Also, it seems that people with low severity symptoms don't build up a lot of antibodies, so they're not sufficiently immune. So out of that 10%, only some count towards the herd.
edit: i think this is logical (for my rudimentary knowledge) given the cause of death is not directly related to the virus. The symptoms of dry cough and fever are all the bodies response. Even the shortness of breath and pneumonia might be attributed to inflammation. Perhaps the asymptotic people just don't have such a strong response for whatever reason.
Article in Dutch (translated title: Mild corona virus symptoms seem to lead to lower creation of antibodies)
https://nos.nl/artikel/2329846-milde-coronaklachten-lijken-m...
https://www.randombio.com/coronavirus2.html
You may not have a lot of them, but you have already evolved targeted killers.
https://www.medrxiv.org/content/10.1101/2020.03.24.20042382v...
> First, we observed three types of antibody responses in COVID-19 patients, strong, weak and non-response. Second, we found that the earlier response, higher antibody titer and higher proportion of strong responders for IgM and IgG were significantly associated with disease severity. Third, the weak responders for IgG antibodies had a significantly higher viral clearance rate than that of strong responders. These data indicates strong antibody response is associated with disease severity, and weak antibody response is associated with viral clearance, which resembles SARS and MERS.
I have a wild-ass guess. I vaguely recall that antibody response and cellular immune response generally tend to be inversely proportional. And that IgM antibody response is an aspect of allergy. So maybe strong antibody responders tend to have weaker cellular responses, and more wet lung problems.