European Union nations risk a new virus surge if they ease restrictions now, health officials warn.

There is a high risk of a surge in new coronavirus cases and deaths in European countries with insufficient vaccination coverage if they relax Covid-19 restrictions in the next few weeks, according to a new report by the European Center for Disease Prevention and Control.

So far, just over 62 percent of the total population of the European Union is fully vaccinated, and only three of its 27 member countries have fully inoculated more than 75 percent of their residents, according to the agency’s data.

That level of vaccine coverage is not enough to forestall the virus from spreading when Covid-19 restrictions are relaxed, the agency warned, especially now that the highly contagious Delta variant is causing almost all new reported coronavirus cases on the continent.

“Countries should continuously strive to increase their vaccination coverage in all eligible age groups, regardless of current vaccination coverage levels,” said Andrea Ammon, the agency’s director.

Anticipating surges of new cases, hospitalizations and deaths if precautions are relaxed soon, Ms. Ammon said it may be necessary to keep Covid-19 restrictions in place until the end of November.

The report comes at a time when most children in the European Union have resumed attending school in person, with no coronavirus vaccine authorized yet for use in children under 12. For this reason, it is especially important for the education system to implement preventive measures, the report said. The European Medicines Agency, the bloc’s drug regulator, said last week that it would decide by early November whether to approve the use of the Pfizer-BioNTech vaccine in children under 12.

As a whole, the European Union is well advanced with vaccination efforts; more than 73 percent of the bloc’s adults are fully vaccinated. But there are considerable differences from country to country.

Eastern nations like Romania and Bulgaria are far behind wealthier countries to the west, putting a large portion of the bloc’s population at greater risk. The agency’s report said it was crucial for those countries to increase their vaccination rates, by understanding why residents are not getting vaccinated and by adopting policies tailored to reach groups with particularly low vaccination uptake.