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.