It is not well known how many doses we need to get near full immunity. Some other vaccines require up to 3 doses https://www.cdc.gov/vaccines/schedules/hcp/imz/adult.html
Now suddenly a 3rd is necessary? What happens when that fails, a second booster? A yearly vaccine like the flu shot (which barely works anyways)? Unfortunately for us humans, nature finds a way.
Speaking of nature, you know what confers full immunity? Having the infection and then recovering or having cross reactive immunity from a previous coronavirus strain.
Natural immunity is the undisputed champion of immunity.