"If you think that you're protected because you had COVID-19 before, that's not the case, and it's really important that people go and get vaccinated even if they've had COVID before," Dr Catherine Smallwood, a senior emergency officer at the World Health Organization's Europe office, told Euronews.
Dr Smallwood called on eligible Europeans to get their booster as soon as possible, saying a third vaccine dose would help its effectiveness.
Her comments come as Europe is amid another wave of COVID, driven in some countries by the more contagious Omicron variant.
"We've seen potentially a five-fold increase in the risk of being reinfected with Omicron as compared to the Delta variant that's also circulating, and we have seen that vaccines prevent severe disease, including for the Omicron variant," she went on.
"So don't think you're protected by natural immunity.".
It has protected the lives of over 500,000 people in Europe alone.
Watch the full interview with Dr Smallwood in the video player, above.