But there is something that works in the opposite direction: if those with some protection become asymptomatic or mildly symptomatic but are still spreaders, then their lack of symptoms might mean they move around more and have more contacts, outweighing their smaller level of infectiousness. We can't know.
If those with existing protection can't become spreaders, then it should indeed mean that the R0 is much higher than expected (It was estimated from doubling assuming 100% were initially susceptible, so if only 50% were initially susceptible then the estimate would be off by a factor 2). But I don't think that this is what the paper is saying. It's saying that some have partial immunity responses that deal with the infection. They still get immunity after their infection, not before.
So those asymptomatic might simply be people whose natural lung operation is strong enough that they can compensate for the covid damage.
OTOH, the killer feature of Covid-19 seems to be triggering an out-of-control adaptive immune response. That's probably why kids aren't dying, as their adaptive immune response is still developing.
The implication (in the best of all possible worlds) is that the antibodies that fight common cold also fight COVID-19. Or it could be a simple correlation and not significant.
What is paper is saying is that previous exposure to coronavirus (the common cold) grants some level of immunity to covid19.
Wrong. There are 200+ strains of various viruses that are all lumped together as the "common cold" only 4 of which are coronaviruses. The vast majority of common colds are not coronaviruses.
> It's not the only virus that's lumped in as the common cold but it is one kind.
Perhaps not having knee jerk reactions and careful examination of the information is what's needed.
It's not saying that. The presence of these antibodies does not necessarily mean any level of immunity against SARS-CoV-2, which is a different virus than the one that triggered these antibodies. The degree to which these antibodies contribute towards immunity against the new virus is unknown, and wasn't evaluated by this study.