Two doses of a COVID-19 vaccine are needed to protect against the Delta variant of coronavirus, the European Medicines Agency (EMA) has said.
The European regulator also called on EU member states to speed up their vaccination programmes amid an increase in infections.
Several European countries have blamed a rise in COVID-19 case numbers on the highly contagious Delta variant, which first emerged in India.
"Preliminary evidence suggests that both doses of a COVID-19 vaccine are needed to provide adequate protection against the Delta variant," the EMA said in a statement.
"Adherence to the recommended vaccination course is vital to benefit from the highest level of protection against the virus," it added.
The regulator reaffirmed its approval of two doses of the Pfizer/BioNTech, Moderna, and AstraZeneca vaccines for European citizens.
According to the European Centre for Disease Prevention and Control (ECDC), the Delta variant of the coronavirus will account for 90% of cases by the end of August.
Studies have indicated that the variant is 40% to 60% more transmissible than previous strains of the virus.
"This makes it essential for countries to speed up vaccination programmes, including delivery of second doses where recommended, and to close the immunity gaps and opportunities for further emergence of variants, as soon as possible," the EMA said.
But the authority also pointed to ECDC figures which suggest that nearly 30% or more of people over 80 have not yet completed the recommended vaccination programme in ten EU countries.
Several countries have recently decided to tighten up their health measures, in particular, to encourage people who are still reluctant to be vaccinated.
The EMA also said on Wednesday that there was good scientific evidence that mixing doses of two different vaccines was "safe and effective" against COVID-19.
"[This strategy] may allow populations to be protected more quickly and make better use of available vaccine supplies," the regulator said.
But the EMA said it was "too early" to say whether a booster dose of a vaccine would be needed in the future.
"There is not yet enough data from vaccination campaigns and ongoing studies to understand how long protection from the vaccines will last, also considering the spread of variants," the body said.
In addition to promoting vaccinations, the European regulator also urged citizens to maintain other measures, such as masks and social distancing, to avoid a resurgence in cases.