All that happens is that they have somewhat smaller protection for a shorter time.

Unless the vaccine or virus is very different from normal ones.

I think early data showed that you still get ~60% protection with the first shot (or was that for the Moderna vaccine?) Still, you get some level of immunity, booster only makes it higher.

The two dose studies provide incomplete information about the single dose efficacy — administration of the second dose ends the data collection for the first dose alone.

The data collection also includes the period of time that the body spends responding to the vaccine (so if you get the shot and get infected that day, the shot gets counted as not being effective).

And then on the other side of it, they don't have data on how long immunity from a single dose would last.

But it's likely that the Moderna and Pfizer vaccines have quite high effectiveness after the initial immune response.

Sure, while it's not perfect, the data for those 3 weeks between the first and second shot still can provide a lower bound approximation for immunity, which I assume is where the 60% comes from.

The point about how long single dose immunity lasts is fair though.

Why would they not get the booster?

Why do people miss their flight?

So much so that airlines can oversell a substantial number of tickets on any given flight and usually the plane departs with empty seats?

Why do people get divorced?

Why do...

Some will forget, some won’t realise they need it, some will get symptoms the first time and not want it. I don’t expect it will be widespread but humans will be human

You must not be a software developer

Mutation?

Mutations happen all the time regardless of vaccines. The vaccine adds evolutionary pressure and will likely give an advantage to the bearers of particular mutations.

If the virus has a smaller base of infected people who grow it, there will be less mutations. From that point of view, the sooner all the world is vaccinated, the better. The risk of a new, more dangerous mutation decreases.