There were three mutations of the original Kalavirus (in 2033, 2037 and 2040).

It seems Foster may have developed a vaccine for the 2040 and the 2033 mutations, but not the original strain.

With 98% of the worlds population dead from the original strain, if you could go back and vaccinate against the mutated strains in the past, you would only be saving ~120 million people. 7 billion would still have died in the first outbreak.

Then there are logistics. The people of 2043 have neither the supplies nor distribution abilities to create a vaccine for over 100 million people, and no infrastructure to distribute it. But given they have a time machine, they could go back and invest in Apple or something, become billionaires and buy a vaccine manufacturer.

How then do they get people to take a vaccine for an unknown virus? Getting almost 200 countries around the world to vaccinate their populations would be difficult, to say the least. And without a treatment for the original strain,  people would still start dying.

It seems that with the virus regularly mutating, any treatment was going to be short-lived, and Jones wanted to save the  7 billion.