The Delta variant is likely to make up 90 percent of E.U. cases by late August, officials warn.

Residents of the European Union should be fully vaccinated against the coronavirus as quickly as possible this summer, the European Center for Disease Prevention and Control warned on Wednesday, as concerns grew that the contagious Delta variant would sweep across the bloc.

Andrea Ammon, the agency’s director, said the variant was expected to account for 90 percent of all coronavirus cases in the European Union by the end of August. The variant has already spread to 23 European countries; in some it is linked to a limited share of cases, but it is responsible for more than 66 percent of new cases in Portugal, which has faced a recent surge of infections. In Moscow, 90 percent of new cases are reported to be the Delta variant, according to the local authorities.

“Unfortunately, preliminary data shows that it can also infect individuals that have received only one dose of the currently available vaccines,” Dr. Ammon said. “It is very likely that the Delta variant will circulate extensively during the summer, particularly among younger individuals that are not targeted for vaccination.”

The Delta variant is unlikely to pose much risk to people who have been fully vaccinated, experts said. According to one recent study, the Pfizer-BioNTech vaccine was 88 percent effective at protecting against symptomatic disease caused by Delta, nearly matching its 93 percent effectiveness against the Alpha variant. But a single dose of the vaccine was just 33 percent effective against Delta, the study found.

After a sluggish start, the distribution of vaccines in the European Union has sped up in recent months. Even so, around 30 percent of residents over 80 years old and around 40 percent of those over 60 have yet to be fully vaccinated, according to the center.

Most E.U. countries have not yet fully vaccinated one-third of their total populations: the average is about 27 percent.

Chancellor Angela Merkel of Germany said on Wednesday that her country’s entire population will have been offered at least one dose of a vaccine by Sept. 21 if vaccine deliveries arrive as planned.

Public health officials have said that Delta may be 50 percent more contagious than Alpha, though precise estimates of its infectiousness vary. The European Center for Disease Prevention and Control estimates that Delta is between 40 and 60 percent more transmissible.

The warning on Tuesday brought a feeling of déjà vu: Last summer, a rise of cases among younger populations in some European countries and the United States led to new lockdown measures and a surge of infections among older people. But vaccines are driving down coronavirus case numbers in most of the United States, and it’s unclear whether Delta will reverse that trend.

The Delta variant has been detected in at least 85 countries to date, and public health experts say a surge of new cases in Britain in recent weeks has been driven in part by the variant. Dr. Hans Kluge, the World Health Organization’s regional director for Europe, said this month that the variant was “poised to take hold” in Europe.

The European Union is set to launch a bloc-wide program for proof of vaccination — the so-called digital green certificate — on July 1. Its purpose is to allow residents to travel freely within the bloc if they have been fully vaccinated, have proof of a recent negative test, or have recovered from Covid-19, although individual countries can impose their own restrictions.

Under recommendations issued last week by the European Union, American visitors would be able to travel to countries of the bloc if they can show a proof of vaccination or a recent negative test.