There has been a continually changing understanding of many aspects of the COVID-19 pandemic. One such aspect is the amount and duration of immunity conferred by immunization and natural disease. We now have more data and experience with both the virus and vaccination. For instance, we once thought, probably erroneously, that vaccination conferred broader and more durable immunity compared to immunity gained after infection. Also, emerging new variants, especially omicron, changed the equations. Undoubtedly, SARS-CoV-2 is a tough, nasty, resourceful and persistent virus that has a talent for evading immune protection from both infection and vaccination; thus, it is very difficult to control. Think whack-a-mole.
Here’s my evaluation of what is known about the amount and duration of acquired immunity. Our understanding will continue to evolve.
Best evidence available from studies with the initial two shots of an mRNA vaccine, although not entirely consistent, is that immunity after infection is roughly the same or greater than immunity from vaccination. Some studies suggest protection against reinfection from natural disease lasts about 6 months and possibly over a year. Reinfections are typically less severe than primary infections.
Pre-Omicron vaccine-conferred immunity against infection, although initially very high, around 95%, waned after about four to six months to low levels of protection but remained high for protection against severe disease and death. Booster immunity — the third dose — restored protection against infection but was similarly short lived.
Hybrid immunity, which is any combination of vaccination plus natural disease-induced immunity, seems to confer the greatest overall protection. These studies do not include boosters. Additionally, two studies reported this spring in the New England Journal of Medicine provide strong evidence that for previously infected individuals, vaccination provides substantial additional and longer-lasting protection.
Omicron deserves special consideration. It has particular propensities to spread and to evade our immunity after infection. The result is greatly rising reinfection rates. Of reported cases, reinfection once accounted for 1% of infections; it is now 10%. The effectiveness of prior infection in protecting against reinfection has dropped from 90% with previous variants to only 56% with omicron. Also, breakthrough infections after vaccination and the percent of hospitalizations and deaths among the elderly vaccinated population have greatly risen too.
Also early data suggest for omicron, third and fourth doses provide improved immunity but relatively less protection against infection. Boosters do remain highly effective for preventing severe infections and death. Boosters are extremely important, especially in the older population and the immunocompromised.
The pandemic is not over yet. We should act more like it. But with an estimated 50% of Americans already infected along with two-thirds vaccinated, there will soon be sufficient “collective” immunity to move from the epidemic to the endemic phase of COVID-19. Then, we hope for a more predictable, controllable and less severe disease experience with a possible influenza-like seasonal pattern only requiring yearly vaccination. But what vaccination will that be? What variants will it contain? And is it possible to create a more universal COVID vaccine effective against all present and future strains?
Multiple studies demonstrate immunity from disease and vaccination to be similarly significant compared to non-vaccinated, non-previously infected individuals against infection, hospitalization and death. But it will be immunization, unlike natural disease, that will continue to move us to the endemic phase while minimizing morbidity and mortality.
Dr. Richard Feldman is an Indianapolis family physician and the former state health commissioner. Send comments to [email protected]