Which seems good and long lasting and cheap, lots of evidence below.
https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(21)00676-9/fulltext
Biological studies
Dan et al (2021) Science, Immunological memory to SARS-CoV-2 assessed for up to 8 months after infection.
https://www.science.org/doi/abs/10.1126/science.abf4063
95% of participants tested retained immune memory at about 6 months after having COVID-19
More than 90% of participants had CD4+ T-cell memory at 1 month,
and 6–8 months after having COVID-19
Wang et al (2021) Science, Ultrapotent antibodies against diverse and highly transmissible SARS-CoV-2 variants
https://www.science.org/doi/abs/10.1126/science.abh1766
Previous SARS-CoV-2 infection,
with an ancestral variant produce antibodies that cross-neutralise emerging variants of concern with high potency
Epidemiological studies
Hansen et al (2021) Lancet, Assessment of protection against reinfection with SARS-CoV-2 among 4 million PCR-tested individuals in Denmark in 2020: a population-level observational study
https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(21)00575-4.pdf
People who had had COVID-19 previously were around 80·5% protected against reinfection
Pilz et al (2021) European Journal of Clinical Investigation, SARS-CoV-2 re-infection risk in Austria
https://onlinelibrary.wiley.com/doi/full/10.1111/eci.13520
Sheehan et al (2021) Clinical Infectious Diseases, Reinfection rates among patients who previously tested positive for COVID-19: a retrospective cohort study
https://academic.oup.com/cid/article/73/10/1882/6170939?login=true
Shrestha et al (2021) Preprint, Necessity of COVID-19 vaccination in previously infected individuals
https://www.medrxiv.org/content/10.1101/2021.06.01.21258176v3
Retrospective cohort study in the USA,
People who had had COVID-19 previously were 100% protected against reinfection
Gazit et al (2021) Preprint, Comparing SARS-CoV-2 natural immunity to vaccine-induced immunity: reinfections versus breakthrough infections
https://www.medrxiv.org/content/10.1101/2021.08.24.21262415.abstract
Kojima et al (2021) Preprint, Incidence of severe acute respiratory syndrome coronavirus-2 infection among previously infected or vaccinated employees
https://www.medrxiv.org/content/10.1101/2021.07.03.21259976v2
Laboratory staff routinely screened for SARS-CoV-2,
people who had had COVID-19 previously were 100% protected against reinfection
Clinical studies
https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(21)00675-9.pdf
Large, multicentre, prospective cohort study
Previous COVID-19 diagnosis,
84% decreased risk of infection
Letizia et al (2021) Lancet, Respiratory Medicine, SARS-CoV-2 seropositivity and subsequent infection risk in healthy young adults: a prospective cohort study
https://www.thelancet.com/pdfs/journals/lanres/PIIS2213-2600(21)00158-2.pdf
Prospective cohort of US Marines,
seropositive young adults were 82% protected against reinfection
Adnan et al (2021) Clinical Infectious Diseases, Reinfection With Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) in Patients Undergoing Serial Laboratory Testing
https://academic.oup.com/cid/article/74/2/294/6251701?login=false
N = 9,119, serial tests
Reinfection rates, 0.7%
So
Risk of repeat SARS-CoV-2 infection decreased by 80·5–100% among those who had had COVID-19
Protection from reinfection is strong and persists for more than 10 months of follow-up, (Hansen et al 2021 Lancet)
Turner, et al Nature, SARS-CoV-2 infection induces long-lived bone marrow plasma cells in humans
https://www.nature.com/articles/s41586-021-03647-4?amp%3Bcode=7bafb609-23c2-4665-804b
Madhuumita et al, Plos One, T cell response to SARS-CoV-2 infection in humans: A systematic review
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0245532
SARS-CoV-2 infection induces specific and durable T-cell immunity,
Nina et al, (2020) Nature, SARS-CoV-2-specific T cell immunity in cases of COVID-19 and SARS, and uninfected controls
https://www.nature.com/articles/s41586-020-2550-z?flip=true
Memory B-cell response to SARS-CoV-2 evolves between 1·3 and 6·2 months after infection, which is consistent with longer-term protection
Some people who have recovered from COVID-19 might not benefit from COVID-19 vaccination
https://www.medrxiv.org/content/10.1101/2021.04.20.21255670v1
One study found that previous COVID-19 was associated with increased adverse events following vaccination with Pfizer
https://www.journalofinfection.com/article/S0163-4453(21)00277-2/fulltext
In Switzerland, proof of recovered infection, in the past 12 months are considered equally protected as fully vaccinated,
https://www.schengenvisainfo.com/news/switzerland-plans-to-extend-covid-certificate-requirement-until-mid-november/
