"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.
Designated "a variant of concern" by WHO, reports of the rapid transmission of Omicron have been reported from at least 38 countries.
Despite preliminary data showing that the symptoms of the new variant are mild and that the death toll, Smallwood believes Omicron "has the potential to develop severe diseases".
"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.
Smallwood specified that the early deaths associated with Omicron were observed in people who had not received a single dose of the vaccine.
"So don't think you're protected by natural immunity."
She called on people to trust the data gathered by the WHO and world scientists.
"Be confident in the data that we have — a billion doses already used in the European region. We have strong data on safety, with over a year of the roll-out of vaccines. It has protected the lives of over 500,000 people in Europe alone. This is really the most powerful tool we have right now at the individual level to really protect ourselves and our loved ones from COVID," she added.
"The good news is that our current evidence really points toward the vaccines continuing to do their job, which means that they'll continue to save lives and prevent people from being hospitalised with COVID-19," Smallwood noted.
Watch the full interview with Dr Smallwood in the video player, above.