"It is of concern that those most at risk of hospitalization or severe COVID are not adequately protected," the European Medicines Agency's head of health threats and vaccines strategy, Marco Cavaleri, told a news briefing.
The European average rate of re-vaccination, or receiving booster doses, was only 29% in the groups of people considered to be at highest risk, such as the elderly and immunocompromised.
Though there have not been major increases in COVID-19 case rates in recent weeks, Cavaleri said that could change during the colder winter months.
WHO data released late on Wednesday showed that 724,000 new COVID-19 cases were reported in the EU over the past week, down 11% from the previous week. In early October, when experts were warning of a new COVID-19 wave, the number of weekly cases reached 1.5 million.
"This virus is maintaining a fast pace in its evolution, and new subvariants of Omicron and its offspring are on the rise," Cavaleri said, adding that these are more resistant to vaccines.
Cavaleri said it was worrisome that available antibody treatments to fight the virus in infected patients are no longer working against new subvariants.
The EMA intends to issue new guidelines for health care providers on the issue of such treatments, he said.
An antiviral drug that is proving effective against all COVID-19 variants and subvariants is Pfizer Inc's. Paxlovid, he said. The drug has also been shown to prevent post-acute, longer-term effects of the disease, he added.
But prompt access to Paxlovid remains an issue in the EU, and the EMA is working with governments to change this, he said.
The Biden administration said earlier this month that the United States will keep in place the public health emergency status of the pandemic, instead of letting it expire in January.
Officials cited the possibility of a winter surge in COVID cases and the need for more time to transition out of the public health emergency to a private market as two factors that contributed to the decision.