[snystrom]

A strong argument against this is the potential to apply selective pressure for virus variants that escape antibody binding. After the first vaccine dose, antibody titer in the blood is lower than it would be after 2 doses. Some of the new variants are better at escaping antibody binding, but with high enough antibody titer in the blood, they can still be cleared. Using only 1 dose risks not having enough antibody to eliminate an antibody-escape mutant.

[xvedejas]

It will be a strong argument when there's more evidence. Do we have any example of significantly stronger viral adaptation in the presence of a vaccine that successfully stimulates the immune system? Even a full course of the less-effective vaccines would still be risking this. The AstraZeneca and Sinovac vaccines only have an efficacy of around 70% after all.

[snystrom]

You're correct that there's never been an experiment on the scale of the current pandemic, but there is a long history of demonstrating that viruses rapidly evolve to escape antibodies, and that this effect is enhanced under direct selective pressure.

Late 2020 there was a paper published where they demonstrate this effect for SARS-Cov-2 in human cells, then they look for those variants in humans and find that they were already infecting humans, demonstrating this selection activity happens in the wild [1]. I totally hear you about the 1 does vaccine argument. I think the kicker is two-fold. First, we do not know whether vaccinated individuals can still be infected with and spread COVID. Second is the rollout is currently very slow. You could imagine a scenario where enough folks are vaccinated to add pressure, but not reduce widespread transmission. If the data comes out that you can't transmit covid after the first dose (and I believe that Moderna has this data, but not ready to publish), I think the answer is clear to delay the second dose. But I think it is the absence of the transmission data, not whether evolution will happen to the virus, is part of the reasoning for the current strategy of completing the vaccination course in the ~3 week timeline.

[1] https://elifesciences.org/articles/61312