(Edit: See update comment below)
I emailed Sarah Constantin about this question. I didn't end up hiring her (my parents decided they didn't want to take supplements--luckily they're working from home now), but she did say:
BTW, the apparent "contradiction" isn't really one. Immunostimulants, taken when healthy, reduce the incidence of respiratory tract infections; once you get an infection like COVID-19, the immune response often gets so strong it kills you.
So I think taking immunostimulants on an ongoing basis is a good idea, but you should stop if you think you might be infected with COVID-19.
Update: Apparently some have been interpreting this Twitter thread as an indication that the virus may take advantage of immune system activity to infect you? Which could mean that immunostimulation is bad?
The supplement effects seem marginal, from your description. And focusing on them may detract from focusing on what is likely to help, which is the laundry list of measures to reduce exposure. So, when you talk to your parents, discussing potential changes in their routine might be a more useful approach.
From the WHO report you linked to (page 36):
There are distinct patterns of intra-familial transmission of COVID-19. It is unclear whether or not there are host factors, including genetic factors, that influence susceptibility or disease course
Something I find interesting to point out.
I've just come across this:
A project that's just starting (110 studies registered as I post) but may yield some interesting data in the future.
Being able to release the least susceptible from lock-down and increasing herd immunity. Targeting limited vaccination supplies to the most susceptible. Although a high specificity antibody test will be the game-changer.
My aging parents live in Santa Clara County and have a relatively fatalistic attitude towards COVID-19 exposure. But they seem open to taking supplements. So I'm trying to figure out what to recommend.
This 2017 post from Sarah Constantin on the prevention of respiratory tract infections says, among other things:
Bacterial immunostimulants are safe and effective in preventing respiratory infections, cutting the risk of infection by 40-50%.
However, this article from National Geographic implies that COVID-19 may kill through immune hyper-reactivity:
...what actually happens to your body when it is infected by the coronavirus? The new strain is so genetically similar to SARS that it has inherited the title SARS-CoV-2. So combining early research on the new outbreak with past lessons from SARS and MERS can provide an answer.
...
After the SARS outbreak, the World Health Organization reported that the disease typically attacked the lungs in three phases: viral replication, immune hyper-reactivity, and pulmonary destruction.
...
...sometimes your immune system goes haywire and those cells kill anything in their way, including your healthy tissue.
...
“So you get more damage instead of less from the immune response,” Frieman says.
This suggests just the opposite: that you want to suppress the immune system, not boost it.
I remembered reading somewhere that immunocompromised populations have a greater mortality rate from COVID-19. If true, this suggests that immunosuppression could be harmful? I tried to figure out whether the immunocompromise claim is based in fact or not. The recent WHO report states:
Individuals at highest risk for severe disease and death include people aged over 60 years and those with underlying conditions such as hypertension, diabetes, cardiovascular disease, chronic respiratory disease and cancer.
...
While patients who reported no comorbid conditions had a CFR of 1.4%, patients with comorbid conditions had much higher rates: 13.2% for those with cardiovascular disease, 9.2% for diabetes, 8.4% for hypertension, 8.0% for chronic respiratory disease, and 7.6% for cancer.
It's not clear how I should interpret these figures. For example, quick online research seems to indicate that diabetic subjects are more susceptible to infections (suggesting immunocompromise) but also that diabetes (esp. Type 1) could be considered an autoimmune disease (so higher mortality among diabetics could be due to the immune system attacking the body even harder than normal).
In any case, my extremely preliminary guess is you should take immunostimulants (with a short half-life) on an ongoing basis in order to try & prevent COVID-19 infection. And if you get it, hopefully your case is one of the ~82% which doesn't ever progress beyond mild symptoms. BUT you should prefer immunosuppressants if your case is reaching the immune hyper-reactivity stage (phase 2 from the National Geographic article). However, I wish someone who actually knows about this stuff would check my work, and I welcome any critiques.