You can get the omega-3 index in vegans to the recommended 4.4 baseline within 4 months using algal-derived omega-3 fatty acid supplements:

https://pubmed.ncbi.nlm.nih.gov/24679552/

“Conclusions: We conclude that vegans have low baseline omega-3 levels, but not lower than omnivores who also consume very little docosahexaenoic and eicosapentaenoic acids. The vegans responded robustly to a relatively low dose of a vegetarian omega-3 supplement.”