We have known for a long time that a COVID infection does not provide lasting immunity. One of the first big examples of this was in Manaus, Brazil. “A study of blood donors indicated that 76% (95% CI 67–98) of the population had been infected with SARS-CoV-2 by October, 2020.2 High attack rates of SARS-CoV-2 were also estimated in population-based samples from other locations in the Amazon Basin—eg, Iquitos, Peru 70% (67–73). The estimated SARS-CoV-2 attack rate in Manaus would be above the theoretical herd immunity threshold (67%), given a basic case reproduction number (R0) of 3.
In this context, the abrupt increase in the number of COVID-19 hospital admissions in Manaus during January, 2021 (3431 in Jan 1–19, 2021, vs 552 in Dec 1–19, 2020) is unexpected and of concern (figure). After a large epidemic that peaked in late April, 2020, COVID-19 hospitalisations in Manaus remained stable and fairly low for 7 months from May to November, despite the relaxation of COVID-19 control measures during that period (figure).” BMJ 2021;372:n394
There are a number of explanations, each of which may contribute to the outcomes that were seen. First, prior infection may not provide robust immunity to newer variants. The relationship between particularly bad waves of cases and the percentage of a particular variant at any given time is easy to see in Brazil. This in itself is a major argument IN FAVOR OF updating vaccines. That doesn’t happen with “natural immunity.”
For the sake of argument though, assume that there are no major emerging variants of concern and that “natural immunity” and vaccination provide equal protection. Those who rely on “natural immunity” fail to understand that immunity wanes over time. In this relatively small sample (n=72), 36% of those who were infected NEVER developed detectable antibodies. This was the case even when high cycle thresholds were used (ie, higher sensitivity in testing). Emerg Infect Dis. 2021;27(9):2454-2458.
Another flaw in this reasoning is that we don’t know what titer threshold would actually provide immunity, suggesting that the 36% value is likely an underestimate of the true percentage that do not have immunity after infection.
We also know that immune protection wanes over time, and as alluded to above, does not provide as much protection against new variants, which we have known since the original emergence of Omicron. In a meta-analysis, “Our findings confirm that past infection affords significantly reduced protection against re-infection by the omicron BA.1 variant compared to previous variants,” which further supports the needed for regular, updated boosters.
Lancet. 2023 Mar 11;401(10379):833-842
A final flaw with “natural immunity” is that it ignores that there much higher risks morbidity and morality after infection among the unvaccinated or undervaccinated. The risk of death alone is 14x higher in the unvaccinated population. MMWR Morb Mortal Wkly Rep 2023;72:145–152.
Even if we were able to prevent these excess deaths from COVID, the data is clear about the risks of COVID reinfections in multiple physiological systems, placing the final nail in the coffin of “natural immunity.” Nat Med 28, 2398–2405 (2022).
Further evidence in a current preprint also indicates that “natural immunity” does not last long, and is less protective against new variants. “Across multiple Omicron waves, protection against reinfection was significantly higher in those previously infected with more recent than earlier variants, even at the same time from previous infection. Protection against Omicron reinfections decreased over time from the most recent infection if this was the previous or penultimate variant (generally within the preceding year).”