There have been striking differences in COVID-19 severity/penetration by region. There are all sorts of reasons posited. This post from Derek Lowe at In The Pipeline, New Data on T Cells and the Coronavirus, suggests a possibility:
And turning to patients who have never been exposed to either SARS or the latest SARS CoV-2, this new work confirms that there are people who nonetheless have T cells that are reactive to protein antigens from the new virus. As in the earlier paper, these cells have a different pattern of reactivity compared to people who have recovered from the current pandemic (which also serves to confirm that they truly have not been infected this time around). Recognition of the nsp7 and nsp13 proteins is prominent, as well as the N protein. And when they looked at that nsp7 response, it turns out that the T cells are recognizing particular protein regions that have low homology to those found in the “common cold” coronaviruses – but do have very high homology to various animal coronaviruses.
Very interesting indeed! That would argue that there has been past zoonotic coronavirus transmission in humans, unknown viruses that apparently did not lead to serious disease, which have provided some people with a level of T-cell based protection to the current pandemic. This could potentially help to resolve another gap in our knowledge, as mentioned in that recent post: when antibody surveys come back saying that (say) 95% of a given population does not appear to have been exposed to the current virus, does that mean that all 95% of them are vulnerable – or not? I’ll reiterate the point of that post here: antibody profiling (while very important) is not the whole story, and we need to know what we’re missing.
It seems that later we will find out that perhaps the Vietnamese benefited from some immunity conferred by a previous asymptomatic coronavirus outbreak? If you’ve been following Spencer Wells (more specifically, here, here, and here on the general hypotheses) on Twitter you know he’s been suggesting this for several months. The pattern seems to extend to neighboring nations too.