This is based on the theory that the immune system response to Oxford's virus vector is mostly negating the benefit of the second dose taken 21 days after the first. Gam-COVID-Vac gets around this by using different viruses for each dose, and has 90% efficacy results from their Phase III trial. Unfortunately, no one can yet manufacture it in anything like bulk quantities, but they have set up manufacturing partnerships with a whole bunch of countries from India to South Korea.