Which seems good and long lasting and cheap, lots of evidence below.

https://www.thelancet.com/journals/la...

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/10.11...

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/10.11...

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/journa...


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/f...

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/...

Shrestha et al (2021) Preprint, Necessity of COVID-19 vaccination in previously infected individuals

https://www.medrxiv.org/content/10.11...

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/10.11...

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/10.11...

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/journa...

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/journa...

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/...

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/s4158...

Madhuumita et al, Plos One, T cell response to SARS-CoV-2 infection in humans: A systematic review

https://journals.plos.org/plosone/art...

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/s4158...

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/10.11...


One study found that previous COVID-19 was associated with increased adverse events following vaccination with Pfizer

https://www.journalofinfection.com/ar...

In Switzerland, proof of recovered infection, in the past 12 months are considered equally protected as fully vaccinated,

https://www.schengenvisainfo.com/news...