This very frequently asked question was addressed, recently, on the IWMF Connect email list.  This particular answer deserves wide coverage, so for those of you who haven’t seen it, it is copied here.  Unlike well-known and well-researched and stable diseases, such as, say, measles, there just has not been sufficient time to do the research into all of the many factors that go into human immunity to Covid-19.

How many antibodies does it take to keep a WMer safe, and is there a test to count them?

It’s an impossibly hard question to answer:

  1. The more virulent a viral variant is, the more immune response is necessary for protection.  So, if researchers did a lot of work right now to find out exactly how much protection is necessary against the variants circulating today, the research would be out of date in the near future, as more virulent strains evolve.
  2. The more viral particles a person is exposed to, the more immune response is necessary for protection.  A certain amount of immune response might be fully protective against the small number of viral particles one inhales walking down the street, but if someone coughs directly on you in a crowded room, the immune response may not be sufficient.
  3. People vary in the number and density of viral receptors.  Even if the virulence was equal, the number of viral particles was equal, and the immune response was equal, infectivity would still vary from person to person.
  4. There’s more to immunity than antibodies.  Antibody levels alone don’t measure the totality of immune protection.  There are certain types of T cells, and more.
  5. Only certain antibodies are protective.  Most current tests measure antibodies to a particular part of the virus, called “spike” (note:  this is different than a spike (an elevation) in IgM or a spike (elevation) in the number of infected people.  “Spike” is the name of a viral protein).  However, not all spike antibodies are protective.  Only a proportion of spike antibodies (probably a small proportion) are what are called “neutralizing antibodies,” that actually stop infection.  Most assays don’t measure Covid neutralizing antibodies; they just measure Covid spike antibodies in general.
  6. Antibodies change with time.  The body starts making antibodies that are not particularly great.  They bind to the virus fairly loosely.  With time, the body makes better and better antibodies (called “affinity maturation”) that bind tighter and tighter to the part of the spike protein that is necessary for protection.  So, the amount of antibody alone doesn’t tell you how good your antibodies are.
  7. Protection depends on a lot more than antibodies and immune cells.  If a person is extremely stressed, tired, or poorly nourished, their susceptibility to disease increases.
  8. As Dr Castillo and Dr Ansell have pointed out, WMers vary enormously.  Some of us have tremendous immune impairment, while others have perhaps no impairment at all.
  9. Those are only a few of the factors involved!

So, you can see that nobody — no matter how much research is done — can give you a precise number on how many antibodies are protective.  That is true even if there were assays that were precise enough to give such a number.

It’s normal in these worrisome times to want a precise answer.  There isn’t one.  All we can do is get vaccinated, practice safe distancing habits, take good care of ourselves, and hope for the best.

Glenn Cantor, DVM, PhD