people in terms of how easy it is to spread. And this sounds like it contradicts what I just said. But, there are 3 key factors which aren't addressed; how this was determined, how reliable is the data, and what is the overall ability to spread the virus.

You see, I haven't been able to ascertain if whatever study was done or statistics reviewed were talking about ALL scenarios or just high transmission scenarios (cough/sneeze or prolonged indoor exposure). This is important because while it is certainly possible that there are some cases where a vaccinated person is just as likely to spread the virus when compared to someone who isn't. It seems similarly unlikely, the rate is the same across the board.

The other component which is the worst omission of all is that duration of infection is COMPLETELY ignored. EVERY. SINGLE. STUDY. I've been able to find indicates a drop in duration of infection of up to 6 days, which means that the period in which someone can infect another is cut almost in half. And the peak of the illness when symptoms show is also cut by a measurable amount.

So, unless the implication is that a vaccinated person is almost twice as infectious during the period when they are infectious to compensate for this contracted period of infection, then the overall infection rate cannot possibly be the same between vaccinated and un-vaccinated people.

Put another way, the argument is basically; if a vaccinated person and one who isn't are both infected and cough in your face, there isn't any measurable difference in your odds of contracting the virus. Thus, there is no drop in transmissibility from a vaccine.

This argument is (effectively) wrong though. The person who is not vaccinated will be coughing into people's faces for almost twice that amount of time. Which in theory gives them the chance to infect almost twice as many people. They are also more likely to be coughing in the first place.

I did a bit of fresh research and found this article which includes some people talking about the research. And, I think it is even better than the ones I found before. Basically, it says that the viral load is the same. And this supports what I've been saying. Viral load is important, but so too is length and severity of infection. The doctors in the article expect to find that vaccines only limit transmissions by about 50% which also lines up more or less perfectly with my own expectations based on the data. And that in turn agrees with this article which references a peer reviewed study indicating that vaccines also help with cross transmission.

The takeaway is this; people aren't smart enough to understand what they're reading most times. And confirmation bias allows us to take little nuggets like this and run with them toward whichever conclusion is more compelling.

There exists a metric based upon which vaccinated individuals and those which are not present the same risk. But that metric is isolated to the period of time during which both individuals have a similar viral loads and symptoms. And since that time period is not the same for the two individuals, they do not bear the same overall risk of transmission. 

All else being equal, the vaccinated person will have that maximum viral load for a much shorter time and be able to infect far fewer people. Which, if you're paying attention, is the opposite of saying that vaccination would have no impact on transmission.