One shot can cover more than one strain. There are two strains of smallpox, and the vaccine covered both, as well as making you immune to cowpox. The measles vaccine covers several strains.
It’s also possible to have more than one thing in a shot. There’s one shot for measles, mumps and rubella, and one shot for diphtheria, pertussis and tetanus. Even more, IIRC, my son got a booster of something called Pediarix, which is DTaP, plus HepB & polio. And wasn’t the second year of H1N1 a shot that had antibodies for H1N1 plus whatever new strain was predicted to come around that year? The only reason two vaccines had been required the previous year was that H1N1 had taken people by surprise.
When multiple strains require different tweaks to cover them, they often include the in one multivalent shot. This is typical for flu, where often the 3 or 4 strains predicted to be most prevalent and dangerous are combined in one shot.
Bearing in mind that developing the multivalent vaccines takes time too.
So far the identified strains of SARS-nCov-2 aren’t so different as to need different vaccines based on their planned mechanisms of attack. But that day may well come. At which point the current vaccine starts working less well than it had been and a new R&D effort needs to be launched to counter the new vaccine-resistant strain(s).
I’m not trying to claim the Cassandra role here. This is just a possibility, and not one widely thought to be imminent.
Can’t recall if this has been mentioned up-post, but one of the things making “Warp Speed” vaccine development so fast for Covid-2 is that a lot of vaccine groundwork had been laid down for SARS (Covid-1).
So, I’m guessing someone’s on this soon, if not already.
Development of pathogen resistance hasn’t been a significant problem for existing vaccines (though the tendency of influenza strains to undergo genetic drift and occasionally profound shifts has obviously limited the effectiveness of seasonal flu vaccines).
While thus far we haven’t seen much in the way of genome mutation from SARS-CoV-2, caution is advised in that we are breaking new ground in developing a vaccine against a coronavirus.