True. Cross-immunity works if the viruses are similar. In this paper, half of the samples (n=37) that were not infected with SARS showed cross-immunity responses. 
https://twitter.com/Aziz71867492/status/1418076705240109056
Sailors who spend a lot of time on ships are said to be careful to take a lot of vitamin D. This is because they don't want to get sick on the sea. Vitamin D activates immune function. It is said to be an essential nutrition for T cells that destroy infected cells.
If you are young, most people will recover from the infection and cross-immunity will work after that. Vaccines may also be effective for the elderly and those suffering from diseases.
We should not expect antibodies from a vaccine against a highly mutated virus. The antibodies don't seem to last very long to begin with. It is important to stay healthy and take vitamin D to keep the immune system working properly.
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