This is an overly simplistic calculation. They are only taking into account the current days vaccination rate instead of calculating the rate of change in vaccinations to extrapolate the future trend. The problem is that new vaccine production is increasing, and distribution channels are improving.

It also fails to include immunity for those who have had covid. While not a perfect measure, proven cases can be used to estimate a persons immunity duration as well as the number of people who had covid but were never diagnosed with covid.

The third variable to consider is the rate that vaccines will become available as some countries reach herd immunity. For example, the US is likely to reach herd immunity by summer based on the above information. Once that happens, more of the vaccine production will go to countries that are still working towards herd immunity.

With that said I would look at this as a worse case scenario for herd immunity and not as any kind of actual case scenario.