Dr Hans Kluge cited a 10% rise in infection numbers over the last week because of “increased mixing, travel, gatherings, and easing of social restrictions”.
He cautioned that the highly transmissible Delta variant, first found in India, is on track to be the dominant one by August in the 53-country region.
There will be a new wave in the WHO European region unless we remain disciplined
Dr Hans Kluges said: "About 63% of people in Europe have not had a first vaccine jab."
“The three conditions for a new wave of excess hospitalisations and deaths before the autumn are therefore in place – new variants, deficit in vaccine uptake, increased social mixing,” he told reporters from Copenhagen, Denmark.
“There will be a new wave in the WHO European region unless we remain disciplined, and even more so when there is much less rules in place to follow, and unless we all take the vaccine without hesitating when it is our turn,” he added.
Dr Kluge said people who want to travel and gather over the summer should continue “life-saving reflexes” like wearing masks.
WHO Europe says people should make sure they get both doses of double-jab vaccines for maximum effectiveness.
Dr Catherine Smallwood, senior emergency officer at WHO Europe, warned governments not to lift social distancing measures amid increased transmission.
She said any such lifting should be accompanied by stronger public-health measures – sharing and sequencing information on new variants, testing, and reinforcing contact tracing.