What We Know and Don’t Know about Naturally Acquired Immunity

Recently there has been a lot of debate over the degree to which recovering from a COVID infection confers immunity from future infections.  Frankly, there has been outrageous misinformation repeated both in the media and on social media by both COVID deniers and COVID doomsters.  There is still a lot we do not know about naturally acquired immunity, but a clearer picture has been gradually emerging.

First, anyone who says there is no evidence of naturally acquired immunity is just flat wrong.  As discussed in this New Times article, reinfections have been astonishingly rare.  BNO News established this on-line registry for reinfections.  So far, only 29 lab confirmed reinfections and 1,713 suspected reinfections are registered on the site – out of 72 million confirmed cases worldwide.

There have undoubtedly been more reinfections than those on the BNO registry.  But I did an extensive Google search for reinfections and there are literally a handful of documented cases.  With over 72 million cases, if reinfections were common we would be hearing about millions of them.  I recently heard a doctor in charge of the COVID response for one of the Texas Medical Center hospitals say that there have been no confirmed reinfections at TMC.

Some reinfections, even with robust natural immunity, should be expected because of the tremendous variation in the immune system between individuals.  This is especially true today because of the number of people whose immune system is weakened by AIDS, radiation and chemotherapy, arthritis medications – not to mention substance abuse and obesity.  Given the fact that such conditions are so widespread, it is remarkable that reinfections have not been more common.

The fact that reinfections are so vanishingly rare is a very strong indication that our immune system is producing a robust natural immunity.  The degree of the protection will vary.  In some, it will prevent the person from getting the disease at all; in others it may only reduce the severity of a second exposure.  But anyone who tells you there is no immunity, or questions whether is exists at all, is either a charlatan or grossly misinformed.

Still, there are some important aspects of natural immunity to COVID that we do not fully understand.

Most importantly, we do know if a person with natural immunity can continue to spread COVID.  When a person with natural immunity comes into contact with the virus a second time, it does not simply bounce off and fall on the ground.  It will attempt to reinfect the person, but the body’s immune response will more quickly fight it off, having been prepped by the earlier infection.  But during that time the person might well be shedding virus onto people around them.  The consensus at this point seems to be previous infection probably reduces that person’s transmission of the virus but does not totally eliminate it.

The second question that has been raised is how long natural immunity lasts.  Last summer there were a slew of studies finding that the number of antibodies were declining within a few months after recovering from COVID.¹  These set off the “hair-on-fire” crowd to conclude that immunity would be short-lived and, therefore, it would be impossible to ever reach herd immunity.  But as explained in this Scientific American article, a decline in antibodies is normal and not an indication that the immunity has ended.  Exactly how long it will last is still an open question, but the consensus seems to be that it lasts at least many months and probably longer.

We also do not know how naturally acquired immunity will compare to the immunity induced by a vaccination – other than the obvious point that you don’t have to go through getting sick to become immune with a vaccine – not a trivial advantage.  Rand Paul made some headlines recently by suggesting natural immunity was more effective than vaccine-induced because the reinfection rate from natural immunity is less than those who vaccinated and nonetheless got COVID.  That is not a sound analysis because the data on reinfections is so limited.

Some previous vaccines for other diseases have proved more effective than natural immunity, but some have been less effective.  I think the better conjecture at this point is that the vaccine probably produces a better immune response across the entire population because these new vaccine platforms (mRNA and viral vector) are so intently focused on COVID’s particular antigen. However, some have questioned whether that focus might make the vaccines less effective against future mutations of the virus than natural immunity would.  We will probably not have a definitive answer on this question for years.  The good news is that both infections and the vaccines appear to be very effective at inducing a robust immunity.

But to me, this is not an “either-or” question.  Personally, if I had recovered from COVID, I would still get the vaccine.  It could be that there will be some cumulative benefit.  And there do not appear to be any significant risks from getting the vaccine.  Why take a chance?

The more interesting question is what effect naturally acquired immunity will have on reaching herd immunity.  More on that soon.

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Note 1 – Some more recent studies have long found antibodies prevalence.  See here and here.