1) The virus mutates or behaves in an otherwise completely unexpected way. Not to be discounted, as the notion that it’s always human ingenuity that triumphs in the end comes from fiction/Hollywood.
It would seem that the not-so-novel coronaviruses that remain in general circulation (HKU1, NL63, OC42 and C229E) and were once more deadly, but now tend to only cause common colds.
2) A vaccine. Don’t hold your breath, as we have never developed one for a coronavirus. Also, beware even of the magic word, as the Oxford University ‘vaccine’ currently being tested won’t stop the vaccinated from passing on the infection.
Medicines might help in other ways, reducing symptoms to the point that we no longer worry so much, or reducing the innate infectiousness of the virus.
3) Herd immunity i.e. roughly 60% of the population becomes immune.
This has become the solution favoured by right-wingers, a nakedly selfish speculation posing as a collective solution.
The trouble is, we have never developed durable immunity to any of the four main coronaviruses that continue to circulate.
The ones that don’t circulate (SARS V1, MERS) didn’t spread so well because only those with symptoms passed them on, and we were thus able to smother them quite quickly.
Also, infection by SARS-COV-2 should not be something that you’d wish on your worst enemy. There appear to be multiple side-effects and longer lasting complications for survivors relating to lung capacity, coagulation, liver function and cognitive impairment. Not to mention gammy toes.
And, as noted above, we have yet to develop any sort of lasting immunity to the four main coronaviruses in general circulation.
4) Somehow the entire world behaves like one big New Zealand.
A little bit late for that...