A
key unanswered question about Covid-19 is how long a person can expect to be immune after recovering from illness. Answering this question is crucial for understanding how extensive our vaccination programs must be to drive down transmission, planning for next year, and identifying long-term strategies for managing Covid-19. A
new study suggests that the answer may be years, not months.
To be clear, the study actually establishes that protective immunity from prior infection by SARS-CoV-2 lasts at least seven months. (Note: this is similar to what we
currently know about immunity conferred by vaccination with the Pfizer and Moderna vaccines.) It does not, however, establish exactly how long that immunity lasts. At the end of this post, I explain how we can extrapolate from their findings to find a (speculative) duration of 4.7 to 8.4 years.
So, to conclude, the study establishes that protective immunity from prior infection by SARS-CoV-2 is pretty robust, at least for seven months. It has not established how long that immunity lasts. However, we can make some rough calculations if we’re willing to go out on a limb and extrapolate. (Read the article for their methodology)
From this study, the protective effect is 0.841, the fraction that fails is around 0.05 or 0.10, and the study duration is 207 days, which gives a waning rate of 0.000328 to 0.000589. Taking the inverse gives a theoretical average time until immunity is lost of 1699 to 3053 days or 4.7 to 8.4 years. Interestingly, this appears to be
about double that of other coronaviruses that circulate in people.
So all in all, this is good news. A long duration of natural immunity means that vaccination programs do not need to be sustained as intensely to drive down transmission, and lends some added flexibility to future planning and long-term management strategies for Covid-19.
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