According to a recent study, people over the age of 80 that were given the second dose of the Pfizer COVID-19 vaccine 12 weeks after the first dose ended up with over three times as many antibodies after the second dose. While this study was only for the Pfizer vaccine it could significantly change the vaccination strategy of many countries.
This is another example that clinical trials of the COVID-19 vaccine was not designed to discover the optimal regimen for public health and we should expect to keep on learning new things.