Production capacity. The equipment to make the effective vaccines in high enough quantity simply doesn't exist yet. The easier to produce vaccines are either le

Production capacity. The equipment to make the effective vaccines in high enough quantity simply doesn't exist yet. The easier to produce vaccines are either less effective (AstraZeneca) or haven't finished demonstrating how effective they are (Johnson & Johnson, CanSino, Gamelya, Sinovac, SinoPharm, and Novavax are still in trials). The risk with a less effective vaccine is that the virus could evolve resistance to whatever particular antibody types the vaccine tends to cause the body to produce, since there would be more vaccinated people who could get infected. Then we'd be back to square 1, trying to develop a new vaccine for the new variant. We REALLY want to avoid that.