In some countries, like the UK, people have been waiting extended periods of time for their second COVID vaccine dose. The reason for this is to be able to administer more people with at least some level of protection before administering second doses. While this may sound like a reasonable plan, some experts warn that there is a risk to the longer wait times between doses.
It is a possibility for an escape variant of COVID-19 to be produced, which means the virus could mutate and become resistant to the vaccine during the extended period of time. However, it is difficult to predict such a possibility and it is extremely rare. What studies have shown is reduced transmission between those who have received the vaccine, and that is what we know for now.