Natural immunity to COVID-19: how long and how strong?
Two new studies provide insight into the strength and length of natural immunity to COVID-19.
As emerging variants rapidly increase the spread of COVID-19 in many countries, such as the UK and Brazil, the proportion of past infections in different populations is becoming significant. These increasing pools of previously infected people may even be enough to assist vaccination programs in reaching the threshold of immunized citizens required to achieve herd immunity – a potential silver lining of the failure to prevent the rampant spread of novel variants in some countries.
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However, before these groups can be factored into our estimations of reaching herd immunity and easing the impact on healthcare systems, two vital questions need to be answered; how long does natural immunity last and how effective is it at stopping re-infection?
Two recent papers from Public Health England (PHE; London, UK) and Rockefeller University (NY, USA) have shed new light on the duration and quality of protection provided by natural immunity after infection with COVID-19.
How protective is natural immunity to COVID-19?
Led by Susan Hopkins, the PHE SIREN study – yet to be peer reviewed – examined a group of healthcare workers in the UK split into a positive and negative cohort. The positive cohort consisted of 6614 people, each of whom had either a prior positive test or were antibody positive. The negative cohort – containing 14,173 patients – had no previous positive test for COVID-19 and tested negative for antibodies for SARS-CoV-2 [1].
Observing these cohorts between 18 June and 9 November 2020, SIREN found 44 reinfections in the positive cohort, compared to 402 infections in the negative cohort. Using these data, the team established that prior infection provided an 83% effective protection against reinfection [1].
The Rockefeller study, led by Michel Nussenzweig, examined the antibody responses of 87 people 1 and 7 months after infection. This study revealed that while antibody levels were significantly reduced at the 7–month mark compared to at 1 month, the levels of memory B cells were the same, if not higher, in some individuals [2].
These memory B cells are produced after an initial infection and can rapidly create antibodies on reinfection with the same pathogen. Further investigation of the B cells in the participants at the 7–month mark indicated that several mutations had been acquired during this time, resulting in higher quality antibodies, more capable of nullifying SARS-CoV-2 and binding to other variants of the virus [2].
Commenting on this finding, Nussenzweig stated that the team was, “…surprised to see the memory B cells had kept evolving during this time. That often happens in chronic infections, like HIV or herpes, where the virus lingers in the body. But we weren’t expecting to see it with SARS-CoV-2, which is thought to leave the body after infection has resolved [3].”
How long does natural immunity to COVID-19 last?
The more recent Rockefeller study seems to indicate that at 7 months the immunity observed was of high quality, due to the continued evolution of the B cells. To explain why these improved B cells, with their more effective antibodies, were stimulated to continue evolving after infection subsided the researchers teamed up with Saurabh Mehandru (Mount Sinai Hospital, NY, USA).
Testing the hypothesis that lingering viruses or sections of the virus caused this evolution, Mehandru studied intestinal biopsies from 14 individuals who had recovered from COVID-19. In seven of these individuals, Mehandru found genetic material and proteins from SARS-CoV-2 in the cells lining their intestines [3]. The presence of these materials could well be what drove the evolution of the B cells, a theory that the team is keen to test further in more people.
Meanwhile, the SIREN study noted that the median protective effect was observed for 5 months following infection. As the study progresses, allowing for a longer follow-up time, it is quite possible that this median will increase.
Both of these studies are encouraging, suggesting that the increasingly significant number of people who have had COVID-19 are likely to be protected against reinfection for at least 5 months, potentially with increasingly effective immunity. However, these results should not be used to remake the case for the pursuit of herd immunity by allowing the unrestricted spread of the virus.
At the time of writing 3.66 million of the UK’s 66.65–million strong population has, at some time, tested positive for COVID-19, just under 5.5%, with a recent modeling study suggesting that up to 12.4 million have been infected as of 3 January – still only 20% at the highest estimate. These figures correspond with over 100,000 deaths – the worst death rate in the world. These data back a conclusion that deliberately increasing infections to reach herd immunity – a potential route to the end of the crisis that was floated, and left behind, in 2020 – is insupportable. What’s more, if immunity is only inferred for 5–7 months, many of those infected in the first wave of the virus may now be vulnerable to reinfection.
If the increased percentages of immunity in the population do indeed represent a silver lining of rising infections, the strain placed on healthcare systems and, ultimately, deaths are the wind and rain that accompany it. The glint of silver by no means outshines the shadowy mass of the cloud.
