You may have antibodies after the first dose of a COVID-19 vaccine, but a second dose is necessary to achieve the desired protection. The COVID-19 vaccines reach their full protective effect after 2 to 3 weeks after the administration of the second dose. At that time, you have reached the highest antibody concentration and you are best protected. The booster dose or third dose also needs approximately 14 days to reach its full protective effect.
The Johnson & Johnson COVID-19 vaccine is an exception, this COVID-19 vaccine reaches its optimal protection after approximately 4 weeks.