Except when they are not similar, that is. This is more common than you seem to think, e.g. TBE - tick-borne encephalitis - is a nasty piece of work of a diseas

Except when they are not similar, that is. This is more common than you seem to think, e.g. TBE - tick-borne encephalitis - is a nasty piece of work of a disease which can leave the victim with permanent disabilities (around 25%), dead (around 2.5% for the strain common here in Sweden, far higher for the strain endemic in Russia), cognitively impaired and more. I got it in 2005 (which made me the "index patient" for this part of the country, it had not been seen before in this area) and it was... unpleasant. Now that I had the disease I am immune for life. My wife and children got vaccinated to avoid my fate. The vaccination consists of three initial doses followed by a new dose every 3d to 5th year, depending on age (the older, the more frequent). The current crop of SARS2 vaccines comes of far worse than the TBE vaccine with immunity waning in a period counted in weeks, not years [1,2,3]. Natural immunity is "from 6 to 27 times stronger" depending on which researcher you listen to. Here's part of the abstract of the largest study (~700.000 test subjects) on these matters: SARS-CoV-2-naïve vaccinees had a 13.06-fold (95% CI, 8.08 to 21.11) increased risk for breakthrough infection with the Delta variant compared to those previously infected, when the first event (infection or vaccination) occurred during January and February of 2021. The increased risk was significant (P<0.001) for symptomatic disease as well. When allowing the infection to occur at any time before vaccination (from March 2020 to February 2021), evidence of waning natural immunity was demonstrated, though SARS-CoV-2 naïve vaccinees had a 5.96-fold (95% CI, 4.85 to 7.33) increased risk for breakthrough infection and a 7.13-fold (95% CI, 5.51 to 9.21) increased risk for symptomatic disease. SARS-CoV-2-naïve vaccinees were also at a greater risk for COVID-19-related-hospitalizations compared to those that were previously infected. [1] https://www.science.org/content/article/having-sars-cov-2-once-confers-much-greater-immunity-vaccine-vaccination-remains-vital https://www.science.org/content/article/having-sars-cov-2-on... [2] https://www.westernjournal.com/johns-hopkins-doc-says-natural-immunity-27-times-effective-vaccine/ https://www.westernjournal.com/johns-hopkins-doc-says-natura... [3] https://www.medrxiv.org/content/10.1101/2021.08.24.21262415v1 https://www.medrxiv.org/content/10.1101/2021.08.24.21262415v...