“We reviewed studies published in PubMed from inception to Sept 28, 2021, and found well conducted biological studies showing protective immunity after infection (panel). Furthermore, multiple epidemiological and clinical studies, including studies during the recent period of predominantly delta (B.1.617.2) variant transmission, found that the risk of repeat SARS-CoV-2 infection decreased by 80.5–100% among those who had had COVID-19 previously (panel). The reported studies were large and conducted throughout the world. Another laboratory-based study that analyzed the test results of 9119 people with previous COVID-19 from Dec 1, 2019, to Nov 13, 2020, found that only 0.7% became reinfected. In a study conducted at the Cleveland Clinic in Cleveland, OH, USA, those who had not previously been infected had a COVID-19 incidence rate of 4.3 per 100 people, whereas those who had previously been infected had a COVID-19 incidence rate of 0 per 100 people.
Furthermore, a study conducted in Austria found that the frequency of hospitalization due to a repeated infection was five per 14,840 (0·03%) people and the frequency of death due to a repeated infection was one per 14,840 (0·01%) people… Due to the strong association and biological basis for protection, clinicians should consider counseling recovered patients on their risk for reinfection and document previous infection status in medical records.
[T]hose studies show that protection from reinfection is strong and persists for more than 10 months of follow-up, it is unknown how long protective immunity will truly last. Many systemic viral infections, such as measles, confer long-term, if not lifelong, immunity, whereas others, such as influenza, do not (due to changes in viral genetics). We are limited by the length of current reported follow-up data to know with certainty the expected duration that previous infection will protect against COVID-19. Encouragingly, authors of a study conducted among recovered individuals who had experienced mild SARS-CoV-2 infection reported that mild infection induced a robust antigen-specific, long-lived humoral immune memory in humans…
Some people who have recovered from COVID-19 might not benefit from COVID-19 vaccination. In fact, one study found that previous COVID-19 was associated with increased
adverse events following vaccination with the Comirnaty BNT162b2 mRNA vaccine (Pfizer–BioNTech)…
Given the evidence of immunity from previous SARS-CoV-2 infection, however, policy makers should consider recovery from previous SARS-CoV-2 infection equal to immunity from vaccination for purposes related to entry to public events, businesses, and the workplace, or travel requirements.”