One of these credible sources is the World Health Organization (WHO), which provides the following summary around the efficacy (how the vaccine performs under controlled conditions) and effectiveness (how the vaccine performs in the greater population) of COVID-19 vaccines:

Efficacy
To calculate the vaccine’s efficacy, the infection rates between those who aren’t vaccinated and those who have received a vaccine, are compared. If the infection rate of the vaccinated is reduced by 80% compared to the infection rate of the unvaccinated for example, this means that the vaccine has an efficacy rate of 80% since 80% fewer people contracted the virus when coming into contact with it. The vaccines registered in South Africa have high efficacy rates, meaning you are less likely to experience moderate to severe disease that may require hospitalisation once you’ve been vaccinated.

It is important to remember that vaccines do not provide 100% protection and that infections can still occur. The expectation is that as more people get vaccinated, there will be fewer people who could transmit the virus.

Typically, a vaccine takes two to three weeks from the date of the final dose of the vaccine to be fully effective. Therefore, it is important that you still take all the relevant precautions to protect yourself and your loved ones. This includes wearing a mask and social distancing.

Effectiveness
Vaccine effectiveness, on the other hand, is a measure of how well vaccines work in the real world. Clinical trials include a wide range of people – a broad age range, both sexes, different ethnicities and those with known medical conditions – but they cannot be a perfect representation of the whole population. The efficacy seen in clinical trials applies to specific outcomes in a clinical trial. Effectiveness is measured by observing how well the vaccines work to protect communities as a whole. Effectiveness in the real world can differ from the efficacy measured in a trial, because we can’t predict exactly how effective vaccination will be for a much bigger and more variable population getting vaccinated in more real life conditions.

Source:
https://www.who.int/news-room/feature-stories/detail/vaccine-efficacy-effectiveness-and-protection