Do vaccinated people spread Covid as much as unvaccinated people?

Denominators (still) matter

Mark Shrime, MD, PhD


A couple of weeks ago, internal CDC data about the Delta variant leaked to the public. There was a lot to process in this tranche of data—like, for example, the fact that Delta is way more transmissible than the original version of Covid-19.

Perhaps predictably, however, the anti-vaccine movement has latched on to one particular finding: the amount of virus (aka the “viral load”) in the noses of infected vaccinated people mimics the viral load in the noses of unvaccinated people.

Because we believe that Covid is airborne—and that it’s therefore transmitted from people’s mouths and noses—an appropriate potential conclusion from this finding is that, all other things being equal, infected vaccinated people may be able to transmit Covid as well as infected unvaccinated people.

That’s not ideal, obviously, but it’s also not where the antivaxxers stopped. They, instead, have concluded this:

Source: @Unwokenarrative on Instagram (who promptly blocked me when I replied to this claim)

This claim is as specious as it is nefarious, so let’s take it point by point.


The fact is, we don’t know if the viral load equivalence means that transmission would be equivalent, all things being equal. Is it an identical virus? In other words, if there are circulating antibodies, are they attached to the viruses in the noses of vaccinated people? Have they somehow neutralized the viral particles? And besides, is the nasal viral load the best indicator of transmissibility?

But, to give the antivaxxers the benefit of the doubt, let’s assume that

  1. Nasal viral load is a good indicator of transmissibility, and
  2. Infected people have identical nasal viral loads, irrespective of their vaccination status.

Heroic assumptions. But even if they’re true, “Forcing people to get vaccinated will not stop the variant from spreading” is a flat-out lie.



Mark Shrime, MD, PhD

Author, SOLVING FOR WHY | Global surgeon | Decision analyst | Climber | 3x American Ninja Warrior Competitor