The difference between preventing transmission and disease

Time for an epidemiology lesson!

“What’s the difference between driving 100 mph and not getting vaccinated?”

Apparently since scientists don’t understand this distinction, I will explain. Readers: I challenge you to provide actual scientific proof that vaccines prevent transmission.

Before I explain why that can’t be shown, let’s be clear about definitions:

Disease: When someone is infected with a pathogen (virus, bacteria, parasite) and has signs and symptoms caused by that pathogen, aka clinically ill

Transmission: When a pathogen spreads from one host to another (in our case, we’re talking about people). Can be airborne, or through fomites or insect vectors, or direct (as in needles).

To demonstrate that a vaccine prevents transmission, one would have to study the person that was vaccinated and their close contacts that have the potential to acquire infection from the vaccinated (or unvaccinated) person. The premise here is that if the vaccine prevents transmission, then no one around the vaccinated person would get sick/infected, but if the close contacts of the unvaccinated person would get sick/infected.

On the other hand, to show that a vaccine prevents disease, you simply compare the incidence (new disease occurrence) in vaccinated vs. unvaccinated people. It is disease within those vaccinated/unvaccinated people.

(All my medical freedom people that are stewing, wait until I’m done making my point! I’m using science to combat “science falsely so-called”.)

Let’s take for example the COVID-19 vaccine. The public health talking heads on TV, and the various elected officials, told you that the vaccine prevented transmission. In order to demonstrate that, you would have to sample the thousands of people with and without said vaccine in a clinical trial, and all of the people those people were in contact with. To keep it simple, you could restrict it to close (household) contacts, where the chances of exposure are greatest. And if you really want to see transmission and you understand that not all infections result in active disease, you could do antibody testing on all those people to see if they were exposed and remained uninfected / non-diseased. Of course you’d have to start with a population of people that had never been exposed to COVID, otherwise you’d have pre-existing antibody responses that would bias your study. You could instead look for the occurrence of disease in those close contacts, but then you lose power, because the events are somewhat rare. Oh, but you’d also have to ensure that those people were not exposed to some other person in the community (what if your vaccinated subject had a spouse was exposed to – gasp! – an unvaccinated subject in a bus!)… so I guess you’d need to keep everyone in their house all the time?

Yep, that’s pretty hard to show. But quite literally, that is the way you’d have to design the study. And that is why the COVID-19 trials actually only looked at other outcomes, like hospitalization.

Could you study population rates of vaccination vs. incidence (an ecologic study) and use that to prove that populations with higher vaccination rates have lower incidence?

No. Let’s disregard the ecologic fallacy for now. Such a study would be full of confounders: differing vaccination strategies by country (timing of vaccination through the life course, timing or boosters or lack thereof, timing of other vaccines, genetic variants of the pathogen that exist in different global locales, socioeconomic differences, differences in general health, comorbidities, and healthcare access…

This is basic epidemiology. This is why community transmission studies are so hard. When you do study transmission from a close contact, using actual molecular typing, often the infection came from another source.

This doesn’t even mention the fact that zero vaccines have 100% effectiveness. Keep in mind that clinical trials are done in nicely controlled, healthy, restricted populations, not the general population that is chronically ill. Vaccinated people get sick all the time. Remember several years ago the Navy ship (where all military personnel are required to be vaccinated with MMR) had a Mumps outbreak? How does that happen?

2 Peter 3:5 KJV says that people are “willingly ignorant”.

So no, driving 100 mph is not the same as not getting vaccinated. By the way, how’s that COVID-19 vaccine working for you? Or that flu shot, for that matter?