Assume that earlier sustained contact and trade means that the New World natives in settled cultures obtain horses and various cattle/stock from the Old-Worlders and these are spread via trade routes over a century or so (probably sheep/pigs/cows). This would certainly expedite both resistance AND the creation of new variants that kill off unprepared Old-Worlders that are NOT part of the sustained contact. It might be able to increase surviving native populations significantly and slow European colonization.
I'm not talking invincible here, but I'm saying say 60-70% die off instead of 90% plus a few return shots to the Old World if you give them several (3-4) centuries. One interesting situation might be that if draft animals reach the farming cultures, large die-offs might encourage the use of draft animals to farm for food because of labor shortages. Of course, I also suspect that reintroduction of horses contributed mightily to the re-emergence of nomadic life-styles but that might only occur west of the Mississippi River valley.
West of this
I'm not talking invincible here, but I'm saying say 60-70% die off instead of 90% plus a few return shots to the Old World if you give them several (3-4) centuries. One interesting situation might be that if draft animals reach the farming cultures, large die-offs might encourage the use of draft animals to farm for food because of labor shortages. Of course, I also suspect that reintroduction of horses contributed mightily to the re-emergence of nomadic life-styles but that might only occur west of the Mississippi River valley.
West of this