All three could push the true total higher, but number two easily could push it lower instead. And I'm not a microbiologist but I think lack of antibodies only means that we won't be able to test for past infection. The seronegative patients (determined by a very precise method for measuring antibodies) in the second cited paper were still able to clear their infections.
I'm sorry, I don't know much about medicine so maybe you could clear up what those studies found. If I understand correctly, they both looked at COVID-19 patients and the first one found 83% had detectable antibodies and the second one found 163 / 173 had detectable antibodies? Or maybe I just completely don't understand it.
Also it looks like they chose people with symptoms (i.e. "COVID-19 patients"). Would we expect the amount of detectable antibodies to be less, more, or the same among asymptomatic/mildly symptomatic people, or do we not know?
I'm not a professional and some of this interpretation may be wrong:
In the first report, 83% of people who tested positive for viral RNA (an active infection) had immune T cells that seem able to neutralize the virus, possibly without antibodies. I don't see any information on whether these patients were also tested for antibodies later, but in theory some of them could have cleared the infection without developing any.
Another cohort of healthy blood donors was tested for SARS-CoV-2 antibodies, and among the samples that did not have them, 34% had T cells that also seem able to neutralize the virus. They note that these cells appeared to work in only one of the two ways that were observed with the PCR-test-positive patients' T cells. The idea is that this one method of neutralizing SARS-CoV-2 could be a carry-over from other human coronavirus infections in the past, and any antibodies generated by this method would be ignored by tests specific to SARS-CoV-2.
Also the seronegative healthy donors skewed female and younger compared to the PCR-test-positive patients, but I'm not sure if that is significant.
In the second report, around 5% of people who tested positive for viral RNA (had active infections), and then recovered, still developed no detectable antibodies. It's possible that more than 5% of exposed people are clearing the infection without developing antibodies, because this sample was taken from confirmed cases, and someone with pre-existing immunity may be less likely to develop symptoms.
Of course, keep in mind that both of these are preprints, and may contain methodological errors.
Also don’t know if people with antibodies can still spread the virus to others or whether antibody levels (not just presence) determines whether you can be reinfected.
- The production of IgG antibodies is mostly delayed until after an infection is cleared.
- This is a random sample of people who were outside, it's not representative of the total population.
- Non-trivial numbers of people may never develop detectable antibodies for SARS-CoV-2:
https://www.medrxiv.org/content/10.1101/2020.04.17.20061440v...
https://www.medrxiv.org/content/10.1101/2020.03.30.20047365v...